What year was the head design changed?

I should know, but my pre-Alzheimer's disease is kicking in and can no longer remember... Was the overall height (external dimension) of the head not changed until the 450's came out? In other words, are all years for 400's and 426's the same (making them inter-changable)? I know somewhere along the way, Yamaha redesigned the head and made it shorter than what was on the 400's and at least the early 426's. Anyone know what year that was?

It was with the 450s...the 400 and 426 castings are the same, but the 400's have a vacuum port on the intake that must be capped to run 'em on the 426's (been there, done that). 400's and '00 426's also had SS valves (and correct, for them, springs) the 01/02 426's have Ti valves and different springs.

Thats good to know.

Are the cams interchangable from 400-426-450? (auto decomp from my '04 450 wuld be nice on my friends '01 YZ426)

You can put a 450 cam in a 400/426. The tooth sizes aren't exactly the same but lots have done it.

Thanks Rich! I picked up a complete 01 WR426 engine. I'll overhaul it this summer and get a good baseline on it before putting it in my trusty 99 WR.

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