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Thank you- letting you know that this has been a valuable thread to read, thank you.

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Originally Posted by old mule
letting you know that this has been a valuable thread to read, thank you.
Thanks for taking the time to comment. The 'Gearbox' tab of my 3.25"-inch (800-page) Gold Star 'Shop Manual' has copies of everything I've ever found about these gearboxes, but the further I went into the SCT2 the more I realized that detailed information on them was lacking. Some people may know everything that has been in this thread, but such knowledge is useless as well as easily lost to future owners if it's not made available to others.

I decided that more precision would lead to more understanding so I checked the calibration of my 4"-5" micrometer and then measured the distance from thrust washer #73 to the raised ring near the ID of Gear H for two T-type layshafts, as shown in the photograph. One is 4.5685", and the other is 4.5760", for an average of <4.5723">. Call that 4.572+/-0.004". Remembering from an earlier post, InC is 4.580+/-0.002". To the extent these small number of data points might be representative, this means we can expect the face of Gear H to sit ~0.008" below the face of the main housing. Even if the tolerances conspire against us, it still will sit ~0.002" below the face.

Also remembering from another earlier post, I found that when #74 is placed on the middle gearbox housing it sits from 0.006" below the surface to 0.0015" above the surface. Call that on average 0.002+/-0.004" below the surface. So, just considering averages, the average T-type layshaft will have an end float of 0.010" (4.580-4.572+.002=0.010) if installed with no gasket, and 0.020" if installed with a paper gasket, i.e. either way would have the desired end float. Actually, almost certainly BSA intended for #74 to sit precisely level with the face, in which case the end float would be 0.018" if installed with a paper gasket.

Of course, it's unlikely any set of components will have these average values, but this information is still quite useful. We can infer from it that BSA intended that the depth of InC in the main gearbox housing, plus a paper gasket, would provide the desired end float. Actually, I infer that their 0.010"-0.020" range isn't so much "desired" as it is an admission of BSA's production tolerances. So, given the choice, I would aim for the bottom end of this range. This information also says that something is amiss if we find Gear H above the face of the housing rather than slightly below since in that case only an out-of-spec fat paper gasket would provide the required end float, which isn't how it would have left the factory.

EndFloat01a.jpg
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Possibly bringing this digression into gearboxia arcania to an end, a simple procedure to determine and adjust layshaft end float in T-type gearboxes prior to applying anything messy to the cases is as follows (a note about Standard gearboxes follows at the end):

Determining and Adjusting Layshaft End Float

To have everything in one place for what follows the internals of T- and T2-type gearboxes are shown in the first image.

1) Assemble the layshaft with thrust washer #73, but without #74, and measure the length with calipers to confirm it is approximately 4.570"+/-0.005"
[Although a micrometer is shown in the second photograph calipers are sufficiently accurate for this measurement that will determine if anything is seriously amiss in the way it has been assembled (e.g. Gear B not pressed fully on the shaft, pressed on without the locating ring in place, or is the thinner one from a Standard gearbox). Any problems need to be located and corrected before proceeding. Although Gears D and F interchange between T-type and Standard you still have to confirm they have the correct number of teeth for your intended ratios.]

2) Insert assembled layshaft into housing with thrust washer #73 but without #74

3) With a flat on the gearbox case use a feeler gauge to determine the Clearance to Gear H (CGH).
[If CGH is negative there is likely a problem with the housing (e.g. the needle bearing isn't flush with the case or the bearing is installed in an unmodified Standard case) or layshaft (e.g. there is an incorrectly made aftermarket washer in place of #99). Any problems need to be located and corrected before proceeding. For the layshaft and gearbox case shown in the third photograph CGH = 0.010". Note, however, that gasket residue or nicks in the Al can give an apparent reading a few thou. too high if you're not careful, and a slightly raised bush inside Gear H can give an apparent reading a few thou. too low if the feeler gauge is positioned over it rather than the raised machined surface on the gear. Also, since the layshaft is supported only at one end at this point it can be slightly tilted so measurements should be made in several positions and averaged.]

