440 Kit

I heard that Wiseco has come out with an oversize piston for the 426. Who should I call to get my cyl. bored and plated? I don’t want a sleeve. Would an accompanying port job be recommended?

The only plating shop I’ve ever heard of is RPM, but I don’t think he’s into thumpers…

Thanks in advance!

Doug- you already need more??? You are da man! I'm just getting to where I'll open mine half way up without shaking :)

You might try Eric Gorr, I don't know if he does thumpers either...


Wiseco just released the 97mm (444) pistons for the 426 rod a couple of weeks ago. I've got mine sitting here :)

Eric Gorr does the piston kit, overbore and plating as a package for $325, with porting it's $425.

I believe Clark is running one of Eric's plated 420 kits .

www.ericgorr.com 1-920-356-0741

Rich Rohrich

[This message has been edited by Rich Rohrich (edited 10-25-2000).]

Powroll also advertises the 440 kit on their website. They say only three kits will be made this year for $1,156.99 each. I doubt more power would do me any good at all but, younger and more aggressive riders could probably make use of it. Then, reliability becomes an issue.www.powroll.com

[This message has been edited by Boit (edited 10-25-2000).]

Eris- sorry, I got you mixed up with mwoldtimer. Anyway. let us know how it works if you do it, I might be interested if it keeps good low end. That's my riding style, lots of low end...


and if i keep be a tryn i mite be abl ta lurn how ta spell...



Thanks, I suspected you still checked this site on occasion. I didn’t realize Eric Gorr performed this service. I will give him a call.

Thanks for the prompt reply!

Mike OK,

Who's Doug? My name is Eric but I'll answer to whatever. Deep sand, high altitude in NM gives you more right wrist courage. Plus there are very few trees to run into...

[This message has been edited by Hick (edited 10-25-2000).]

I wondered why the Powroll was so much…

It sounds like the bore is left the same and the crank is stroked, although maybe it is bored and stroked, it does come with a piston kit. That may actually be a better idea since the YZFM is so oversquare (or undersquare, whatever) to begin with. Even so, that is a lot of money for a new crank and a piston kit. Boring makes me less nervous, after all, a brand new cylinder is only around $250. Offset grinding a crank seems more iffy…

I say “only” $250 because that isn’t much relative to the money I’ve spent already (in OEM and aftermarket parts).

Besides, I already mailed my cylinder to Mr. Gorr :)

MikeNeb, ( :D)

I’ll let you guys know how it runs when I get it back together in mid-Nov. (I’m waiting on some other parts as well). I also ride a gear high, especially in whoops. Maybe this is a habit carried over from my last bike (KX 500), but the 426 seems to really shine down low. I would imagine the extra displacement will help out more here, although it will now be even more oversquare so maybe it will help the top. It will be hard to compare directly, I don’t ride with any other 426s, but I will try to be objective.

I may have to borrow a bike, I am itching to ride so bad and my bike has been apart more often than not lately. I’d ride anything right about now. Even an ATV. Well, for a little while, anyway (wheelies are always fun on my buddy’s TRX).

[This message has been edited by Hick (edited 10-26-2000).]

Yeah, the Powroll description only mentions stroking the crank. Stroker Speed Equipment did a great job on boring and stroking my '94 KLX250R. The boring included sleeving the cylinder instead of the Nikasil. That bike is still running strong so if the stroking is done correctly, the result can be just fine. For comparison info, my KLX is now 338cc's and the HP output is around 42 compared to a mild 27 stock. With this engine and the Stroker suspension mods, the bike is a lot of fun to ride. The 6 speed transmission is great for trails but is a little too closely spaced for MX work.....lots of shifting on the track.

Replika Maschinen has a pretty interesting site for some engine work with their thermal coating and porting. This would be something that appeals more to me since it would decrease wear and increase reliability. I also talked Best MC about cylinder head/valve work and got some good info on a patented valve modification called anti-backflow cutting. This was developed for the sportbikers to increase midrange and would lend itself well with the 5-valve Yamaha head. From what I can understand about this mod is that the exhaust valve edges facing the pipe have small square(or flat) cuts in them. Evidently, these small cuts are supposed to decrease exhaust gases from backflowing into the combustion changing the scavenging characteristics and enhancing the midrange. Since this technology is patented, I haven't been able to track down where to have this done. Best MC isn't allowed to do it.www.replikamaschinen.com.

