The BSA A7-A10 Forum
Bikes, Pictures, Stories & more => Other BSAs, Other Bikes, Cars, Machinery & Tools => Topic started by: RDfella on 22.07. 2018 21:04
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Chaterlea - here's a pic of the M21 with own engine. Sure I've posted it before. Also pics of latest engine I'm about to fit in A10 frame.
And thanks again everyone for your help. Must say the suggestion that the breather was not allowing air to enter and thus causing a partial vacuum makes sense.
ATB
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Further to Chaterlea request:
M21 engine specs:
Heavily modified BSA B31 heads and barrels. ZB31 conrods. Fiat pistons. Own crankcases, crankshaft, cams, timing gear etc. Since converted front brake to 8” s/s with own tls backplate.
Latest engine:
Modified Yamaha XT cyl heads, modified Yamaha 550 pistons. VW Golf conrods. Own crankcases, crankshaft, cylinder barrels, timing gear, final drive sprockets etc. BSA semi-unit gearbox bolted to cases at rear. Drive belt tensioned with a Chevrolet timing belt tensioner.
On both engines, my own patterns were cast in Sheffield in LM25. Crankshafts were machined from EN40B billet. All machining done by myself.
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Hi RD,
Fantastic work *respect* *respect* *woo* *woo* *wink2* *wink2*
Earlier today there was discussion on belt driving a plunger twin, well you just went and done it *smile*
Is there going to be a cush drive somewhere in the drive line?
John
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I'm not planning on a cush drive. I know belts like the one I have don't 'give' but hoping never the less that it won't be as severe as a chain. Basically didn't have much choice, as I'm aiming for 100bhp @ 9,000 which would be beyond a 1/2" chain. And I don't like twin or tripple chains as they never load properly due to gearbox mainshaft flex.
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That's the goodest looking M21 I've seen..... *clap*
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Hi RD,
TBH I wonder if the BSA transmission would stand up to anything like 100BHP *eek*
I have a BNR primary and clutch that was removed from a 600cc Gold Star as the owner fitted an electric start
It had not done any great mileage but the engine pulley teeth are quite worn in one area
I think this is where the power pulses pushed on the belt teeth??
SRM at one time were fitting belt pulleys to BSA cush drive components, so its doable
John
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I did consider the issue of whether the BSA box could stand the strain. As it's torque which is the destroyer here, I deliberately chose to use a fairly large engine sprocket in order to reduce torque to the box. The fact the engine is short stroke (92mm bore / 75mm stroke) will mean power is at fairly high revs (assuming it runs!!!) also helping ease the strain on the box.
For anyone attempting to fit a belt drive to a bolt-up A series, I've made a couple of observations (presuming of course that no kit is available).
My vee twin has an idler to adjust belt tension. This is located on the rear primary case which, in my case, is a separate casting bolted to the crankcase. As I recall, the A series in question has the rear primary case as part of the LH crankcase (and a Weller tensioner for the chain) - which would make fitting an idler extremely difficult. I would therefore suggest belt tension is adjusted by shimming the gearbox. Helpfully, belts tend not to need as much re-adjustment as chains.
Secondly, the belt I chose was the only one capable of taking the power I am hoping for. Unfortunately, the only sprockets available for this particular belt are in cast iron, which, of course, is totally unsuitable for this application. I therefore had LM25 alloy cast over a splined steel centre for the engine sprocket and then cut the teeth in a mill using a dividing head. The clutch sprocket was machined likewise, but out of EN32 steel. It was then machined out to accept a BSA clutch basket which I riveted in. To avoid some of the work involved, and with the A series output being less than half of my requirements, I’d strongly advise anyone attempting this to choose a belt for which alloy sprockets are available. Of course, that still leaves the problem of fitting an alloy sprocket to the engine mainshaft, but that should not be insurmountable.
Hope the above helps,
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Hmmm- interesting, but shimming the box isn't really much of an option in a stock frame, due to the mounting points. An idler where the stock tensioner would be most obvious if possible...
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duTch,
With an appropriate choice of pulley sizes, one should be able to manage with shims between 0 and 5mm. Not familiar with the mounting arrangements on those bikes, but surely an elongated hole somewhere to allow would be easier than trying to mount an idler. Remember, when kick-starting and on the overrun, that idler is taking a fair load. The stock tensioner is only really there to stop rattle, and is pretty flimsy so doubt there's enough material in that area to get a good fixing for a pulley. Of course, nothing's impossible, but the easiest option is usually the best.
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Chaterlea,
I see no reason why a cush drive couldn't be incorporated into a belt pulley. After all, it's just a different type of sprocket.
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Hi RD and All,
A while ago I built a unit Triu*** engine for a customer
It had a Hayward belt drive, The outer case had been cut about (custom bike) so everything in there was full of sh1t
I got a new primary cover and ran the belt setup in oil with suitable belt
I used the standard Tri blade tensioner, It has been run fine since
So I don't see why a plunger A with a similar belt should not do the same
I need to get a different engine pulley for my BNR clutch to up the gearing and must investigate getting a cush drive fitted to it
John