4) With thrust washer #74 in place use a flat and feeler gauge to measure the Clearance on the Middle Casting (CMC), keeping in mind it could be either positive or negative.
[For the casting shown in in the fourth photograph CMC = 0.005". If CMC is much larger than this it is likely the casting has been altered in which case a thicker bespoke washer may be required, and if CMC is proud of the surface by more than a few thou. it is likely due to an out-of-spec. aftermarket washer that is thicker than 0.113". Any problems need to be located and corrected before proceeding.]

5) Keeping track of positive or negative signs, calculate the Raw End Float (REF = CGH + CMC)
[For the combination shown in the third and fourth photographs REF = 0.010"+0.005" = 0.015". This says if the two cases are clamped together without a gasket the end float would be 0.015".]

6) Use REF to determine what additional layshaft shim, if any, or gasket thickness is needed to obtain the Desired End Float (DEF) of 0.010"-0.020".
[As can be seen from the fifth photograph, when the above two cases are bolted together without a gasket the measured end float is 0.016". This is the same as the 0.015" from the separate casting measurements within the variation expected from using feeler gauges. So, if this combination of layshaft and cases were to be used, these two cases should be sealed only using gasket cement to result in a value within the range of DEF.]

The situation of the REF being less than the DEF can be easily addressed with paper or composite gaskets. However, I can't remember ever seeing a discussion of what to do if the REF is larger than the DEF since washers #73, #74 and #99 only were made in one thickness each.

Although #73 and #74 are hardened steel, keep in mind that one side each sits directly against soft Al, and the equivalent of these hardened steel washers in a Standard gearbox are the soft (compared with hardened steel) bronze brims of the top hat bushes. Since none of these relatively soft surfaces has a wear problem it means the thrust against them is small. So, if needed, I wouldn't hesitate to make a #73 or #74 (but not #99) thrust washer from bronze of the necessary thickness to achieve the DEF.

For bespoke thrust washers I would use a high strength phosphor or aluminum bronze because there's no reason not to, although light duty SAE 660 bronze probably would be fine. Ideally, they would be surface ground to make their faces accurately parallel. However, it is very unlikely the brims of the top hat bushes in a Standard gearbox are accurately perpendicular to the layshaft, and they don't have problems, so the tolerance of a lathe should be fine for producing these washers.

Note on Standard type gearboxes: In these gearboxes the brims of the top hat bushes serve the shimming function of washers #73 and #74 so the end float procedure described above applies with this in mind. In Step 1) the measurement should be 4.400"+/-005" for a Standard layshaft.

Gearbox_shims.jpg EndFloat01a.jpg EndFloat01.jpg EndFloat02.jpg EndFloat03.jpg
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Before leaving the subject of end float, I started assembling the Spitfire's gearbox yesterday and one of the first things to do was to check and adjust the end float. I did a test assembly and found it out of range at 0.026" without a gasket, making me initially think I'd have to machine a thicker thrust washer from bronze to reduce it below 0.020". I then realized there was another, even better, option. The first photograph shows Gear B pressed up against the circlip that locates it. Looking closely it can be seen that the groove for the circlip is considerably wider than the circlip, allowing another way to reduce the end float.

If a ~0.014" gasket is added the 0.026" end float would increase to 0.040" which is way out of range. However, if I added a wire to that groove the gear could not be pressed as far onto the shaft and the result would be to extend the far end of the layshaft further from the gear, i.e. it would reduce the end float. A 0.039" wire along with a gasket would reduce it to ~0" if the gear currently were pressed fully on and if it were then pressed back on with the same force after adding the wire. Actually, because the dogs on the gear are somewhat tapered on the bottoms, a wire of diameter less than that of the circlip would let the gear be pressed further than calculated from the wire diameter alone so this wire should result in a positive end float. It seemed worth spending a few minutes with various wires to find the best result.

The second photograph shows the shaft after I pressed the gear partially off. As my first attempt I added an additional "circlip" of 0.039" stainless steel wire, the thickest that would barely fit in the groove. The third photograph shows it after I pressed the gear back on.

