you fellas with a 420 kit

Hi guys. I want to know how many miles your 420 kit has lasted. I have heard 2 weeks to 6 months. I want to hear from the guys with good luck as well as the bad. thanks


1st time 10 rides, 2nd time 10 rides. I believe the first failure was due to a lean condition in the midrange (detonation). The second failure was certainly due to overrev from missing some shifts during a race. Just to be safe, I used a 426 rod, cylinder and piston this time, as I hear the rod is the weak link in the 400.


Eric Gorr informed me his 420 kit has suffered ZERO failures.

I will be having Eric do my engine this winter...for real this time though :)

Talk to Eric!

ive probably put over 2000 miles on motor since 420 and still going strong.2000 more miles. and it will probably be ready for set of rings.

cool guys. I just wanted to know. It is looking like i wont be able to get a new bike for quite some time. We are getting married and we are closing on a house next friday. So i was wondering what my options yould be on a rebuild if my motor died. My friend who is a mechanic said dont do it. Not vert reliable. So i asked about just rebuilding it if it died. He said i dont know they never seem to die. LOL. he has not seen as many bikes as we do on here but he sure is a yamafreak. But he helps me with everything for free so i cant complain. I hear there is a 290 kit for the 250F. he just put that one in and it they are waiting to see how long it lasts.

the dirt rags were whinning about the reliability of the cast iron sleeve kits that stroker and thumper make.

The gore kit utilizes a renikasiled bore so the heat transfer issues are not present.

I have about 50 dyno runs and 1400miles of wfo baja on my 444cc wr426.

No breakdowns just popped it apart for more tricks...

lewischris,here is my secret.i dont wait till motor dies to save a lot of money in the long run.

here is my secret. You wait if you dont have the money. If it does break you dont ride till you have money. lol


why did you bore it out. Did you blow it up?

No ! no blow ups here. If so I'd have said.

The new cylinder was preped by removing any cast imperfections. Then it was bored cleaned again then honed to spec for the piston.

Started with a new cylinder for sake of argument.


I've had my used 99 since early 2000.


-JE piston (actally a 416)

-new barrel bored and plated

-yz spec cam, and cam timing

-rejetted, with throttle stop removed

-airbox lid removed, uni air filter

-scotts ss oil filter

-dmc exhaust system

-grey wire

-yz seat and tank

after breakin I have about 1200 miles runs great no problems.

- very happy, going to hollister hills this afternoon.


Bill, I am sure you covered this before, but if you dont mind...

Your bike has been on the Dyno. What was the before and after with regards to the 420 kit? Thans!!!!

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