Need some HELP ASAP Yz450F Rebuild!

mxzxracer16 · Aug 9, 2007

Hey guys this is my first time rebuilding a YZ450f the bike is an 2006 I installed the new crank and have been putting everything back together! Well anyways I got the bike all put back together and I went to kick it over and NOTHING the kick starter wouldnt even budge. Well I tore it all back down thinking a ring must not be aligned correct on the piston. Well I took the top end apart and here is what I've got. The entire bottom end rolls over so smooth and easily but when I go to install the piston into the cylinder it fits but there seems to be alot of drag on the piston threw the cylinder!?!? Is that common am I missing something or what guys? It is a new wiseco piston I put the old one back in and same thing is happening, if you put a wrench on the crank shaft you can turn the engine over when the top ends on but it is not free rolling at all it to give you an idea of what the drag feels like it feels like your fighting a compression stroke constanly and that is with just the cylinder on not the upper head. PLEASE PLEASE PLEASE some help would be great!

Ol'89r · Aug 10, 2007

mxzxracer16 said:
The entire bottom end rolls over so smooth and easily but when I go to install the piston into the cylinder it fits but there seems to be alot of drag on the piston threw the cylinder!?!? Is that common am I missing something !

Did you mic the cylinder and the new piston and check clearance? Also, did you gap your rings. The rings should have a end gap of at least .011" when placed into the cylinder without the piston.

rmc_olderthandirt · Aug 10, 2007

mxzxracer16 said:
I put the old one back in and same thing is happening, QUOTE]

You put the original piston back in, with the original ring and it is still too tight to kick over?

Definatly assembling it wrong then. If you are using a new ring in the both the new and old case then I would agree with Ol'89r that the ring gap might be too tight.

The first thing to do is to make sure that the ring is properly positioned. I don't know about a four stroke but on a two stroke the ring end gap is locked into one spot to assure it doesn't align with the exhaust port. It not only has to be in the ring grove but the end gap needs to be in the right spot. There is probably a tiny little "tab" down in the ring groove on the piston that locks the ring in position. If the end gap isn't over this tab then the ring will be held out of the groove too far and it will be really tight!

If the ring isn't too big (proper end gap) and is positoned properly then I start looking at how the connecting rod is positioned. You mentioned that the crank was replaced so it is possible that something is wrong down in the bottom and it doesn't bind up until you force the connecting rod into the position it has to be in.

Rod

Need some HELP ASAP Yz450F Rebuild!

mxzxracer16

Ol'89r

rmc_olderthandirt

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