Shock absorber sprocket grinding against the crankcase

[owain]

So whilst the engine is at the machinist, I thought I'd mention some damage that I saw when I took apart the engine and hopefully find out how to prevent it from happening again...The damage being that the engine shock absorber sprocket has been in direct contact with the crank casing and has left very obvious scoring marks. There is still plenty of metal left on the crankcase but I don't want any metal on metal nonsense going on once I've put the bike together.

My first thought was that the previous owner failed to place a spacer between the sprocket and crankcase but after looking at the diagrams for the 1950 plunger A10 (with a twin a duplex chain sprocket) doesn't show a spacer...Just some tiny wee shim. Any thoughts on how to stop this from happening on the rebuild. I'm just thinking of slapping the thickest shim available in (+0.030"). Any thoughts?

[morris]

They all do that sir 
You'd be extremely lucky to find a cover that's not scored.
I used a lockwasher like this one;
http://www.degroot-bsa.nl/index.php?route=product/product&path=47_80&product_id=1135
and a rattle gun to lock it up. Some use locktite but I haven't.
There's a hole fore a split pin in the shaft, but don't count on that to hold the nut.

[chaterlea25]

Hi,
Crankcase or primary cover??
The shims in the parts book are there (or not) to align the sprockets, even more important with duplex chain
When your crank and new main bush job is done there should be no more than 0.003in. end float on the crank
Read the books and service sheets

BSA's should not be "slapped together" 

John

[mugwump]

The witness marks are also often a sign of the chain breaking and coming adrift at some time.

[TT John]

I take it you mean the drive sprocket from the engine, There is a special  lock washer that fits in the keyway under the nut, I usually tend to get a screwdriver and lever it as its backwards arsed, after tightening up to the limit you have to get a longish punch and tap the lock washer into the recess, plus replace the split pin in the end of the shaft.

Hope this helps, TTJohn

[duTch]

  I think owains' is a Plunger model, and the nut/inner sleeve/spline is a different setup is different and doesn't use a  washer- but I can't see how that would affect the crank sprocket rubbing on the crank-case anyway I say it needs a shim(s) between the bearing and the inner sleeve/spline. I have a  couple of inner sleeve/splines that have different diameters with thicker/wider flanges, I'd say to accommodate the seal, which came along in '54 (?)

[TT John]

  I think owains' is a Plunger model, and the nut/inner sleeve/spline is a different setup is different and doesn't use a  washer- but I can't see how that would affect the crank sprocket rubbing on the crank-case anyway I say it needs a shim(s) between the bearing and the inner sleeve/spline. I have a  couple of inner sleeve/splines that have different diameters with thicker/wider flanges, I'd say to accommodate the seal, which came along in '54 (?)

My 1957 A10 is also a plunger model but still has a lock washer on the drive sprocket but as you say it should not affect the crank case, perhaps he needs a thicker gasket between the inner & outer chain case.

[duTch]

 

...... perhaps he needs a thicker gasket between the inner & outer chain case.

 I can't see that achieving anything relevant ?  It may be possible the flange of the T/s main bearing/bush is not thick/wide enough
 
 

...The damage being that the engine shock absorber sprocket has been in direct contact with the crank casing and has left very obvious scoring marks. ......

 owain, it may be a good idea to post a pic to show the damage to clarify damage location ?

 

The witness marks are also often a sign of the chain breaking and coming adrift at some time.

  How often does a primary break on an A10 ?  I did have one come apart on my RR after a roadside clutch fixup whereby I forgot to reinstall the circlip....hitched a ride back 30-40 miles on a RD400, circlipwas right where I left it. Put it back on and did the remaining  1100 mile+ return trip to Sydney 
  Didn't think to look for scratches though