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1948 Gearbox: Converting early to late mainshaft? #735428
05/15/18 6:35 pm
05/15/18 6:35 pm
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South Pasadena, Ca. USA
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Dicky Mitchell Offline OP
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I have a '48 rigid gearbox (66-3141) with the shorter main-shaft. I'd like to switch over to a better 4-spring Triumph style clutch like so many Gold Star riders do, however I know that these conversions don't seem to work with the earlier short shaft. I was wondering it there are any later main-shafts that can be fitted to this box that are of the longer style that would work with this clutch upgrade? Its difficult to tell from the photos I've found if there is a difference other than length between various main-shafts. I'll be having the box rebuilt so I thought I'd see if swapping main-shafts was an option first and if so, which one would work?

Thanks

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Re: 1948 Gearbox: Converting early to late mainshaft? [Re: Dicky Mitchell] #735459
05/15/18 10:19 pm
05/15/18 10:19 pm
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Originally Posted by Dicky Mitchell
I'd like to switch over to a better 4-spring Triumph style clutch like so many Gold Star riders do, ...
I don't have the answer to your question, but as for 'better', I question that. I realize it would benefit from having the photographs PhotoBucket deleted, but I described in detail how to rebuild a 6-spring clutch so it will work properly. It's hard to imagine a Triumph 4-spring clutch working any better than the rebuilt 6-spring clutches on two of my Gold Stars that covered ~1000 miles each this past fall (and I have a Triumph with a 4-spring clutch so I have a basis for comparison).

Re: 1948 Gearbox: Converting early to late mainshaft? [Re: Magnetoman] #735514
05/16/18 8:50 am
05/16/18 8:50 am
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The problem with 6 spring clutches is flexing of the pressure plate. and 4 spring clutches for that matter.
It would appear that the holes were pressed with no regards to the orientation of the grain direction.
Thus if you have lucked it in and have a plate where hole 1 & 4 are directly in the rolling direction then hole 1 the centre hole and hole 4 form a bending axis and there is no way the plate will lift without flexing.
I found this out when I etched a 4 spring pressure plate that was giving me grief.
So some 6 springs can be set up to work beautifully, even though it is very flawed design and others can not without adding some sort of stiffener to the pressure plate.


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Re: 1948 Gearbox: Converting early to late mainshaft? [Re: Dicky Mitchell] #735545
05/16/18 2:47 pm
05/16/18 2:47 pm
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Rickman Offline
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Another conversion to consider;
I have heard the "best" conversion to consider, was to utilize a Notrun clutch on the short BSA shaft...? It was supposed to be the thing to do in the day...

Re: 1948 Gearbox: Converting early to late mainshaft? [Re: Rickman] #735547
05/16/18 3:04 pm
05/16/18 3:04 pm
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South Pasadena, Ca. USA
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Dicky Mitchell Offline OP
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I haven't heard of this option? Do you have any additional info as to which year/make/model clutch? This sound interesting

Re: 1948 Gearbox: Converting early to late mainshaft? [Re: BSA_WM20] #735550
05/16/18 3:45 pm
05/16/18 3:45 pm
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Originally Posted by BSA_WM20
It would appear that the holes were pressed with no regards to the orientation of the grain direction.
Could you post a sketch of what you think is happening, taking into account the three-dimensional structure of the cap? Remember, hundreds of thousands of these clutches were produced during the 1950s so its surprising a fundamental design flaw as you're suggesting wasn't revealed at the time. However, there have been 50+ years since then for the clutches to be bodged and re-bodged and they aren't immune to that. Anyway, I can't picture how grain orientation would affect deflection in the actual three-dimensional structure of a cap so a drawing would help. Thanks.

p.s. I just looked at my 6-spring restoration thread to find some information and all the deleted Photobucket images are back(!). Has anyone read something recently about this, i.e. is this a temporary anomaly, or has Photobucket announced a change of their policy on 3rd party Hosting back to what it used to be?

Last edited by Magnetoman; 05/16/18 4:11 pm. Reason: p.s.
Re: 1948 Gearbox: Converting early to late mainshaft? [Re: Rickman] #735558
05/16/18 4:55 pm
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Originally Posted by Rickman
Another conversion to consider;
I have heard the "best" conversion to consider, was to utilize a Notrun clutch on the short BSA shaft...? It was supposed to be the thing to do in the day...




