Britbike forum Forums British motorcycles in general Bsa gold star forum Gold Star Engine Rebuild: Bottom End

Having addressed in detail rebuilding a Gold Star's magneto, carburetor, 6-spring clutch, and gearbox that leaves the engine. So, this is the first of what will be five separate threads covering the entire rebuild of a Gold Star engine (or any pre-unit B-series single) based on my rebuild of a BB34A. A few months from now, when I hope to have finished this sequence, it will have separate threads detailing the step-by-step rebuild of the:

Crankshaft
Bottom End
Cylinder
Head
Rocker Box

Although there are a few detail differences (e.g. breather and rocker boxes), for the purposes of rebuilding an engine the procedure is essentially the same for all Gold Stars from the late ZB with separate rocker box through the DBD. Even with the one-piece head and rocker box the early ZB is essentially the same as well. Further, although my 1956 BB34A "Alloy Clipper" engine has a 'B33' bottom end so is lacking a drive-side bearing retaining plate, otherwise the engine is basically the same as a BB34GS (nb. this plate is shown in the Spares manual for the B32 and B34 so variations exist).

The next three images show, respectively, the bottom ends of a BB34A, BB34GS, and DBD34GS.







Comparing these three images in detail shows the great similarity of all three engines. Because of this, even though my 1949/56 BB34A-based "Alloy Clipper" is in the Projects forum it seems worthwhile to repeat the engine rebuilding information here in expanded form, pointing out where any important differences affecting the rebuild of a Gold Star engine exist (e.g. lipped inner roller bearing on my BB34A whereas Gold Stars have larger, non-lipped bearings).

Since my BB34A came to me in pieces, each thread in this series will be the inverse of usual rebuilds, i.e. they'll end with 'disassemble in reverse order'. Also, as people add to, or correct, what I've written I'll make the appropriate changes in the relevant posts so the information in this thread will be as accurate and self-contained as possible. However, this has to be done within the 5 images/post limitation so I already know that breaks between posts won't always be in ideal locations. Finally, I supplemented the photographs I took while rebuilding my BB34A with additional photographs of parts from the shelf, some of which aren't in great condition, so don't assume all of them are from my engine.

Assembling the Bottom End

If you follow the 17 parts of this section in order it will keep you from having to unbolt something you've already bolted together as you assemble the bottom end.

Supplies needed:
-- Assembly lube
-- Camshaft lube
-- Yamabond 4 or equivalent
-- Permatex Permashield Gasket Dressing and Sealant
-- Loctite 262 (red, high strength)
-- Loctite 680 (green, retaining compound)
-- (2) Sump gaskets (or gasket material to make your own)
-- Timing cover gasket
-- Oil pump gasket
-- (2) ¼-20 UNC socket head cap screws or BSW bolts drilled for safety wire
-- Safety wire
-- Oil seals for cams and crankshaft in timing cover (two for BB and three for later models)
-- Oil seal for magneto pinion
-- Tape or rubber bands to hold studs

1. Inspect all mating surfaces and clean and deburr all tapped holes.



2. Rebuild, true, and balance the crankshaft for the piston that will be used.
[this will be the subject of a separate multi-part section in another thread that I haven't written yet]

3. Inspect the following:

-- Condition of tappet faces, and ensure they have minimal runout when rotated. I don't know how much wear is acceptable, but I didn't bother refacing ones for my BB34A that were worn by ~0.002" in the center.

A tappet with runout of the face of nearly 0.020" (left) should be replaced but less than 0.002" (right) is probably fine for all but an engine being prepared for racing.



-- Clearance of tappets in guides (tappets should be free to move but with no perceptible side-to-side clearance). The gauge in the photograph shows '0' but the actual measurement was 0.0002". If it is much above 0.001" you should consider finding a replacement.



-- Condition of cam and idler gear spindles and idler thrust washer.



-- Condition of cam faces. As long as the surface is smooth some pitting on the base circle of a cam isn't of much concern because the tappet isn't pressed against it there. Pitting on the lobe can be tolerable as well as long as the surface is smooth since what matters is the pressure (=force/area) and the pits only remove a minor amount of area.



-- Presence of all "fixed" components (e.g. studs).

4. Remove and replace tappets, guides and spindles if required.

-- Remove the old magneto oil seal before heating the cases.