The fourth photograph shows that after adding this wire the end float, with gasket now in place, has been reduced to 0.015". I expect after bolting the case together this may reduce by a thou. or two. Anyway, with the end float adjusted I can move on with assembling the gearbox.

Layshaft_offset01.jpg Layshaft_offset02.jpg Layshaft_offset03.jpg Layshaft_offset04.jpg
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A few years ago I turned a broken gearbox case and middle cover into Swiss cheese so I could see what was going on inside. In light of the measurements I made over the past week, yesterday I made a slight modification to convert it into a precision instrument for inspecting the operation of T-type internals. It does the same for Standard internals with an appropriate spacer.

Anyway, the five photographs show the configuration of the gears in my SCT2 in each of the positions of the shifter cam in the order 1st, N, 2nd, 3rd, 4th. Enjoy flipping through the images and watching what each gear does as you shift up and down. Gears G & H are nearly hidden in these photos but can be easily seen by moving one's head, as can be seen that gear pairs A & B, and G & H are always in full contact with each other.

Less than full contact occurs with the middle set of gears in 2nd and 3rd. This contact is determined by the cam along with the fact each fork moves a pair of gears together coupled with the need for the dogs of one set of gears to be sufficiently engaged to transmit the torque while simultaneously those of another set are sufficiently clear of each other.

Cutaway_gears_1st.jpg Cutaway_gears_N.jpg Cutaway_gears_2nd.jpg Cutaway_gears_3rd.jpg Cutaway_gears_4th.jpg
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Hi MM,

I'm not familiar with the SCT2 gearbox as used on your Spitfire Scrambler, however it looks similar enough in operation to the bikes I've previously worked on including Nortons, BSA's and others.

Your gearbox looks to be in good condition with little if any wear on the engagement dogs and gears. Whats not obvious from the photos is the condition of the cam plate, selector forks and detent spring, though no doubt you have these components under control and are using the best available.

If your gearbox is anything like the one used on BSA unit singles, then the correct engagement of the detent spring with the cam plate is essential for correct gear engagement, the gearbox cutaway observation holes will be useful for checking this. Additionally the tension on the detent spring can affect the gear change operation though I'm not sure how this is adjusted on your box.

As previously noted, shimming of the gears is needed and ideally I would try to adjust things so that the dogs are engaged as much as possible although this may not always be possible.




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Originally Posted by gunner
Whats not obvious from the photos is the condition of the cam plate, selector forks and detent spring,
Actually, what I had installed for those photos were the plate, forks and springs that are part of the ventilated case. However, the plate and forks are easy to swap so I will be doing careful measurements with the actual SCT2's components.

Originally Posted by gunner
Additionally the tension on the detent spring can affect the gear change operation though I'm not sure how this is adjusted on your box.
There's a bolt-type adjuster that sets the load on a spring and BSA suggests adjusting it such that one thread is left exposed.

Originally Posted by gunner
I would try to adjust things so that the dogs are engaged as much as possible although this may not always be possible.
The possibilities for making adjustments are less than might seem apparent at first. Gears A/B and G/H are fixed in their positions so once the end float is set there's nothing more to be done with them. The shifter forks lock C/E together and D/F together so each pair is forced to move in lock-step. Referring to the photographs in my previous post, you can see there is a small space between E and G in 3rd gear so a shim could be placed on the shaft to offset that fork a little to the right. However, if there were done, the dogs on C would then engage less with A when in 4th.

The cam plate is the one other place where there could be problems. If it were made incorrectly it might, say, shove a gear pair the correct distance one way in a given gear, but not far enough the other way in a different gear. I took a photo I'll use later, showing the differences between a 'normal' and a reversed cam plate, applied some Photoshop magic, and produced the composite photograph below.

The reversed plate (i.e. RRT2-type) is in green with its cam slots in red, and the Standard plate is overlayed on it at 50% transparency. Note that the slots match extremely well between these plates, which means the relative positions of the gear pairs would be the same. However, I haven't yet checked to be sure there are no issues with the SCT2's cam plate, such as the slots being identical but the notches for the detent being incorrect. All will be checked before it goes together, but I've had no garage time today and will have very little tomorrow..

