WR400F bigbore kit?

allens do a wiseco 445 and 455cc kit. i don't know much about them. i think they are reliable. i wouldn't go any bigger than 420cc. as long as ir's not for a '98 you'll be ok.

why? well there is a maximum squish area and if you bore out the barrel you make the squish wider and that slows the bike from revving out in top gear usually. feels like your pulling a caravan but only in fifth gear.

my advice is that if do go to 445cc you have the combustion chamber machined by someone who knows what they're doing. that on it's own needn't be expensive, but to get full value out of your big bore kit you want larger inlet and exhaust valves.

now if the machiner has just done your cylinder head , he might as well cut all your valve seats properly!!!

now your bike will rip your arms off



I will be doing the 420 kit this winter. Eric is world reknowned and has been wrenching for a long time. One of our members, Clark Mason, had the big bore kit.

I asked Eric about lower end longevity. He said the lower end life is not affected. Eric also claims his nikasil cylinder lining is superior to OEM.


'99 WZ/YR (you choose!) with ALL YZ mods, de-octopused, DSP Doug Henry airbox w/ velocity stack, FMF PowerBomb header, Stroker SX-1 silencer, SS front brake line, OEM YZ tank, IMS YZ seat.

Kevin, I had mine done by A. Bernard in Thedford Mines, Qc. Cost for bore + Nikasil : $300.00 CAN. If you are interested I can get you their phone, fax, etc. Factor in the exchange rate and you are looking at 195.00 USF! Add a few bucks for shipping back and forth. I don,t think there would be duty but you may want to ask. B.T.W. this aftermarket plating is more durable than stock and their machining precision / tolerances / service is above par to anything I have seen yet.


Just curious, did your jetting get affected?


'99 WZ/YR (you choose!) with ALL YZ mods, de-octopused, DSP Doug Henry airbox w/ velocity stack, FMF PowerBomb header, Stroker SX-1 silencer, SS front brake line, OEM YZ tank, IMS YZ seat.

I have the 97mm piston in my '98 WR. Took a stock 426 cylinder and had it bored 2mm. Also had the top of the cylinder cut .020" to bump the compression. Didn't do a thing to the head. All this was going with Eric Gorr's reccomendations. Bolted it together and it will virtually pull your arms out of their sockets if you can get traction. A mountain of power from idle all the way to the rev limiter.

But I got the last 97mm Wiseco piston that fits a 400 piston pin. All the new 97mm Wiseco pistons fit a 426 piston pin. To get 97mm out of a 400 cylinder, you need to install a steel sleeve. This is where some reliability problems come from. The biggest I'd do a stock 400 cylinder is 94mm. That works out to a 416 (called the 420 kit). Wiseco sells a 94mm low compression piston and a 94mm high compression piston. I've done both and the high compression piston motors run better everywhere.

Just my personal findings so far....


MX Tuner

Hi,does anyone know a company making bigbore kits for my wr400 in England or USA that are any good.

Kevin, yes I did a few jetting changes as follows : 185 main, 50 pilot, 100 pilot air, 2 turns out ( per Stoker recommendations ) Did not alter the head as mine is the 416 kit. ( 420 ) It is the regular 12:1 comp. P8 Wiseco piston I beleive. E-series, 12 discs with high-boy header and 14/50 gearing with O-ring drive chain. The bike ia same as yours, a '99 YZ. If you do go ahead and re-do your top end you should also change the timing chain as a precaution.

My bike runs great,has a tan coloured plug but seems to eat more gas ever since I put in the 185 main, may go back to 180 or try a 183. I will eventually try the BK mod and James Dean jetting with that other needle he recommends. Look at Bill's trailer when he posts, it's all there. As it is I get very little if any bog but do clean the AC pump diaphram often. I do feel thet the fuel squirt is very long however.

I had an advanced 426 rider try my bike a few laps so as to listen to my exhaust note and found it embarrisingly loud! His feed- back was that it was the ulimate in smoothness! ( his bike seemed trashed however )

Hope this helps!

[This message has been edited by Hugh LePage (edited September 12, 2001).]


I just put an FHP needle in it again w/ a 168MJ. I have shortened the AP rod by ~5mm to reduce the stroke of the AP diaphragm. This also delays the AP timing relative to the slide opening. I need to do extensive testing to come up w/ definative results for posting.

The snap off the bottom is substantially improved, probably dealing w/ the AP. I do have the plans to make the AP FULLY adjustable, both the AP timing and the stroke of the AP. I had been running an EKN.

Kevin, thanks for the great info! I will get back to you in a few days when things space out a bit and will then start on the mods.

Thanks mx tuner for hooking me up with Rich.

Rich hooked me up with a 97mm piston for my 426. Along with the piston, I got the adjustable cam sprockets and along with 104lca intake and 106lca exhaust settings I am running a Rohrich fuel blend of b37 and mtbe adjusted according to ambient temps.

So the question is mx tuner, what are you running for jetting in your bike?

What needle, main, pilot and pilot air jet are you running?

James Dean or Taffy, if you have any ideas about base line jetting for a 97mm piston set up, I would appreciate it.

Thanks, Bill


you should be looking for a white plug. the days of tan plugs are gone. your bike seems very rich, but if it runs fine, great!

the trouble when they're rich is that they often just rev slower so there SEEEMS to be no problem.


if you were happy with your jetting before i would say it will remain the same or come down just one on the main jet.

you may also know that i have the VT camwheels as well.


When I first built my 450, I checked around. Everyone who had built a big block said go way rich, especially on the pilot jet. The thing wouldn't hardly run. I ended up going back to stock '98 WR 400 jetting except for a YZ needle. The bike runs brutally strong, revs perfectly from any rpm or throttle setting and gets killer mileage. The exhaust discs are a nice even grey color. I've never looked at the plug.

It will outdrag a KTM 520 and thats with spotting it about 40 lbs extra weight. This baby is too much fun. Sorta like driving a V8 Pinto.

Sounds like it's jetted pretty close to me.


MX Tuner

Ok you guys got me all freaked out here now.

I am really really happy with my WR right now, but I am also a road racer, my WR never really sees the dirt, except when I get too happy sliding into/out of a corner (yes on the pavement) and run wide into the dirt. This big bore stuff perked my interest here and now i need a real answer...has anyone got any dyno data on these big bore kits to show what kind of power increase is possible? I need to know before I go out and spend a bunch o' money on a kit to get more power......(frothing at the mouth trying to figure out a lie to tell my wife about where the next $1000.00 went)....???


01' WR426,YZ timing,EMP #2,#48Pilot,#100PAJ,#172 Main,BK mod,stock pipe-no baffle, open airbox,Gray-Wire Pulled,K&N Filter, throttle stop mod,CA Street Legal,Pure SuperMotard 17 in. wheels, 15/46 gears,ProTaper bars,

Eric Gorr's price is ~$350.00. His web page shows he has cylinders ready to go out the door. Talking to Eric, he said go w/ the lower compression to avoid race fuel, and although he can do it, forget a porting (valve) job.

The 420 kit is the most bang for the buck.


'99 WZ/YR (you choose!) with ALL YZ mods, de-octopused, DSP Doug Henry airbox w/ velocity stack, FMF PowerBomb header, Stroker SX-1 silencer, SS front brake line, OEM YZ tank, IMS YZ seat.

Apparently you haven't ridden a 450.


MX Tuner

I use to ride a Ninja 1100...does that count? :)

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