-- Removing a tappet requires removing the associated guide and spindle. The guides are a tight fit in the case and will be difficult to remove without a special tool unless the case is heated. Heat the case to ~175 ℃/350 ℉ before removing a guide, then reheat the case before removing the spindle. This requires an appropriate tube to withdraw the spindle.

Fantastic, thanks! Finally, a dissertation that will most likely quantify the 'how much is too much' questions I usually have when looking at 'pre-loved' parts and their wear..

Go to it!

-- Support the timing side bearing race when heating and when installing spindles to keep it from falling out. Even if it doesn't fall out it can move out of position, which might not be noticed.

-- Don't forget to have the replacement tappet in place before you install the spindle.

-- Make sure the flats on the cam spindles face up when installed. Also, the spindles are not identical so see the following post for important additional information.



5. Install bearing and races in the cases.

-- Heat the cases to ~175 ℃ /350 ℉ before installing the ball bearing and roller bearing races. The Gold Star drive-side roller bearing race is without the lip that is on non-Gold Star races.

-- If the cases are worn such that the bearings are not press fits in them, Loctite will not cure the problem because differential thermal contraction will quickly destroy the bond. Electroplating the bearing to increase the diameter is the solution if the wear isn't excessive. Otherwise, the case has to be machined and fitted with an insert.



-- Install the circlip for the ball bearing after the case cools.

-- Use assembly lube on the ball bearing.

-- When installing the timing-side race, make sure all three spindles are supported on something suitable when you tap the race into place.

6. Install oil pump drive spindle and locating pin.





-- Use assembly lube on the shaft.

7. Fit oil pump with gasket, tighten gently, and lockwire the bolts.

-- Use a very thin coat of Yamabond 4 gasket sealer on the pump and top of the gasket so as not to clog passages when the gasket is compressed, and don't omit the top plate washer.

-- Use Loctite 680, slowly snug each bolt while checking that the spindle continues to turn without binding, and lockwire the bolts in their final positions. Do not tighten the bolts as much as you normally would for Fasteners of this size.



-- A ¼-20 UNC socket head cap screw is easier to drill for lockwire than a hex-head BSW bolt as used by BSA. By test fitting the pump, gasket and screws first, and marking the heads with paint, it ensures the holes will end up in a convenient orientation rather than blocked by the oil pickup.

-- Check that the spindle turns freely (it turns CCW when the engine operates)

8. Install the magneto oil seal.



9. Do a "dry assembly" of the crankshaft in the cases.

-- The drive-side roller bearing on a Gold Star is larger than that on a BB34A, its outer race doesn't have a lip, the race is held by a retaining plate bolted to the case, and there is an additional distance piece. However, there are more similarities than there are differences.

At this point in the assembly of a Gold Star engine the retaining plate should be bolted in place, followed by placing distance collar 65-1874 on the shaft next to the flywheel. Other than those two components, the rest of the assembly proceeds in the same way as for a BB34A. Next, the inner portion of the roller bearing is installed on the shaft (which should require somewhat of a press fit), followed by the "top hat" distance collar 65-1396. The BB34A also uses a distance collar between the two bearings for the same purpose, although it's a simple "pipe" rather than a "top hat." Although part numbers and diameters differ, shown in green in the next photographs are the only two components of a Gold Star that require different assembly than a BB34A.



Although I used a BB34A case for the next photograph, it roughly shows what the plate would look like on a Gold Star case.



-- Hold the studs in their extended position using tape or rubber bands.

-- Insert the crankshaft in the drive-side case.



-- Attach the timing-side case using the four ⅜" engine mounting studs or four 3"-long ⅜" bolts to temporarily clamp the cases together.



-- When the crankshaft is pulled tight against the drive-side bearing the connecting rod must be centered on the joining face between the cases.



-- If the rod is too close to the drive side make a shim to fit between the oil flinger and flywheel. If it's too far from the drive side check to see if the ball bearing is fully seated in the case, and that the oil flinger, distance collar 65-1874, and inner race of the roller bearing are fully seated against the flywheel.

-- By shoving the crankshaft to its two extremes, check the end float to be sure it is greater than 0.010" and less than ~0.10"



-- Temporarily pull the crankshaft tightly against ball bearing using cush drive components and check that the crankshaft still turns freely.

The components used in the cush drive varied over the years so may be mismatched in a basket case. Even if not mismatched, the fact several thicknesses of distance pieces and alignment washers are listed by BSA means the final fitting of the cush drive has to wait until the engine and gearbox are in the motorcycle so that the correct alignment of the primary chain can be achieved.