People have been cutting anti-reversion troughs in exhaust valves since the 60s. It works quite well, and can be effectively done without violating anyone's patent.

The 97mm bore is an excellent match for the valve area in the YZ. It lowers the peak torque and horsepower rpm, and eliminates the low speed torque trough that comes from the oversize head pipes that are so popular. The downside of the big bore is there isn't a good way to get enough valve area in the engine to support 11,000 rpm operation, so it's a poor choice for road racers, wide open dessert and some flat track applications. For most of us, it's the holy grail.


It sounds like a good match for my riding style, I rarely encounter the rev limiter except in a few sand washes, but I’m also considering swapping to a WR 5th gear for the occasional bomb run. Plus I have a big PC headpipe.

Actually, since I opted for the 13.5:1 piston I’m guessing the higher compression will provide as much of a boost in output as the increase in displacement. I talked to Mr. Gorr about this and he didn’t think I’d have any problems with the higher compression since I rarely ride below 4,500 ft.; I’m guessing 13.5:1 at this elev. is equivalent to 12.5:1 at sea level (or maybe even less).

But I have a question, I was thinking of using a 50/50 mix race/pump gas at least during break in since I figure that is when I can least afford any pinging. Isn’t race gas bad for the seals (or acc. pump diaphragm) in the FCR? Is using race gas during break in a good idea or just a waste of time and money?


I can’t wait!

The seal thing is just nonsense that MXA conjured up. I've run some of the most evil liquids know to man (including race gas) through my YZF with no carb issues. The fast Ducatis have been running the FCRs for years, without seal problems as well.

If you're riding at altitude, I'd just jet it for the pump fuel you're going to use and not worry about it. The race gas will most likely cause you to rejet when you stop using it.

There are lots of reasons to run race gas in the YZF, but octane requirement isn't really one of them at altitude.

Good luck. You're going to love the difference.

i know that the americans like everything to be bigger but if you were to keep your money in your pocket for a moment you will see the folly of the big 440cc motor.

trouble is it won't crack crankcases, it won't have sealing problems at the head, it WILL rev quicker (compared to the 440 but not the 12.5 to 1) & most of all it doesn't give you bigger balls!

singles crack crankcases if over revved & not in good balance. they crack like eggs.


the YZ cylinder head has a squish clearance that can be machined closer. i would think anywhere between .12" to .30" depending on what you find when you check it.

to take say, .016" off would give you a compression of guess what! 13.5 to 1!!!you will have the bike running the way the manufacturer intended with no burning taking place out wide.

it's a bit like buying something you already own! cost? well in england i was quoted £15 to skim the head.

while you're there you could alter the cam timing to the figures i have posted giving you more stomp. the cost? well er um it's free actually!

if you take the engine to a tuner & get the engine top end set up sweet you'll have a brilliant engine that's RELIABLE.

the big boys change cases & engines regularly, bikes even. even if they crack a case they can spit out their dummy out & get a new set free that will be rebuilt by someone proffesionally. when are you going to do yours? how much money have you got?


[This message has been edited by Taffy (edited 10-31-2000).]

I've noticed you Brits love to speak before you have all facts :)

i know that the americans like everything to be bigger but if you were to keep your money in your pocket for a moment you will see the folly of the big 440cc motor.

If you do a careful analysis of the entire system, it's easy to see the value of the 97mm piston for specific applications. Having tested 92mm & 94mm bores with various clearance volumes, and cam timing, I feel comfortable telling folks that the big bore is an excellent upgreade. The 97mm engines I've been tracking haven't shown themselves to be any less reliable than the stocker, but they are harder on clutches.

Rich Rohrich

thanks for giving us all the facts then rich!

so before i say anything else why don't you tell me first.

it's reliable when an expert puts it together & with the right people giving the right info but not everybody is as lucky as you clearly are. you only need one season out of your bike before you sell it on & it becomes someone elses problem.

pound for pound (£) i would rather see owners who intend keeping their bike years & put many miles on them to keep it simple & not throw their money away.

what i was offering costs peanuts compared to the 440 kit & i stand by my decision that people shouldn't get involved unless they have stacks of lucra.