RRT2_cam_overlap.jpg
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This past weekend I decided to disassemble another T-type gearbox so I would have a complete set of photographs of the entire rebuilding process (plus another freezer baggie full of gearbox parts...). Although I've written what follows specifically for the T-type, the differences from the Standard type are small so the information should be of general use to anyone with a pre-unit swinging arm BSA model.

The disassembly follows in the next few posts, broken up by the five-image limitation per post, but eventually all the king's horses and all the king's men will put a coherent gearbox rebuild thread together again. Maybe...

There is one photo per numbered line item in what will follow in the next few posts. To avoid headaches with numbering I don't mention the photograph number in each line description (e.g. Step 7) doesn't explicitly mention the "seventh photograph") so the reader will have to figure out that the, say, 2nd photograph in a given post goes with the 2nd Step in that post (again, for example, Step 7 will be in the 2nd post and will be illustrated by the 2nd photograph in that post because 7-5=2). Clear?

Disassembly of the Gearbox

1) The gearbox will be easier to disassemble if you mount it in a sturdy vise with soft jaws to protect the Al. Although it isn't essential to remove the inspection cover in order to disassemble the gearbox, the cover is held by two screws.

2) Remove the three screws and four nuts that hold the outer case. The arrows point to an additional screw and nut (hidden by the kickstart lever in this photograph), neither of which is removed. Also, leave the kickstart lever in place for the moment.

3) Rotate the kickstart lever somewhat to relieve pressure on its stop, then use the lever in that position to help wiggle the outer case off the four studs. A few gentle(!) taps with a soft mallet on the projection for the speedometer cable may be needed to help break the seal to the gasket. I'll deal with rebuilding this outer case sometime in the future so for now set it aside.

4) Flatten the lock washer on the drive sprocket, hold the sprocket with a chain wrench, and use a punch to loosen the nut. Since hitting the nut also in effect hits the ball bearing, instead of a punch I use a sprag socket I made for this purpose along with an impact wrench. Usually the chain wrench isn't needed when the impact wrench is used. Leave the sprocket in place for now.

5) Use a large screwdriver to shift the gearbox out of neutral into any gear. You may have to rotate the gearbox's input while doing this to get the gears to align and allow the shift to be made. Unless a reverse shifter plate is used (e.g. in an RRT2) neutral is engaged when the red recessed dot on the shifter quadrant is aligned with the recessed dot on the housing when viewed straight on. However, likely by now the paint is long gone; the red paint you see in this photograph was applied by me.

Disassembly010.jpg Disassembly020.jpg Disassembly030.jpg Disassembly040.jpg Disassembly050.jpg
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6) Remove the circlip from the end of the layshaft.

7) Bend the lock washer on the mainshaft flat and, with a chain wrench on the drive sprocket, remove the nut. An impact wrench makes this easier but, unlike when removing the nut on the drive sprocket, you will need a chain wrench to hold the mainshaft.

8) Remove the ratchet mechanism, spring, and washer.

9) With a large screwdriver lever the shifter quadrant back into neutral, which is where the recessed dots on the shifter quadrant and on the housing align when viewed straight on. For a reverse shifter plate (e.g. in an RRT2) move the shifter quadrant to its highest position (1st gear), then move it down one click to be in neutral. You can confirm any gearbox is in neutral by checking that the the input shaft and sprocket turn independently of each other.

10) One screw at the top still holds the middle cover in place but it is often hidden by the gasket and so easily can be missed. With the "hidden" screw removed, slide the middle cover off the four studs.

Disassembly060.jpg Disassembly070.jpg Disassembly080.jpg Disassembly090.jpg Disassembly100.jpg
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11) With the middle cover removed, slide the mainshaft out of the case. The drive sprocket now can be removed as well since the chain wrench won't be needed for the remaining disassembly.