10. Separate the timing case, apply Yamabond 4 or equivalent, and bolt back together.

-- A cheap artist's paintbrush makes it much easier to apply a relatively uniform thickness of gasket cement.

-- Bolt the cases loosely, tap them with a soft hammer or plastic handle of a large screwdriver to rotate them slightly with respect to each other if necessary to ensure there is no step at the joint, and make sure crankshaft still turns freely. Again, use the four ⅜" studs or bolts to clamp the cases together, although they will have to be removed later to mount the engine in the frame. Also use three 5/16" studs or 1¾"-long bolts, which won't have to be removed later. Tighten the bolts or studs fully.

11. Fit the two studs with magneto or Magdyno straps.

-- There is no reason for these studs to be very tight, and that only would make it harder to remove them later if you wanted to swap straps



12. Fit sump filter and cover plate.

-- The lip in the screen around the hole for the pickup faces upwards



-- Use gaskets on both sides of the screen. Apply Permatex and allow 20-30 minutes for the solvent to evaporate before assembling the cover-gasket-screen-gasket-engine sandwich.

13. Fit the crankshaft timing pinion so that it meshes with the oil pump spindle.



-- Make sure the key for the pinion is in place when you install the pinion.



-- Because the key doesn't allow the pinion to rotate on the shaft, rotate the crankshaft to draw the pinion worm fully into the oil pump spindle.

-- Install the lock washer and nut and continue screwing the nut on finger tight as the pinion worm advances into the spindle.

-- After the pinion is fully seated and the nut tightened somewhat with a wrench, rotate the engine through several revolutions to make sure there is proper clearance between the worm and oil pump spindle.

-- Apply assembly lube to the meshed gears.

14. Fit cams and idler gear.

-- Use assembly lube and cam lube.

-- Set timing marks correctly (line-to-line on inlet and dot-to-dot on exhaust).

-- If there is a wider flange on one side of the idler pinion it should be installed toward the inside, against the thrust washer that's trapped under the spindle.

15. Fit outrigger plate.

Since the shafts are constrained on both sides of the cases by bearings, if they are not precisely coaxial the ends will precess in a circle. This isn't too much of a problem for the drive side, but for the timing side it means the bush in the outrigger plate will be quickly enlarged by the TIR of that circle. In addition, the rivets holding the bush may become loose.



-- Remove nut and lock washer after ensuring the pinion is fully advanced into the spindle worm

-- Use assembly lube on the bush and pinion.

-- Fit outrigger plate, lock the crank, and tighten the pinion nut.



16. Check operation of the oil pump.

-- To confirm the pump is operating correctly, use a small oil can to add oil to the inside fitting (the outside fitting is marked 'Return') and keep topping it up while turning the crankshaft through ~10-15 revolutions (CW from timing side) until oil comes out the small hole directly below the crankshaft.



17. Fit new seals to timing cover.

-- Clean the passages in the timing cover before installing the new seals. The BB34A and BB34GS timing covers (bottom in the photograph) feed oil into the crankshaft through a short pipe so the cover only has seals for the cams. Crankshafts on later Gold Stars have a nose that fits into a third seal in the timing cover (top). The crankshaft oil seal (62-2505) is not shown in the DBD parts manual but it installs in the cover the same way as do the two cam seals, which look the same but have a different part number (66-1923) (these seals are missing from the top cover in the photograph).



-- Attach the timing cover. The cover is held by twelve BSW Posidrive (Phillips-like) screws, which can be replaced by 1¼"-long ¼-20 UNC socket head cap screws. Socket head screws are more convenient to work with but caution has to be used not to apply too much torque and strip the threads on the engine. If you have a suitable torque wrench, tighten them to 4 ft.lbs. when "permanently" attaching the cover and gasket, but for now don't use gasket cement and just gently snug the screws.

-- The magneto pinion and the magneto or Magdyno remain to be installed once the engine is complete and back in the motorcycle. If you have a ZB or BB the magneto pinion looks like the following, while a CB or later will have a peg protruding from the face to drive a rotary breather.



---------- [End bottom end rebuild] ------------

Thanks for the positive reinforcement. Aside from the time writing the material it took me nearly an hour just to upload it tonight. I'll wait until tomorrow to go through the images to see if I managed to get the right ones in the right places.