Yes, the mainshaft out of the semi-unit twins is splined at the clutch end and will accept the Norton Atlas clutch center. I have a gearbox that is stamped RRT2 and was raced for many years with this setup. I believe Eddie Dow sold the shaft in his accessory catalog.



Bill B...


Note: This is for the later swingarm PU gearbox, not the early kidney shaped rigid/plunger gearbox.

Last edited by Boomer; 05/16/18 4:59 pm.

Boomer
Re: 1948 Gearbox: Converting early to late mainshaft? [Re: Dicky Mitchell] #735569
05/16/18 7:54 pm
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The Eddie Dow brochure states the the splined mainshaft to accept the AMC / Norton clutch, can be fitted into STD T and RRT 2 gearboxes. This I find rather strange, as the splined type shaft in the brochure, is depicted as having an oil scroll the full length of the section which runs in the sleeve gear, as would be compatible with the fully bushed type gear.
The RRT 2 mainshaft is only scrolled on the outer bush section at 0.810"OD, and the section which runs in the needle bearing is up 0.0025" at 13/16" OD.
It would obviously fit into a needle bearing type sleeve gear, and be the correct size at the bushed outer end. But it would be running a scroll in the needle bearing, and 0.0025" down on size.
I have not personally fitted one of these shafts with an AMC clutch, but the scenario with the needle type sleeve gear has crossed my mind.


Brian

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Re: 1948 Gearbox: Converting early to late mainshaft? [Re: Dicky Mitchell] #735585
05/16/18 10:06 pm
05/16/18 10:06 pm
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There is a person who would know about this conversion, but he is NOT online, I have to go to him and get the info.....

I'll try to do that some time this summer, but he isn't close by...

Maybe Boomer has the best info...?

My guy didn't say a thing about needing the "T2" type trans.....

I decided NOT to pursue this conversion, because of the cost of the other shaft, the whole new clutch assembly, assorted gaskets, and whatever I couldn't think about right off...

Re: 1948 Gearbox: Converting early to late mainshaft? [Re: BSA_WM20] #735595
05/16/18 11:27 pm
05/16/18 11:27 pm
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Sydney, Oz
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Shane in Oz Offline
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Originally Posted by BSA_WM20
The problem with 6 spring clutches is flexing of the pressure plate. and 4 spring clutches for that matter.
It would appear that the holes were pressed with no regards to the orientation of the grain direction.
Thus if you have lucked it in and have a plate where hole 1 & 4 are directly in the rolling direction then hole 1 the centre hole and hole 4 form a bending axis and there is no way the plate will lift without flexing.

I hadn't thought much about it before, but the clutch pressure plates have a step. To a first approximation, the effective thickness of the plate is the depth of the step.

Re: 1948 Gearbox: Converting early to late mainshaft? [Re: Dicky Mitchell] #735615
05/17/18 3:05 am
05/17/18 3:05 am
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Kerry W Offline
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Long time since I've had both s/arm (late) and rigid/plunger (early) gearbox internals in my hands, but I seem to recall that the design of the gears, splines and engagement dogs was quite different between the boxes, so it would surprise me if retro-fitting a later shaft into an early box was feasible. That said, I never actually tried putting gears from one box on the shafts of the others.

I've been wrong before..

KW


No generalisation is wholly true, not even this one.
Oliver Wendell Holmes
Re: 1948 Gearbox: Converting early to late mainshaft? [Re: Dicky Mitchell] #735639
05/17/18 11:06 am
05/17/18 11:06 am
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David Cass1 Offline
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If your rigid is '49, is it a 350? if so, the 6 spring unit should work fine. change the plates and set it up as per MM's guide lines.
Be surprised if you have any major issues

Re: 1948 Gearbox: Converting early to late mainshaft? [Re: Dicky Mitchell] #735712
05/17/18 10:58 pm
05/17/18 10:58 pm
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Hi All,
The 48 gearbox the OP was asking about has no relationship to either the semi unit or the later pre unit (53-63?) gearboxes
I know that the 46/47 ish B31 boxes are different to both the M type and the later A,B,M types
I think the OP is asking about the 48 rigid and the later rigid / plunger boxes with the clutch lever horizontal on top?
The later semi unit splined mainshaft will not fit the earlier box
I have not come across the different length mainshaft issue? but then again may have not have dealt with a 48???/
On 49/50 bikes I have fitted Triumph type 4 spring clutches, on one I reversed the sliding plate so it would engage with the adaptor boss
and packed out the cush drive, on another with a random autojumble shaft adaptor the primary chain alignment worked out perfecty