12) Remove the selector shaft by first removing the grub screw and then pulling the shaft out from the front of the case. Since the grub screw seats in a deep groove in the selector shaft it is safest to remove the screw completely before trying to remove the shaft. Although the shaft may be a tight fit you can easily tap it out from the back of the gearbox using a drift.

13) With the selector shaft removed you can now remove Gears C and E along with the selector forks. Although the two selector forks were identical when new, because of wear it's best to mark them in some way so you can return them to their original positions when you reassemble the gearbox.

14) The complete layshaft with its thrust washers at either end can be removed as a unit.

15) If you want to remove the shifter cam plate, first remove the indexing plunger. Although an RRT2 has its indexing plunger installed at the top of the case rather than the bottom, all five T-type cases I have (for SCT, SCT2, ASCT, and STDT) are drilled and tapped at the top as well as the bottom so this feature of the main case is not unique to the RRT2.

Disassembly110.jpg Disassembly120.jpg Disassembly130.jpg Disassembly140.jpg Disassembly150.jpg
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16) Since the selector shaft already has been removed the cam plate now can be simply pulled off the short pin. It's best to remove this plate even if you only wanted to remove the plunger since otherwise it easily could fall to the floor when you handle the case. A nut with lock washer on the outside of the case holds the pin in place, and a slot in the head allows use of a 90° screwdriver to keep the pin from turning as the nut is removed.

17) Remove the snap ring and then the oil seal. Using two screwdrivers allows one of them to pry the notched end of the snap ring out of the groove and the other to lever it up and over the edge of the case.

18) If the sleeve bearing needs replacement heat the case to 225 °F and tap or press the bearing out with a socket or drift of suitable diameter. The press fit at room temperature is ~0.002" so, as shown in the inset, if you do not heat the case there is a good chance you will ruin the case by the force required to remove the bearing.

19) If the layshaft needle bearing (or bush) needs replacement it also should be done with the case heated to 225 °F. If the bearing is of the open-end type there's a big enough gap between it and the end cap that a 17 mm blind bearing puller can be used. If it is of the closed end type the bearing and the outer cap will have to be tapped out from the inside using a suitable drift.


These 19 steps complete the disassembly of the major components of the swinging arm-type gearbox, although several posts sometime in the future will be needed to address the middle and outer covers. As I wrote earlier, in parallel with these posts I've been pulling together into a Word document all the relevant material I've written on gearboxes and organizing it in a more logical sequence than in this thread. However, that document already is up to 68 pages, including 111 embedded thumbnail images, so how to deal with it after it's finished is an open question.

Disassembly160.jpg Disassembly170.jpg Disassembly180.jpg Disassembly190.jpg
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Section I, the 19 steps to disassemble a pre-unit swinging-arm gearbox, was just completed. Next will be Section III, the steps needed to assemble one. I'm skipping Section II for now, the steps to inspect and repair all the internal components, because I've already covered some of this in posts spread over the past few years, and all of which will be in the final massive document. Anyway, what follows assumes all the necessary repairs already have been made and the components are ready to be reassembled.

As with the disassembly, there will be five photographs in each post, each illustrating the corresponding step.

Addendum: I realized I left out a description of the installation of the sleeve gear oil seal, which is now step 0 below. However, I can't add an image for this step because of the 5 images/post limit so instead refer to the final post of the disassembly process.

Assembly of the Gearbox

0) Follow step 17 of the disassembly in reverse to install the oil seal.

1) With the gearbox housing mounted in soft jaws on a vise, oil the bearings and the various components as they are installed.

2) Although the inside face of the sleeve gear is pulled tight against the inner race of the bearing, any oil that gets past can find its way out of the gearbox via the splines. Although this leakage path is unlikely, a thin coat of gasket sealant makes it even more unlikely.

3) Push the sleeve gear into place. It will be a snug fit so it might have to be lightly tapped into place with a piece of wood. Install the sprocket, lock washer and nut.

