January 17, 200917 yr Hi Bultaco Sherpa T M49 - Does anyone know if there is meant to be anything special about the teeth on the engine sprocket, or are all mine just worn? The teeth appear to be the normal involute form (as in any chainwheel) but with the outside diameter then reduced. Has the proximity of the chain tensioner anything to do with this? It may well be that mine are worn and I shall have to look at replacements. Thanks Sparks2
January 17, 200917 yr Hi - The chain on an M49 should be a 428 if the sprockets are original and there is nothing special about them. Normal wear appears as a 'hooking' of the tooth as the wear undercuts the drive side of the tooth. Abnormal wear is usually because of slack chain, stretched chain or mis-aligned wheel. If you think your sprocket has abnormal wear check the alignment of the wheel by removing chain guard, standing back from the bike and looking along chain. It should appear parrallel from rear sprocket to engine sprocket. To get an idea of what you are looking for slacken wheel and twist snail cams to purposly mis-align chain. Wear from misalignment will appear as chain shaped wear on either side of the sprocket (both sides should be smooth). If the chain is stretched the obvious indicator is the snail cams are at their thickest adjustment (however some bultaco owners run with the wheel at this adjustment to give the longest wheelbase for extra traction in muddy conditions). Also the chain tensioner will be slack. If you are unaware of the chain and sprocket history I would change them all and have the wheel aligned by somone who can guide you through the process. If the chain comes off you run the risk of the chain fouling the rear mounting of the magneto case and ripping it off! possibly with a piece of the crankcase just by the gearbox mainshaft bearing. If you do upgrade consider changing to a 520 chain and sprockets. These are stronger. Only buy good quality chain and remember to face the split link away from chain rotation. Also by a new chain tensioner complete. The pivot wears as quickly as the rubbing block and if worn will drag the chain out of line when under tension. Forward tensioners are hard to get unfortunately. Bultaco UK or Miller have all the items you need. Tim
January 17, 200917 yr Author Hi Tim - Thanks for your reply - But I said ENGINE (ie primary drive sprocket) not gearbox (or final drive) sprocket. I'm well aware of the problems associated with final drive alignment. As you say, the predominant characteristic of all chainwheel wear is hooked teeth (as far as i am aware) but I can only repeat that my engine sprocket appears to have had it's outer diameter reduced. The only logical explanation I have is that it is just wear associated with the proximity of the primary chain tensioner however it is wear I have not seen the like of before. Regards Sparks2 Edited January 17, 200917 yr by sparks2
January 17, 200917 yr HiTim - Thanks for your reply - But I said ENGINE (ie primary drive sprocket) not gearbox (or final drive) sprocket. Regards Sparks2 Pictures? They are worth a thousand words...
January 17, 200917 yr Author Hi Swooshdave Yes i agree. But it's not an easy item to photograph as there is an outboard flywheel (solid with the sprocket) and then an inboard flywheel welded on after the sprocket was machined. So the teeth sit in there own dark little slot, flywheels solid to both sides. Regards Sparks
January 17, 200917 yr Flash + Macro. I think you description of "reduced diameter" is hard to picture. Do you mean the teeth are shorter than you expected?
January 17, 200917 yr Author Hi again Swooshdave Just imagine you put a chainwheel (any chainwheel) in your lathe and the then turn off half the tooth depth - thus reducing the outer diameter. Has that made things any clearer. Regards Sparks2
January 17, 200917 yr The primary drive crankshaft sprocket is normally a little truncated on Bultaco's from memory. Now I'll have to go look at one to check myself! Update: Just found a single sided bultaco flywheel in my store pic attached. The tooth profile does look quite truncated, and it's not wear. Not sure which engine that flywheel is from, possibly a smaller Frontera or Pursang. Wayne... Edited January 17, 200917 yr by wayne_weedon
January 18, 200917 yr Author Hi Thanks Wayne. Thats just what I mean. I also have a M198 primary drive with very little use, I shall dig it out next week and have a look at that. Have you any idea why the teeth should be truncated? Regards Sparks2
January 18, 200917 yr HiThanks Wayne. Thats just what I mean. I also have a M198 primary drive with very little use, I shall dig it out next week and have a look at that. Have you any idea why the teeth should be truncated? Regards Sparks2 It may of been done to Regina (original chain manufacturer I believe) specs and possibly reduces snatch on the runoff off that small driving sprocket, considering the close proximity of the roller tensioner. That chain is a little non standard, but whatever the reason I think you have nothing to worry about.
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