The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => A7 & A10 Engine => Topic started by: crojack on 13.02. 2020 18:55
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Hi,
I have a 52 plunger A10 with a engine that has the crank sticking through the front of the cases. Not the best, but impressive to see how the conrod wrapped itself around the crank!
I have a line on a 58 A10 engine all together and in good shape. Will it bolt into my 52 frame with no mods?
Will the 52 tranny work with the 58 engine?
I tried some searching but couldn't find an answer.
Thanks for any help!
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I believe any year plunger A10 engine will fit another plunger year, but the swinging arm crankcases are different and aren't interchangeable. I'm sure someone with more knowledge will be along soon.
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G'day crojack.
The quick answer is NO.
The semi unit plunger motor and frame are completely different to the later pre unit motor and swing arm frame.
Most of the motor top end and internals are interchangeable.
Cheers
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Thanks!
If I understand correctly,
my 52 is a semi unit.
58 is a pre unit
meaning on mine, a semi unit, the transmission mounts to engine cases
on the 58, the transmission mounts to frame.
Is that correct?
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Correct, the semi-unit i.e. bolt on gearbox was used until about 1957 and was fitted into the plunger frame. BSA introduced the swinging arm frame in 1954 and a redesigned pre-unit engine and gearbox to suit, so both types were available during the 'overlap' years 54-57. I have seen semi unit engines fitted in swinging arm frames but never the other way round. Plunger crankcase sets and engine internals are not particularly difficult to get hold of and, as previously posted, most of the internals are the same or interchangeable.
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It's probably thrown a rod due to oil starvation to the drive side big end. First step is to strip the crank, clear out the sludge trap. Get the crank checked for truth, good chance it will do again after a regrind. All detailed on this Forum.
If you go with the later motor, all the engine internals will interchange, but you need a nice set of plunger cases to get back on the road. '58 crank should be the later so called "Big Journal" crank, with a better choice of big end bearing shells available.
Have a look on the Forum for the various cranks and the care and attention they need, sludge traps are different on early small journal cranks. You will need to retain the existing plunger transmission, clutch etc. The crank, rods, pistons, barrel and head will all swap over. The running clearance of the timing side main bearing bush is critical for oil distribution to the crank, plenty on the Forum about this when swapping cranks between different crankcases.
Swarfy.
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Wonderful information. Thanks everybody!
Anyone know if engine stands are available in the states?
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Are the cases on your motor repairable? Might be worth posting a picture. If not you may be able to source an odd case or another set. I do have a spare set off '55 plunger cases if you get stuck albeit not mint but damage all repairable
If you can source a good later crank you'll have a stronger motor but the small journal cranks from the plunger bikes can give excellent service too, just as long as you dont want to hot rod it.
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I think the cases are toast. I do have a set of cases that are good. I was just hoping to get the 58 motor and swap it in to move me further along on this bike.
I wonder if it's worth getting just to have the extra parts since it seems most of the insides will swap over? Or maybe just for the large journal crank and barrels and top end?
What's a solid complete, not sure if running, A10 motor go for usually?
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G'day crojack.
If you have a good set of plunger cases use them. Things to check are the holes for the timing side bush and the drive side bearing are round and within tolerance, they can be flogged out of round. A new main bush can be made to fit and line bored/reamed to fit crank journal. Fit new cam and idler bushes, also line reamed.
If finance allows I would get the 58 and use the crank and top end. The head steady is different but stay's can be made to fit the plunger system (done that on my A7 plunger with 57SS top end). Then sell off the 58 cases.
Cheers
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your 52 would be small fin, so quite a different appearance. totally interchangeable though. the late semi unit BA plunger motors were big fin and look fine.
All the internals can be swapped and the top ends. As Musky mentioned you can always flick on the cases.
As to what its worth... a moot point, iron heads are pretty cheap (some on ebay for like 35gbp) whereas alloy heads can be way more and repairs on them can be very expensive too.
A brand new set of conrods alone isnt cheap. I recently bought a complete assembled bottom end (LJ) with rods pistons and timing side for I recall around $400nzd.
An iron motors going to be cheaper than a super rocket motor even though for general use an iron motor is quieter.
catch with buying a complete motor is knowing what condition its in although you americans seem to have all the relatively unmolested stuff. Hopefully not on maximum oversize/undersize etc.
You'll want to flush the crank and replace main bearing shells and seals in any engine strip. cam followers are usually worn.