4) There is no torque specification for this nut but later BSA manuals specify 50/55 ft.lbs. for the 1" fork cap nuts. Based on this I torque the nut to 60 ft.lbs. using the sprag socket I made along with a chain wrench. Without such a socket you will have to use a hammer and drift. Once the nut is tight bend the lockwasher to lock it in place.

5) If the short spindle for the cam plate was removed, reinstall it with its lockwasher and nut on the outside, using a 90-deg. screwdriver to keep it from turning, if necessary. Once the nut is tight bend the lockwasher to lock it in place. Slip the cam plate on the spindle, check that it rotates freely, then move it to the neutral position (CCW to the first detent, then CW to the shallow neutral detent next to it) and install the plunger.


Note: PhotoShop, Adobe Bridge, Paint3D, and Windows all show the photographs in the correct orientation but Britbike has rotated all of them in this post by 90 degrees. I don't know why this is the case and have no control over it..

Assembly010.JPG Assembly020.JPG Assembly030.JPG Assembly040.JPG Assembly050.JPG
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Great work MM and just a couple of comments which may be useful:-
- the gearbox sprocket looks slightly hooked to be, maybe its the camera angle or you're using an old sprocket for demo purposes.
- there's clearly a line on the selector cam where the detent spring has been rubbing. Its often worthwhile reliving any high spots in this area and also inside the selector grooves. This will help make the gear change smoother.


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Originally Posted by gunner
gearbox sprocket looks slightly hooked ... a line on the selector cam
You're right on both counts. But...

As I assembled the SCT2 gearbox (which already is finished, although I've only posted the first five steps so far) I made notes. Sometimes I took photos of certain steps on the SCT2 as soon they happened, other times my hands were too greasy to use my iPhone so an hour or two later I consulted my notes and used parts from the shelf to reconstruct those earlier steps, and yet other times I realized when drafting the material that an additional, or a different, shot would be better and I reconstructed it. Anyway, what you see isn't (necessarily) what the SCT2 got.

An unedited example of a scene I reconstructed today from non-SCT2 parts to better illustrate a step I took a few days ago when assembling the SCT2 is below. Getting the shifter fork to capture Gears C and E as well as insert in the shifter plate can be a fiddly process which sounds even more complex than it is when described in words. So, I reconstructed the scene from another point of view to make it clear what the words mean. An edited version of this shot will appear in a few days when the post reaches that point in the assembly.

A very distressing development just occurred while I was typing this. I got a text from the organizers saying that the 2020 Cannonball will be for pre-1930 motorcycles. Which means my 1928 Ariel again qualifies. Damn, so much for the hoped-for excuse that a different machine would be needed...

IMG_9768.JPG
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Quote
A very distressing development just occurred while I was typing this. I got a text from the organizers saying that the 2020 Cannonball will be for pre-1930 motorcycles. Which means my 1928 Ariel again qualifies. Damn, so much for the hoped-for excuse that a different machine would be needed...


Well, at least you now know the Ariels weak areas and have the time, patience and skill to address them.


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Note: this post only has 4 steps in it because step 10 will have two photographs and I don't want the 5 photograph limit per post to break them up.

6) If a reverse cam plate is installed instead of a standard one, rotate it CW to the first detent (to the right of the shallow neutral detent) which is 1st gear and install the plunger at the top of the gearbox rather than the bottom.

7) Screw the plunger in such that it leaves one thread exposed after tightening the lock nut.

8) Install the complete layshaft assembly along with the shifter fork in the orientation shown and with the fork overlapping the facing raised ridges of Gears D and F. The round projection on the side of the shifter fork facing the shifter cam should be in the slot of the cam farthest from you. This photograph shows Gear H and thrust washer #74 on the layshaft, but they should be removed for the next step. Leave washer #99 in place or, if you remove it, do not forget to replace it.

9) With Gear H and thrust washer #74 removed from the layshaft, push Gear C all the way against sleeve Gear A as well as against the housing at the left (i.e. not directly in line with sleeve Gear A), and hold the second shifter fork in the position and orientation shown to allow clearance for the raised ridge of Gear E in the next step.

