Is the wearing rate of the sprockets equal?which one would wear more rapidly? I`m asking because compared to the chain and rails,seems like sprockets get changed on rare occasions...Should the "sharp teeth" be sufficient reason to replace a sprocket,if no broken teeth are present?And does it affect the chain longevity after all?

The chain gears rarely, if ever need replacing. That's the purpose of a true roller chain. If your chain gears are worn to the point of replacement, the motor will need much more than gears.

The crank sprocket likely wears more quickly as it, by virtue of being smaller, turns at twice the speed of the cam sprocket.

Actually the cam sprocket will wear faster because it has slighty less oil delivery than the crank sprocket. The crank sprocket does turn faster but that is not a factor in sprocket wear. The cam sprocket is also much taller and not as rigid as the smaller crank sprocket. I have never seen a badly worn crank sprocket. I have seen pics of bad cam sprockets.

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From what I remember from Beagle, the sprockets will start to see accelerated wear when the pitch of the chain deviates from the pitch of the sprockets. This occurs when the chain elongated due to wear. The result is that all the torque load is concentrated in the first two or three sprocket teeth causing the teeth to wear at a much higher rate. The remaining teeth are unloaded and are simply accommodating the chain for the ride in a semicircle.

So, with a chain that has not worn excessively, the sprockets will probably last forever.

that wore like that on my 72 harley 350 (benelli) and broke.

i am pretty sure i have never seen a chain that wore like that on any of my benzes. hmmmm, maybe the 74 240 i changed the chain and sprockets. that engine had very excessive wear like it had suffered infrequent oil changes or was run with the oil bath air cleaner dry....or probably both!

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Any motorcycle guy will tell you that a streched chain will wear the sprockets. Then, when they get nice and sharp, they will even wear out a brand new chain.

I, also, have never seen a crank sprocket worn out on any MB. Only cam sprockets.

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True, but a motorcycle chain runs open, in the dirt, little to no lubrication. Not a valid comparison. Every construction guy knows that a bulldozer will wear out it's tracks, and that which drives the tracks.

And vice versa???And what if one of the sprockets has already worn and left in there(just sharpened teeth,no damaged though).I guess the pitch of the sprocket doesn`t correspond to the pitch of a newly installed chain anymore? And it`s not the cam or crank one,it`s the injection timing device,number 142 on the pic:
http://www.detali.ru/cat/oem_mb2.asp?TP=1&F=110104&M=621%2E918&CT=M&cat=020&SID=07&SGR=001&SGN=01

I don`t want to let anyone mess with it ...or maybe it`s not that difficult to R&R...

The pitch of the sprockets doesn't change as the sprockets wear. The pitch of the chain does change. My guess is that a new chain would work acceptably well with worn sprockets because the pitch would match and the excessive sprocket wear would be reduced. Naturally, the existing sprocket wear would remain. Whether this would cause additional wear on the new chain is doubtful.

i wouldnt put a new chain on a worn sprocket. the pitch i would think would change. as the sprocket wears the bottom of it would get closer to the axle of the sprocket and that will make the pitch smaller. (assuming that the pitch is the distance between teeth.)

although the chain will wear a lot more and be more noticable...

tom w

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.