450 cams in a 426

[Jamir]

OK, today I was talking smack with a guy who is a mechanic for a local custom bike builder south of KC. He is known for being one of the most knowledgeble mechanics in the area and I trust what he says. But he told me something and I wanted to see if anyone has heard about this. He said that if you drop 450 cams in the 426 or the 400, they fit perfectly and they will allow those bikes to start much easier and will allow auto decompression.


Is this true or did he take advantage of my niavity?

 

[biglou]

It be true. Exhaust cam only, though, for the auto-decomp. I'll email you a .pdf file. There has been some bickering over how exact the fit is, and Dr. D. specifically said, "Don't do it", but there are several on ThumperTalk, including a shop owner that have done it with good results so far (several months of riding). I think there is a huge thread stuck to the top of the 400/426/450 forum over there.

 

[JasonJ]

Its true, but dont tell everybody, the cams are already on back order :O.
Dirt Bike Mag admitted they were wrong in saying the 250 retro fit would not work and even featured an article on how to do it in the April issue. They say somewehre else in that issue I think that it wont work but,,,, it dose in fact work and seems to be working fine for some miles now. Just make sure you get the right cam for your timing. YZ or WR.

 

[DEANSFASTWAY]

YUPPER Weve been doing these for awhile now.And on the 250s (camswaps)weve only once had to reshim a valve and it was a bike with alot of time on it . When the Yamaha machines cut there seats and sets up the valve depth the heads are all very close .On the assembly line They use like green or blue markings on the cyl head and the assembler knows which shim to start with. Seems to always be blue or green dots near the lifter bore. Had a 250F drop a intake seat and screwed up the motor really bad and also had a brand new 03 250F that leaked coolant through the spark plug drain hole only when it got cold like 30F degrees out . Brand new bike first we thought like a head gasket or something . but it was a porous casting on the head itself . YAMAHA covered it under warranty and wanted the head back . I think YAMAHA should build that motor in like a 350cc that would really be good in the woods .