Assembly060.jpg Assembly070.jpg Assembly080.jpg Assembly090.jpg
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This post only has four steps because the first two photographs go with step 10, leaving only three more that can be included.

10) While continuing to hold the shifter fork with your right hand in the position shown in step 9, use your left hand to insert Gear E, push it against Gear C, then lower the shifter fork to capture the adjacent raised ridges of Gears C and E. After the fork has captured the two gears it might require a small manipulation to ensure the round projection on the opposite side of the fork is seated in the cam slot nearest you. I realize this operation sounds fiddly, and it can be, but it's fairly straightforward if you follow the instructions. The first two photographs in this post illustrate step 10 as you will see it, and if you could look down through the top of the case before the fork is fully seated in the cam.

11) When both forks and the shaft are correctly installed, ensure washer #99 is still in place on the layshaft and then replace Gear H and washer #74.

12) Insert the mainshaft and then insert the selector fork shaft through the two forks, which will require moving each slightly to align them as you insert the shaft.

13) You might have to tap the shifter shaft to get it fully into the housing, which is when the end is flush with the outside. If you overshoot, tap the shaft back to be flush with the housing. Then put Loctite on the retaining screw and tighten it to lock the shaft in place. In addition, you could fill the cavity above the screw with silicone sealant to be extra careful since if the screw vibrates loose the shaft could migrate out causing the gearbox to cease shifting.

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14) Confirm the shifter cam plate is still in neutral. Then, in a test fitting, install the middle cover using the screw at the top and a spacer on the stud diagonal from it to tighten it. Use whatever gasket (or none) that you used previously to set the end float of the layshaft. It's OK to let the shifter quadrant find its own position in this test. Make sure the shafts rotate freely.

15) Measure the end float to confirm it is still the same value as you previously determined. If not, find the problem and address it. The end float should be in the range 0.010"-0.020".

16) Remove the middle cover, apply gasket sealer to the surfaces, wait whatever time is recommended by the manufacturer (20-30 min. for the Permatex 'Permashield' that I use), install the gasket, and slide the middle cover partially on, leaving a gap of a bit over ¼" from the main case.

17) With a standard, non-reverse, cam plate that is in the neutral detent, hold the shifter quadrant with the two dots aligned when viewed straight on.[*] Use a soft mallet to gently tap the middle case to lower the gap to a bit less than ¼" at which point the teeth of the shifter quadrant will start to mesh with those of the shifter cam. You might have to move the quadrant slightly up or down to get the teeth to mesh. After confirming the two dots are still aligned tap the cases together. If the dots are not aligned while there is still a gap, widen the gap enough to release the teeth, re-align the dots, and try again until you are able to install the middle cover with the dots aligned.
[*]BSA Dealer Parts and Service Bulletin No. 82 dated 19 March 1963 says an unknown, but small, number of shifter quadrants were produced with the dot in an incorrect position which could result in not being able to select all four gears. The Bulletin says there isn't "any way of accurately describing how the quadrant might be checked". Because of the rounded outer edges of the quadrant it's not possible to give accurate figures but when using calipers I find the center of the dot 0.25"+/-0.01" from the nearest edge and 0.65"+/-0.1" from the other edge.

18) If a reverse cam plate is installed the dots are not used. Confirm that the reverse cam is in the 1st gear detent, move the quadrant up until there is a ~5 mm gap between it and the case, and tap the case to lower the gap to less than ¼" at which point the teeth will begin to mesh. If correct, the gap between quadrant and case will be ~5 mm as the teeth start to mesh but if off by a tooth it will be ~12 mm. After the case is fully in place the gap will be nearly zero as the teeth are fully meshed but if off by a tooth the gap will be ~7 mm.

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Last edited by Magnetoman; 01/06/19 5:14 pm. Reason: info. from BSA Parts & Service Bulletin
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19) Once the quadrant and cam are correctly meshed tap the cases together and clamp them with the screw and a spacer on one of the studs.