Looks like you've met your match :):D


It's good to see someone post with a practical approach, and I can see you won't be deterred.


Maybe a few extra cc's and a 13.5 compression would help on second gear starts. I'll be looking forward to your test report. :D

[This message has been edited by holeshot (edited 11-01-2000).]

you only need one season out of your bike before you sell it on & it becomes someone elses problem.

Nice try, but I keep all my personal bikes pretty much forever, and pay for all the research from my own bank account so reliability and return on investment is of paramount importance. There is no trick to doing the 420 (94mm) upgrade. You send the barrel off to a professional for plating, you get them to skim the barrel on the lathe, and you set the squish clearance with the appropriate thickness base gasket. Performance per dollar (or pound) is excellent. On the other hand changing the cam timing is a snake pit unless you have access to fairly sophisticated test facilities. Given the myriad of differences in, pipes, jetting, fuel, and compression on these engines, it's silly to think that a generic prescription for LCA is of any value. Only serious testing will show the value of subtle changes in the cam timing, and the ability to press off the stock gear and repress it repeatedly is a SERIOUS unknown at this point. I have reports of failures from people, but not knowing the exact procedure that was followed it's hard to know where the fault lies. The Falicon gears appear to be of excellent quality, but press fitting a gear to a hollow shaft is a dicey engineering excercise.

Like you, I'm a firm believer in the blueprinting approach to performance, but given what I've seen of the relative strengths and weaknesses of these engines since their inception, I can say with complete comfort that the 94mm overbore is the best dollar/performance value out there ASSUMING IT'S DONE CORRECTLY as in NO STEEL SLEEVES.

BTW, what the engine really needs is a more aggressive opening ramp, not an LCA change :) I think you'll find that that was the driving force behind the Ti valve addition on the new engines, and NOT the ridiculous idea that it would lower the Mean Friction a substantial amount.



you're moving the goalposts you naughty boy!

we are on about a 440cc bike aren't we! we are on about a 97mm bore &/or the stroked crank aren't we!

folks bored out their motors to 420cc on the 99 models & the little ends went tits up.

my point is that it is very expensive to bore & stroke the engine & they shouldn't do it until they've done the simpler stuff first, i still question the reliability & will do for another year. of course we need someone to be the guinea pig & you just got the job.

i was going to respond to your comments on the cam timing & then i realised that you've changed some of my previous comments. that i won't put up with.


Originally posted by Taffy:

if you take the engine to a tuner & get the engine top end set up sweet you'll have a brilliant engine that's RELIABLE.

the big boys change cases & engines regularly, bikes even. even if they crack a case they can spit out their dummy out & get a new set free that will be rebuilt by someone proffesionally. when are you going to do yours? how much money have you got?



I’m just a poor dumb broke hick who doesn’t know any better but $325 didn’t seem like much. To answer your question I’ve already had to order new cases, bearings, and a new crank and am waiting for them to get here.

My bore looked fine, but I was pondering a new piston and rings. The big bore kit seemed like a good deal when compared to staying stock.

Since the altitude I ride at probably robs quite a bit of output relative to sea level the bore and compression increase will probably just bring my bike back to stock, sea level output. In the event that my YZ cracks a case it will surely be because I’ve goofed the rebuild or Yamaha goofed their manufacture, not because I’m a stupid Yank who thinks bigger is better.

But it usually is… :)

And where am I to find a good tuner? The head mech. at the only bike shop in town (and the relative best in a 200 mile radius) told me I needed to use a press to install my crank, this after a junior tech. told me to use a hammer, so I doubt they are well versed in cam timing or cyl. quench. I may be a stupid hick but I’m smart enough to keep my bike away from the likes of them. I don’t expect they are capable of delivering anything “brilliant” or “reliable.” Costly, suspect, and catastrophic is what I would expect instead.

So I’m not sure if your suggestions apply to my situation.

But thanks for the advice nonetheless, consider it duly noted.

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