MM, Good writeup, thanks,I enjoyed reading this, but could you please elaborate a bit more on point 8 that said "install the oil seal" ?

I have found that the space between the mag pinion and the crankcase housing is too narrow to fit an oilseal, and I fabricated an alternative solution that is not 100% oil tight.
Not sure if I use the correct seal, Do you have a part nr. or seal dimentions ?

Well, I cheated on that point. There isn't an oil seal under the socket because I don't currently have one. However, I installed an oil seal in at least one of my Gold Stars without problem some years ago so it must be a matter of finding one of the correct thickness.

An oil seal is on my list of parts to buy, but since there's no urgency to have anything on that list in hand I'm waiting to see if other items get added to try to avoid paying shipping on two orders (yes, I realize that's futile, but hope springs eternal).

p.s. it's difficult to make an accurate comparison, but nearest to the "concept" of this thread is a 4-part series in Classic and Motorcycle Mechanics magazine 25 years ago in which there are ~19 tiny (~2"x2"), muddy images corresponding to the 30 I have here. A Haynes manual covers assembling the lower end with ~14 photos. Anyway, I'm not aware of anything that already exists in print or on the web that leads someone through the rebuilding process the way this thread does so I'm sure at least a few people will find it useful.

Magneto Oil Seal - 20 X 38 X 5

The specific brand in my hand is "Narwhal"

The cavity is 5.7 mm deep so Rich's suggested 5 mm-thick seal wouldn't even protrude from it. The cavity is 1.50" diameter (38.10 mm) so I assume that extra 0.004" is within the ability of the oil seal to accommodate the extra crush.

Thanks very much MM for the great record of build. This is going to be very helpful in the future for me (hopefully) with my CB build.

Kevin

Many years ago, as a 'get it running because the next race meeting is imminent' teenager, I had such a mag pinion seal that was sufficiently loose in the case to not be retained...so, knowing that a) the seal would not need changing anytime soon and that b) my ability to prize the material I used to retain the seal out was unquestionable, I mounted the mag, setting the height with shims, and positioned the seal on the pinion. I then pushed the pinion onto the mag armature after applying a quantity of Araldite epoxy to the case, thereby hopefully positioning the seal in the optimum position.

It worked.

I'm guessing I wasn't the first or last to effect such a solution!

cheers As for the mag seal some time ago there was a post about using a face seal for the mag pinion that type of seal is self centering . but does any one have that info hunter

The seal actually measures 38.10 mm diameter, 4.75 mm thick.

I seem to remember putting something on the outer edge of the seal just be sure, likely one of my usuals of either 3 Bond product or Loctite 518. It didn’t fit loose, took some pressure to instal.

Offline bsalloyd pointed out something well worth addressing. If you take both cam spindles out they may look identical but they're not. As the photograph shows, in addition to the flats for the tappet that the spindles are resting on, each spindle also has a flat ground into the part that is pressed into the case.



These flats apparently are there to minimize the small, but significant, distortion that otherwise would be placed on the crankshaft's roller bearing race, so they need to face toward the crankshaft. If remove both spindles and forget to mark which is which, remember those flats need to face toward the crankshaft when you press them back in (and the other flats have to face upwards).

On the 3rd diagram referencing the DBD lower end item # 8 which is a 66-661 bearing shim .020". Is it always used or is it used in the event there is some lateral play in the crank once the cases are buttoned up. Secondly, does the shim sit between the case and the outboard bearing? thanks. bill

Bill

The shim #661 is always used. Out board of the outer bearing there is a small cavity in the case for that shim. My understanding is that the shim basically covers the open part of the bearing and helps to prevent the oil from migrating through the bearing into the chain case.

The crankshaft is pulled up against the outer bearing so shim #661 does not position the crank. All play is taken up when the cush nut goes on.

Gordo

Thanks Gordo. I appreciate you taking the time to let me know. bill

I'm sorry I didn't notice these last few posts until now. The disadvantage of it being a 'sticky' is it's located at the top so a superficial glance can miss it unless it's noticed before the next post comes in. But, I'm glad Gordo noticed it and gave you the answer you needed.

Since most of the replacement 6205 bearings have integral seals, why not remove the inner seal for oil lubrication and leave the outer one in place to slow down leakage to the primary. I have been doing this for years on similar applications. Seems better than a washer.