20) Before the gasket sealer has set, use a large screwdriver to shift the gearbox up and down through the range to confirm it clicks into five detents total (four gears plus neutral). If you have the inspection cover off you can watch the operation of the two pairs of gears as they are moved by the selector forks.

21) Attach the circlip to the end of the shifter shaft.

22) The end float should not have changed, but confirm it still is within the range 0.010"-0.020".

23) Use a large screwdriver to put the gearbox in any gear and install the kickstarter ratchet assembly. In order from inside to out this consists of a washer, bush, spring, pinion, ratchet, lock washer, and nut. Based on the thread being 9/16" I use a chain wrench to hold the sprocket and torque the nut to 40 ft.lbs. Once the nut is tight bend the lock washer to lock it in place.

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24) Install the kickstarter quadrant in the middle cover and make sure the mechanism operates smoothly since wear of the second tooth of the quadrant can cause it to jam (the first tooth is partially ground off) and a too-tight bush under the ratchet pinion can cause it to bind.

25) Install the kickstarter quadrant in the outer cover with the notch hooked over the spring.

26) If you removed the nut for the kickstarter spring or the special fastener for the speedometer drive, replace them now. The speedometer drive mechanism has to be correctly oriented in the housing in order for the unthreaded portion of the fastener to pass through it to keep it from rotating.

27) Do a test fitting of the outer cover to make sure there are no issues. Install the kickstarter lever, tension it against the spring to the ~10:00 position (~12:00 for the RRT2-type quadrant) for the lever to clear the stop, and fit the outer case over the four studs. This may require wiggling the kickstart back and forth as well as rotating the mainshaft to allow the speedometer gears to seat.

28) If all went well with the test fitting, remove the outer cover, apply gasket sealer to the surfaces, and wait whatever time is recommended by the manufacturer (20-30 min. for the Permatex 'Permashield' that I use).

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29) Shout Hurray. and take a dram.

Well done MM


71 Devimead, John Hill, John Holmes A65 750
56 Norbsa 68 Longstroke A65
Cagiva Raptor 650
MZ TS 250
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Originally Posted by gavin eisler
29) Shout Hurray. and take a dram.
Not so fast, buckaroo, there still are two steps remaining:

29) Reinstall the cover as in step 27. Four nuts with washers along with three screws (two 2" at the front and one 1â…›" at the rear) hold it in place.

30) Install the circlip shown by the arrow followed by the selector lever and check that the gearbox correctly selects all the gears. If you removed the drain plug or inspection plate during the rebuild now would be a good time to reinstall them.

These 31 steps (counting step 0 that I added later) complete the assembly of the major components of the swinging arm-type gearbox, although several posts sometime in the future will be needed to address the middle and outer covers.


Surely the effort that went into these gearbox posts deserves a double dram.

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Definitely. Slainte.


71 Devimead, John Hill, John Holmes A65 750
56 Norbsa 68 Longstroke A65
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Sláinte mhaith*, MM!

THis was well worth the double dram, which I will raise myself this evening. It has inspired me to start the restoration of my own 59 DBD34 scrambler with the gearbox, and I'm already into the middle cover.

Relax, now, and dream of Cannonballs.



*Gaelic: "Good health!" as spoken by the Celts in Gavin Eisler's world, and in mine, across the water.

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Originally Posted by NYBSAGUY
Relax, now, and dream of Cannonballs.
Should I dream of small or large Cannonballs?

For the past week I've been at an impasse of sorts with the Spitfire. With the rebuilt gearbox resting comfortably it would seem the engine should be next. However, once I start on it the workbench will be occupied for the duration and I'd like to get the Ariel's engine taken care of first. Unfortunately, the backordered exhaust valve blank still hasn't shown up. I'll reassess my options with the Ariel's head and the Spitfire's other components (e.g. rebuilding the forks) over the next few days and decide what to do next.

While straightening up the work area today I decided to lay out all the "special tools" I used in rebuilding the SCT2 gearbox (i.e. excluding screw drivers, sockets and spanners). Unless I forgot something, the 20 I used are shown below.

SpecialTools.jpg
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