John

Re: 1948 Gearbox: Converting early to late mainshaft? [Re: Magnetoman] #735832
05/19/18 3:06 pm
05/19/18 3:06 pm
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Originally Posted by Magnetoman
Originally Posted by BSA_WM20
It would appear that the holes were pressed with no regards to the orientation of the grain direction.
Could you post a sketch of what you think is happening, taking into account the three-dimensional structure of the cap? Remember, hundreds of thousands of these clutches were produced during the 1950s so its surprising a fundamental design flaw as you're suggesting wasn't revealed at the time. However, there have been 50+ years since then for the clutches to be bodged and re-bodged and they aren't immune to that. Anyway, I can't picture how grain orientation would affect deflection in the actual three-dimensional structure of a cap so a drawing would help. Thanks.

p.s. I just looked at my 6-spring restoration thread to find some information and all the deleted Photobucket images are back(!). Has anyone read something recently about this, i.e. is this a temporary anomaly, or has Photobucket announced a change of their policy on 3rd party Hosting back to what it used to be?


[Linked Image]

Sorry about the rough drawing, I did try to mark up a photo of a plate and when that was not working even tried Illustrator but it would be next year before I got it done.

This is what happened.
I have measured it on a 4 spring pressure plate fitted to WD B40.
Bought 3 B40GA's from a military surplus sale. Two changed quite fine but one would never change clean.
If backed off enough to get a clean change it slipped and if tightened enough to not slip it dragged.
I copped a bollocking in the lunch room at the lab one time because of the massive crunching that morning so my poor mechanical skill became the topic of the day.
Now there were 3 engineers, 3 chemists 2 metallurgist & 2 undergraduates ( one was me ) thus all sorts of theories got tossed around and some money ( and night shifts ) got tossed on the table so a full investigation was sanctioned.
Engineering confirmed that the plate did in fact bend on the bike and few days latter I came in with the offending plate and we did a macro etch on it which confirmed that two holes and the centre were all aligned in the rolling direction and thus on a long grain axis.
We also measured the thickness of the plate and it was indeed thinner to the left & right of the center line which is actually what you would expect from an open die pressing of cold rolled steel plate.
IF you search "Cupping test" you will note that pressings tear along the grain lines when stretched across them unless it is a high energy process where it takes on a cold forging microstructure rather than a pressed microstructure or fully closed dies both of which were way too expensive a process for BSA to use to make a pressure plate.
For the heck of it I brought in the other two pressure plates and neither had any two holes in the same grain axis and both were different from each other which suggests that they were randomly placed in the die when the holes were punched.
This is what you expect if an operator was taking the already cupped pressure plates out of a stillage and punching the spring holes or if the entire plate was done in a single pressing using pre cut round blanks.
Burman were aware of this so they used a 5 spring pressure plate thus it was impossible to get 3 holes on the same grain axis.
I have always thought that the 6 spring BSA clutch was a knock off of the 5 spring Burman clutch modified just enough to avoid patient violations.

Eventually this problem was overcome by the use of cross rolled drawing steels, but that did not come into popular use till well into the 60's

And from the above I made a reasonable assumption that the same would happen on a 6 spring, particularly as over the years I have come across some 6 spring pressure plates that just can not be made to lift clean while others are very mal adjusted, even missing a bolt and work quite fine.

The fundamential flaw I was reffering to was the floating clutch drum which moves during declutching and the fact that the squareness of the drum is dependent upon the backing plate, which is again a cold rolled steel pressing sitting against a lip which is way too thin to support it properly and frets where it sits against the edges of the lip.

And FWIW the image is via photo bucket who are now charging $ 1.99/ month or $ 16.00 /pa for their premium service with 3rd party Hosting.
None of what I have posted using photo bucket had ever vanished but there were precious few of them.
They seemed to target the high volume users

Last edited by BSA_WM20; 05/19/18 3:24 pm.

