has anyone had an issue with their cam sprocket? A friend who's neighbors  tensioner 'gave up' after 38,000 Miles. yes! And he has had the bike from new. Anyway he noticed a hair line crack between the bolt holes. BTW luckily the tensioner failure didn’t trash the engine. The friend maintains the bike for his 76 year old neighbour and the reason for the tensioner longevity is probably the fact that the old guy thinks that 50mph is enough for him  

Thanks! - I sent a spare sprocket to my friend - from my spare engine. What do you think of that mileage? Its usual to replace the tensioner around 17,000 miles isn't it?

We don't replace the tensioner ,we extend it , the chain is the problem ,when it stretches the tensioner falls out, but the chain is only stretched to half it's service life ,extending the tensioner( see Verslagen) allows us to use the chain for the other half. Replacing the tensioner with a new one(without extending) will do nothing unless your also replacing the chain.

Yes 17.000 to19,000 seems to be the limit on older bikes, although we have seen newer bikes,  2005 and up stretch the chain at as little as 6,000 miles ,a lot depends on how the bike is driven ,short trips tend to stretch the chain faster due to more heating and cooling cycles.

Regarding the amount of 'dish' in the sprocket this is what my friend said:

the pics don't show how much the sprocket has "dished" so to give you some idea of the extent i put the sprocket onto a piece of glass,pressed on the outside edge & under the opposite outside edge i was able to easily insert 1.26mm of feeler gauge blades.

Sounds  like a good explanation   he was so lucky not to trash the engine

Mama mia that's one spicy meatball.  You didn't elaborate in your post, what happened to this thing?  Did the chain come off the sprocket or did it simply snap?  Did the tensioner fall apart and jam everything up?

What sort of checks has your friend done to verify that other parts of the engine weren't damaged?  I can't believe that thing failed with such vengeance and nothin else took a lickin.

What's the current status?  Did he install your replacement sprocket yet?  If he hasn't, I think he should do some checks before he tries to run it (or even rotate it for that matter).  If the chain broke, or even jumped the sprocket, there's a good chance the piston kissed some valves.  He should verify that the valves close all the way and aren't bent.

With the chain off and cam removed all valves should be fully closed and hard on their seat.  Remove the carburetor and exhaust header.  Visually inspect the valves through the ports to see if you can tell if they are fully seated.  Shine a flashlight down the spark plug hole and look through the ports to see if any light is detectable.  If all looks OK, you can pressurize the cylinder with air and verify that there aren't any BIG leaks past the valves.  Install an adapter in the spark plug hole and connect an air hose to it.  Apply about 50 psi to the cylinder.  Listen at the intake and exhaust ports to verify that you don't have any big leaks past the seated valves.

If a valve is bent it will not close all the way and you will either see it through the port (if its bent a lot) or hear the air leaking by (if its bent just a little).  Either way, if there's any bent valves the head has to come off.

Dave wrote on 08/10/18 at 04:25:55:



Abused ?
Ahhhhhh yes, and that is an understatement to say the least.
And I agree with the last comment by Dragbikemike that the forces involved that did this damage were tremendous.
I’ve seen a lot of engines from parts bikes, most with toasted engines and several where parts were torn apart and thrown about the cylinder.  But I have never seen a cam gear damaged like this before.
This is remarkable.