very odd problem with timing chain

Newbie here!!

 

I have a 99YZ400F  that i have been putting back together for a while, got to the point where I'm putting the cam chain on. I get the chain over one cam sprocket, go to put it over the other cam sprocket and the chain is so tight that it pops both cams out of there journals. I checked, and double and triple checked that the cam guides are in correctly and that they are the correct ones even. The chain tensioner is not even installed, i have ordered two other cam chains from rockymountain ATV and both have come up to short. 

 

I cannot find any specs as to what the stock chain should be, so i have no base to go off of. The chain that came off the bike, when you lay it out and pulled like you would an old fan belt to find the length, measures 14 7/8 inch, the chains im getting from rocky mountain atv are measuring 14 3/4 inch. 

 

I have a hard time believing that this thing stretched that much!?!  but I am no expert. 

 

any advice guys!?!  I have no idea where to go from here... 

You cannot assemble the cam chain onto the cam sprockets with both cams seated in their bearings, for one thing.  The correct procedure is to run the chain over the exhaust cam and verify the correct timing mark position of that cam relative to the crank.  Then, position the intake cam in the center of the head, between the cam saddles, and place the chain over that with 12-14 link pins between the two "12 o'clock marks" and roll the cam back and down into place.  If it comes out timed wrong, reverse it out and adjust the chain position accordingly.

 

An easy mistake to make is to loop an "extra" link under the bottom of the crank sprocket.  This is only visible/correctable if your stator plate is out of the way, which means your flywheel would also be off, which means no timing mark, so you'll have to find TDC with a probe in the plug hole instead, but it's a common mistake, and makes the chain one link too short to work. 

 

Remember to seat the cam caps by hand, don't beat them into place or pull them down with the bolts, torque them evenly in steps and do not exceed the specified torque values.

-grayracer513-  i'll give it a try, thank you for responding.  What about the issue of the old chain i just took off being shorter then the OEM Yamaha packaged chain i just got though?  anyone ever ran into that!?

It could be a number of things that in the end led to an error, but it's a near certainty that the new chain is not longer than the old.  One thing you might look at is whether the bike appears to have a 1.5mm (.060") plate under the cylinder against the crankcases.  There is a small possibility that it has a stroker kit in it.

-grayracer513- i'll give it a try, thank you for responding. What about the issue of the old chain i just took off being shorter then the OEM Yamaha packaged chain i just got though? anyone ever ran into that!?

Your previous post said the old chain was 14 7/8 (14.875) and the new one is 14 3/4 (14.750). If this is correct then the NEW chain is in fact shorter. The first post contradicts this one.

Edited by xcbuckeyeguy

Xcbuckeyeguy- yes I apologize, miss-type. Old chain is longer than the new one. I counted the links on the the new chain and old and they are the same link number. So it looks like the old chain did stretch, I'm surprised they stretch that much but this is my first 4 stroke bike.

It's not actual plastic stretching, it's the cumulative wear of each of the pins and the bores they run in.  .002" x 120 pins is .240", almost a quarter inch.

It's not actual plastic stretching, it's the cumulative wear of each of the pins and the bores they run in. .002" x 120 pins is .240", almost a quarter inch.

And this is why the chain should be replaced when doing a top-end. Failure to do so is bad news, especially on Yamahas with their non-replaceable sprocket on the crank. Wear on that sprocket from a worn chain means a new crank.

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