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Trevor
Re: 1948 Gearbox: Converting early to late mainshaft? [Re: Dicky Mitchell] #735837
05/19/18 4:44 pm
05/19/18 4:44 pm
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Originally Posted by BSA_WM20
Sorry about the rough drawing,
That's what I thought you meant but if you look at the photograph below of an actual Actuating Cap, your drawing leaves out essential features of both the three-dimensional structure stamped into it as well as the other holes.

The Cap isn't a flat plate between the center and the springs, as you have drawn, which is important since the three-dimensional stamping has a huge effect on the stiffness of the steel for the same reason the sides of a cardboard box are very much stiffer than the same amount of uncorrugated cardboard. Also, note from your drawing that if you rotate the plate slightly the angle between the line through the holes and that of the rolling direction only changes slightly as well. Since because of the hexagonal symmetry the maximum angle that can be reached between those two lines is only 30-deg. your explanation would require both an unrealistically large difference in stiffness parallel and perpendicular to the rolling axis as well as an unphysically-dramatic falloff, rather than only slowly changing as the sin of the angle. The effect you're suggesting cannot and does not affect the operation of the 6-spring clutch.

Originally Posted by BSA_WM20
Now there were 3 engineers, 3 chemists 2 metallurgist & 2 undergraduates ( one was me ) thus all sorts of theories got tossed around and some money ( and night shifts ) got tossed on the table so a full investigation was sanctioned. Engineering confirmed that the plate did in fact bend on the bike.
Yes, the Cap does flex by a measurable amount (using a gauge that reads to 0.001") as I discuss in my rebuild thread, but not by enough to affect the operation of the clutch.

I can't help but mention the time staff at the Stanford Research Institute examined Uri Geller for six weeks but couldn't figure out how he bent spoons, implying that his mind-bending psychokinesis action-at-a-distance prowess must be real. It took a fellow magician to show how he had done the trick, which didn't involve new physics.

The point being, engineers (excepting a subset of mechanical engineers), chemists, metallurgists, and undergraduates do not on the face of it have the right set of skills to conduct the right experiments to correctly understand how a BSA Actuating Cap works. If they did indeed conclude the Actuating Cap was bending by an unacceptable amount because of some metallurgical alignment revealed by etching it's because they didn't realize that the alignment was coincidental. since it could not be the actual explanation for the reasons given above.

Originally Posted by BSA_WM20
The fundamential flaw I was reffering to was the floating clutch drum which moves during declutching and the fact that the squareness of the drum is dependent upon the backing plate, which is again a cold rolled steel pressing sitting against a lip which is way too thin to support it properly and frets where it sits against the edges of the lip.
Here I disagree as well. All clutches wear, and all are built to a price. Although the 6-spring isn't immune to these factors that doesn't point to any fundamental flaws in the design.

It's easy to "armchair re-engineer" many features on all of our bikes, but having studied 6-spring clutches more extensively than I've seen written about anywhere else, my conclusion is it's a perfectly reasonable design that is no more "flawed" than any other component made a half-century ago. Properly refurbished it can continue to function well for decades to come, just as the tens of thousands of them did at the time they were produced in the 1950s.

We've certainly hijacked this thread and overstayed our welcome so we should let this drop here. I would encourage you to let it drop totally since you've internalized an explanation that is simply wrong.

Attached Files 034Cap_roughness.jpg
Re: 1948 Gearbox: Converting early to late mainshaft? [Re: Boomer] #735839
05/19/18 5:17 pm
05/19/18 5:17 pm
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Originally Posted by Boomer
Originally Posted by Rickman
Another conversion to consider;
I have heard the "best" conversion to consider, was to utilize a Notrun clutch on the short BSA shaft...? It was supposed to be the thing to do in the day...




Yes, the mainshaft out of the semi-unit twins is splined at the clutch end and will accept the Norton Atlas clutch center. I have a gearbox that is stamped RRT2 and was raced for many years with this setup. I believe Eddie Dow sold the shaft in his accessory catalog.



Bill B...


Note: This is for the later swingarm PU gearbox, not the early kidney shaped rigid/plunger gearbox.





I did not imply that the Norton clutch was something for the OP to consider. I was replying to Rickmans post. Please see my edited note above.


