Have just split the 2nd drive sprocket on my rebuilt 348!

The sprocket fits on a taper shaft.

I think the left hand thread nut is tightening as I ride the bike and forcing the sprocket further onto the taper shaft.

The original sprocket also used a 'woodruff key'..

The replacement sprockets don't have the woodruff key groove as the taper shaft provides enough friction (or so i'm told)

I'm torque loading the nut to 50 ft lbs as per the manual..

 

Has anyone else experienced this problem???

Edited December 28, 20169 yr by 79cota348

I have never heard of this on the 348 primary gear. I think that when a woodruff key is used the taper will be steeper, no key shafts have a flatter taper in my experience. It could be you have a mismatch between shaft and sprocket tapers causing excessive loads at the contact point. Are your gears / crank genuine Montesa?

Could the gears be over hard / brittle/

I have that gearbox output shaft arrangement on my 348 and have not had a problem with the sprocket splitting or slipping. My sprocket is a TALON purchased in the 2000s.

Some thoughts came to mind when I read your post:

Maybe faulty sprocket (possible incorrect heat treatment process)

Maybe a lubricant was used on the taper during fitting of the sprocket

 

A clear photo showing the fracture surfaces of the sprocket should show if the problem is a faulty sprocket.

Hi fella's, appreciate your reply.

(dadof2) All of the gears/crank are original, I have recently replaced the crank seals, clutch plates and various ignition parts but other than that all of the engine is original, I haven't split the engine at all. 

(feetupfun) Both sprockets I have used are Talon 10T. I have had the same thought about faulty sprockets but Talon products are usually top quality.

No lub was used during assembly.

I'm now thinking that it maybe the 'locking tab washer' thats at fault.. It is the original and appears to be opening at the point where it contacts the sprocket (not where it contacts the nut) I have torqued the nut to 50 ft-lbs both times and I'm sure the torque wrench is correct.

At the moment, I only have pictures of the 1st sprocket that split but it definitely has witness marks on the inside edge where it has been rubbing against the engine case.. It must have been under a massive amount of force to move it that far up the taper shaft!!

I will post pics of the 2nd sprocket when I get time to have a look at it. 

Any thoughts???

 

 

I think you are right about the nut becoming tighter when you ride, causing the taper to break the sprocket. 

The only thing that can cause the nut to rotate relative to the shaft while you are riding is if the sprocket spins on the shaft, so maybe 50 foot pounds is not enough to prevent slippage.
I see that the sprocket came with a yellow coating which may affect how well the sprocket locks onto the taper (the angle of the taper is designed for steel/steel contact)

Edited December 29, 20169 yr by feetupfun
typo

If it was me I would let TALON know about you issue. If the problem is the coating on the bore of the sprocket, it would only cause problems with bikes that use a taper mount sprocket, and there are very few bikes that use a taper to mount the gearbox sprocket, so they may be unaware of the problem.

About the tab washer, yes this mode of failure is more likely because the tab washer connects the sprocket with the nut, making it more likely that the nut will move relative to the shaft. However even without a tab washer, the nut may still rotate with the sprocket when it slips on the taper

Edited December 29, 20169 yr by feetupfun
typo

I misunderstood your first post, I thought it was the primary drive gear you were having problems with. I know of several problems with output shaft final drive sprocket but none include the sprocket splitting.

Problems included sprocket bore being too large. The sprocket is then nipped onto the gearbox bearing not the taper. In one case at least this caused gearbox failure. These were not Talon.

Some others failed because the bore was too rough. Started rotating on the shaft.

I have never known a problem with a genuine Mont sprocket, but once any has failed on a taper the replacement should be lapped in with fine grinding paste.

Thanks fellas. 

I intend to talk to 'In Motion' after the Christmas/New Year break, that is where I bought the Talon sprockets. I will also talk with Talon..

Do you know of any genuine Montesa sprockets with woodruff key slot being available??

I will lap the next sprocket to the shaft to try and remove the yellow coating. Will also try an extra 10% on the torque figure (i.e. 55ft-lbs)

Thanks again gents.