Getting The Most Power Available

I would like to know if anyone has any real good experience with big bore kits and reliability?

I am at the point where I would like some more power and I have done all the free mods and some not so free. I have looked at Eric Gore's page and see there are kits available from him but after all is said and done is it really worth it? can the tranny handle it? do you get enough extra power to justify the expense? do they leak oil, gimme some hints here fellas, I need to be able to spin the back tire up comming off a corner on the pavement (Mine is SuperMotard tricked out) anyone done this or anything like it? thanks!

I talked to Eric at length, and Clark Mason (retired from WR TTalk, but occasionally a guest speaker!) about this mod. I scored my cylinder 2 years ago, and "have been busy...???" and just haven't gotten to it.

MXA did the big bore article on the 400YZF, White Bros 450 kit, and claimed it drastically reduced bottom end life, although theirs hadn't disintegrated, YET. One of our guys (MotoGreg?) had the insider scoop on this and claimed it was written out of non-sense. I think Greg regualrly dines with the rich and famous mx factory guys. Rich Rohrich, and MXTuner both have the big bore kits and have had reprtedly zero problems. Eric Gorr told me he has experienced zero problems w/ his 420 kit, in which he offers low or high compression. He also offers valve porting, in which he told me to SAVE my money. When a tuner tells me to forgo on one of his mods, it tells something of his integrity!! Eric told me, as I have read as well, of all the mods available, the big bore kit gives the most bang for the buck.

At ~$400 Eric's price, it is cheaper than my exhaust system, Power bomb and stroker silencer or my DSP airbox. Eric claims his Nikasil IS superior to Yamaha OEM, and I have no reason to doubt him. Eric ported my 91 Honda CR250 and I was very happy with it. He also has NEVER failed to reply to any of my e-mails or he has never been too busy to chat on the phone about any of his work. Eric has been in the industry for a long time. He was the head engine guy at TUFF Racing for quite a while.

I don't know if this answers anything, but maybe I shed a little light...???

I enjoy my 97mm(440cc) wr very much.

I can send you the dyno results if you like, although it is a very large file.

The power becomes more linear than a stock 426, with a broad usable delivery.

Once I got it jetted properly.

I have about twenty gallons of fuel through it so far with no problems.

If you ever get the chance to ride one of these conversion kits I think they sell themselves.

If you are ever this way I would be happy to let you ride mine.

Been out on business for the last couple weeks fellas sorry for the delay in posting my thanks for the replies. I will surely call Eric Gorr on Monday to speak with him about the big bore kit, 54 rear ponies seems like a decent amount for sure, I think with the mods I have done mine is probably somewhere in the 45-48 hp range eh ? so 6 more hp at the rear wheel should do the job one would think.

At $400.00 this is an affordable upgrade... I guess then the next wuestion would be how hard is it to do your self? I am pretty darn mechanically inclined having built several car engines and being a privateer road racer who does all his own repairs/pit work. any further advice?


It bolts right on to a 426 bottom end. Same thing as doing a routine top end rebuild. You'll be impressed with the increase in low and midrange power output. It is sooooo much fun to ride.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now