Bill B...


Boomer
Re: 1948 Gearbox: Converting early to late mainshaft? [Re: Dicky Mitchell] #735846
05/19/18 7:39 pm
05/19/18 7:39 pm
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The later swing arm BSA 6-spring clutch got bad reviews. The plunger mainshaft was used to fit a Norton clutch.


Amateur Loctite enthusiast.
Re: 1948 Gearbox: Converting early to late mainshaft? [Re: triton thrasher] #735847
05/19/18 7:53 pm
05/19/18 7:53 pm
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Originally Posted by triton thrasher
The later swing arm BSA 6-spring clutch got bad reviews.
By reliable people who are known to have the skills to properly work on them[*] and who have provided verifiable details to support their conclusions, or by magazine writers repeating hearsay[**]? Given that most of what we think we know comes to us from magazine writers (and the rest from the internet...), unless something credible comes to light to the contrary I have to trust my own experience and judgement on these clutches.

[*] In 'Gilding the Goldie' Eddie Dow writes that "clutch drag is sometimes complained of. Usually this means that the tongues on the steel plates are burred and their slots in the clutch hub damaged." He advises how to fix this problem that is due to wear as well as that of a heavy clutch due to "faulty adjustment of the thrust screw inside the gearbox." Nowhere does he suggest there is anything wrong with the design or that an improvement is possible with a different clutch.

[**] In a 1996 magazine article Brian Carter, whoever he is, writes that "Fitting the later 4-spring A10 clutch transforms riding of the bike." Given that my rebuilt 6-spring clutches work as well as any of those on my old or modern bikes, I can't imagine what "transforms the riding" could possibly feel like. For my rebuilt 6-spring clutches to be any better Uri Geller would have to teach me how to operate them by telekinesis.

So, who are you going to believe, the lies that Eddie Dow and I are telling about the 6-spring clutch, or what you read in magazines?

Last edited by Magnetoman; 05/19/18 8:48 pm. Reason: added [*] and [**]
Re: 1948 Gearbox: Converting early to late mainshaft? [Re: Magnetoman] #735887
05/20/18 9:18 am
05/20/18 9:18 am
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The 6 spring BSA clutch is in fact a design that is over eighty years old. What may be the original item is working just fine in my '39 M24. I don't care for the double row of ball bearings and would prefer rollers from a design perspective, but there you are.

Tangential to the first post here, and if the engine is particularly powerful, the very early postwar B31/2 gearbox is what our gang calls the lightweight box. We built a ZB34A rigid special not so long ago with DBD34 internals and were careful to weld strengthening gussets in the mounting areas of that gearbox, and we also beefed up the plates. Incidentally, we used the BSA single spring clutch in that special, which with four small grub screws in the clutch nut to level the spring, can be an excellent item.

Gerry

Re: 1948 Gearbox: Converting early to late mainshaft? [Re: Magnetoman] #735897
05/20/18 2:18 pm
05/20/18 2:18 pm
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Originally Posted by Magnetoman
Originally Posted by triton thrasher
The later swing arm BSA 6-spring clutch got bad reviews.
By reliable people who are known to have the skills to properly work on them[*] and who have provided verifiable details to support their conclusions, or by magazine writers repeating hearsay[**]? Given that most of what we think we know comes to us from magazine writers (and the rest from the internet...), unless something credible comes to light to the contrary I have to trust my own experience and judgement on these clutches.

[*] In 'Gilding the Goldie' Eddie Dow writes that "clutch drag is sometimes complained of. Usually this means that the tongues on the steel plates are burred and their slots in the clutch hub damaged." He advises how to fix this problem that is due to wear as well as that of a heavy clutch due to "faulty adjustment of the thrust screw inside the gearbox." Nowhere does he suggest there is anything wrong with the design or that an improvement is possible with a different clutch.

[**] In a 1996 magazine article Brian Carter, whoever he is, writes that "Fitting the later 4-spring A10 clutch transforms riding of the bike." Given that my rebuilt 6-spring clutches work as well as any of those on my old or modern bikes, I can't imagine what "transforms the riding" could possibly feel like. For my rebuilt 6-spring clutches to be any better Uri Geller would have to teach me how to operate them by telekinesis.

So, who are you going to believe, the lies that Eddie Dow and I are telling about the 6-spring clutch, or what you read in magazines?


I'd be silly not to give consideration to all the points of view, wouldn't I?


Amateur Loctite enthusiast.
Re: 1948 Gearbox: Converting early to late mainshaft? [Re: triton thrasher] #735899
05/20/18 2:27 pm
05/20/18 2:27 pm
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Originally Posted by triton thrasher
I'd be silly not to give consideration to all the points of view, wouldn't I?
Yes, if the points of view are properly weighted, where that weighting factor often can be 0.

Re: 1948 Gearbox: Converting early to late mainshaft? [Re: Dicky Mitchell] #735900
05/20/18 2:28 pm
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Hi Gerry,
Quote
We built a ZB34A rigid special not so long ago with DBD34 internals and were careful to weld strengthening gussets in the mounting areas of that gearbox, and we also beefed up the plates. Incidentally, we used the BSA single spring clutch in that special, which with four small grub screws in the clutch nut to level the spring, can be an excellent item.


Was that using the later type swing arm gearbox housing or the earlier rigid/plunger casing?

The single spring clutch was specified by the AA on the M21 outfits until the end (1961??)
The only downside of the single spring clutch is the weight !
I like the idea of adding the grub screws to get even lift, as I have found problems with the springs being "off"

John

Re: 1948 Gearbox: Converting early to late mainshaft? [Re: Dicky Mitchell] #735907
05/20/18 3:39 pm
05/20/18 3:39 pm
Joined: Nov 2011
Posts: 4,360
U.S.
Magnetoman Online content

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Magnetoman  Online Content

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Joined: Nov 2011
Posts: 4,360
U.S.
Originally Posted by ZeBe
I don't care for the double row of ball bearings and would prefer rollers from a design perspective, but there you are.
That was my initial reaction as well, but I changed my mind after spending time with the clutch. Whereas more expensive, more robust rollers would seem to be better than the pair of ball bearings, good engineering uses the minimum of materials and fabrication expense to get the job done in a reliable way. That's what the ball bearings do.

The Burman clutch on my Valentine Page-designed 1928 Ariel uses rollers, but sometime in the decade between it and his design of your M24 he or someone else at BSA realized the same job could be done with less expensive ball bearings. The BSA clutch design places the centerline of the pair of ball bearings directly under the sprocket and in line with the drive sprocket. The only time the bearings come into play are when the clutch is disengaged, at which time the clutch basket spins freely and the bearings are subjected to essentially no load. The ball bearings are up to that task, whereas rollers would be significant overkill.

While the larger surface area of the rollers would seem to be better for keeping the basket somewhat better aligned than the side-by-side ball bearings, the inertia from the mass of the entire clutch assembly spinning at ~250 rpm (~500 rpm engine at a stop light x 20T/43T) automatically takes care of that. I'd say using ball bearings instead rollers in that location is a clever bit of good engineering.

Re: 1948 Gearbox: Converting early to late mainshaft? [Re: chaterlea25] #735908
05/20/18 3:42 pm
05/20/18 3:42 pm
Joined: May 2013
Posts: 30
Ireland
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Ireland
Hi John,

It was the early rigid B shell and plates, which we beefed up a little. The frame was 47/48 and the attractiveness was the light overall weight. A downside is the smaller headstock cups and cones, one of which is not available and has to be ground specially. From memory this is the lower cup which of course wears the greatest. It is possible to improve matters somewhat by removing it and rotating 90 degrees, which is what we used to do when skint! The single spring dry clutch is indeed heavy and has the added weight of the bowler hat, but it does work well. The first gear torque of this lightweight special is great fun. There is an article on M20s in the current TCM which mentions that the four grub screw mod. was an old DR mod sanctioned by the army in 1958!

Gerry

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Re: 1948 Gearbox: Converting early to late mainshaft? [Re: Dicky Mitchell] #735926
05/20/18 5:26 pm
05/20/18 5:26 pm
Joined: Feb 2014
Posts: 431
Cork Ireland
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Cork Ireland
Hi Gerry,
I did not know that the later gear cluster could be fitted into the earlier case ???
A friend of mine has a NOS close ratio gear cluster for the early gearbox in his stash of bits
Of course he has a couple of ZB's to try them out on as well

John

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