417/420 Reliability

I interested in a 417 kit running 13.5 compression and want to know of those of you who have the 420---any reliability issues at all?? I just installed a Hinson Clutch basket and new friction plates and springs and I want more bottom end. I'm now seriously considering the 420 so talk me out of it!!


Clark I'm planning on doing the same thing this winter on my 00' WR. I've done lots of reading about the big bore upgrades and it seems like the best one out there is by White Bros. Yamaha and several other people have questioned the reliability of the bottom end in our bikes :)

One of the dirt rags had done some aftermarket upgrades to a 426 and had talked with Yamaha about it and they couldn't promise too much as far as keeping all the parts inside the crankcase intact. You know as well as I there have been some crank failures out there, so the posibility is there. I'm sure Yamaha has grenaded plenty off these motors with big bore kits to find the optimum displacement. The same mag had a YZ upgraded to 420 with the White Bros kit and had run it all season with no problems. White takes your existing cylinder and head and upgrades it to 420 and installs new valves in the head as well as doing some porting work. They said the power was awesome :D This kit is what my game plan is for the end of this season. Let us know if you find anything else.


00'WR400 75'Yamaha 250 enduro

White e-Series S-bend 12 discs, Yz timed, rejetted to Clark specs, throttle stop cut, lid removed, Topar Racing top clamp, Tag Metals T2 bars, Scotts damper, Devol disc guard and frame guards, lights removed, Cr routed Fastline brakeline, Acerbis Rally pro hand guards fastened to the top clamp, Gel grips Live in Southwest Ohio


I've been running the 420 kit along with porting for about 6 months now - so far everything is holding together - no problems. I hit the MX track about 6 to 8 times a month, so I've been running it pretty hard. I am running with the high compression piston 13:5. There is tons of low end, mid, and top. I was told that the White brothers Big valves would not make a difference with the 420 kit so I did not go with the valves.

James that sounds great! I was wondering one thing...What are you running for gas? I'd figure at that compression there must be some predetonation going on.

What did you finally end up with for jetting compared to what you were running before the conversion?

How fast did they get your motor parts back to you?


00'WR400 75'Yamaha 250 enduro

White e-Series S-bend 12 discs, Yz timed, rejetted to Clark specs, throttle stop cut, lid removed, Topar Racing top clamp, Tag Metals T2 bars, Scotts damper, Devol disc guard and frame guards, lights removed, Cr routed Fastline brakeline, Acerbis Rally pro hand guards fastened to the top clamp, Gel grips Live in Southwest Ohio

Clark and all other performance aficionados,

A few thoughts on the big bore idea:

Dirt Bike ran a WB 420 modified WR and a stock WR in a desert race a year or so ago (titled Tortise and Hare). Bottom line the 420 had major engine problems and the stocker finished. Dirt Bike strongly recommended against the 420 kit and strongly recommended the oversize valves. They said the oversize valve on the stock 400 made a big difference.

MXA had an article on oversize kits for the YZ400 a few months ago. Bottom line, not worth the effort. Reliability goes way down. Considering they were focused on track riding, I would not consider an oversize kit from a reliability perspective when off-road riding out in the middle of nowhere.

My thoughts: First go with oversize valves. If that does not work and you really want more performance and have the money, then get the YZ426 and put in 1st and 2nd gear set from a WR and what ever other parts you want.

Eric in WA


’99 WR400, YZ timing, IMS Tank, YZ Seat, Acerbis Handguards, Scotts Steering Damper, Renthal Bars, Devol Rad Guards, Dunlop 755F & 756R Tires

’99 Polaris 400 Scrambler –“Trail Couch, Toolbox and Lunchwagon”, ’99 Honda Z50R, ’97 Honda XR70R, Kubota L275DT and an assortment of farm equipment.


check out this web site. http://www.worksracing.com/motorcycle.htm

if you are looking for power the dyno tests this guy has on the site sure looks like he has the HP. It's a less work just send in the head and $400 later you bolt it back on and your done. Although we all like to wrench on our bikes so the work might be the fun part. Just another option!!

I'v already done the works CNC porting and want more low end. Im inclined to go for the 13.5 to 1 compression, 417 displacment and work the the squish band down to around 0.040". My only hesitation is 1) bike works good now and 2) I worry about reliability.

What re-kinled my intrest in all this is I went riding with the guy who purchased my build 97 Husaberg FE600E and of course had to put up with easy electric starting out standing power a true wide ratio six speed and be blown away all day. I should have NEVER sold that bike!


Clark, I rode a '00 Husaberg 501 w/electric start last weekend. It shifted like a vintage bike and the brakes and ergonomics were not up to the level of the WR. The big hole under the seat where the airbox usually is looks odd too. And the Mr Bubbles tank shroud.

-On the flip side, the torque was great and the hydraulic clutch was awesome, as light as any 125's.

-Overall, the WR still has a better package.

The HP claims for porting from the link above seem to be in line. Anyway, my WR hit 54hp on the dyno at a cost of 900 bucks. I am running 101 Oct. race gas. Jetting is: MJ 178, PJ 48, with stock YZ400 Needle.

Would I do it again? I don't think so. I purchased the WR with the intent of desert riding, but that has changed to MX. The WR is now a YZ420 with WR gearing. The gearing actually works great at the track, start in 2nd hit 3rd and leave it there.

I have decided to race the WR for the next year, convert it back to desert bike, then purchase a new MX thumper.

Clark I am thinking of having the CNC porting done this winter. What did youthink of the results? Is it worth the money? Where in the power band did you see the results?


Just a few thoughts on those big-bore write ups in MXA...

Why would they say that a 420 kit is perfectly reliable but a 426 kit scares them? A whole 6cc changes it from an anvil into a grenade?


More like payback to their big advertisers.

You ever notice how much of the WB stuff they use on their bikes?

I mean, the rod and bearings were the same on both bikes. How much of a difference can a 26cc enlargement, everything else being stock, make over a 20cc increase with big valves?

I believe Rich has a big-bore in his, probably along with other mods, so he should be able to provide insight.

(Tongue planted firmly in cheek, well partly planted, any of you that have had you're sense of humor removed read no further :D)


I think you've taken the poor "Sow's Ear" WR and made it as close to "Silk Purse" YZ as you can! :D It's time to sell it to one of your riding buddy's and get a new bike to

start working on. I think you need one of the new YZ440's (2001 model )that would give you the extra power you're after with a stock motor then you can put WR gears

in it and duel sport it! NOW THERE'S a PROJECT!!!

( Engineer's just LOVE projects :D)

You know on second thought, it probably can't be done, you'd have to split the cases and rewire the bike, put a coil on it, yup, too hard can't be done.

(psssst, the rest of you.....

:) Engineer's can resist a project that "can't" be done!!)


Chris in the Mojave

'98 YZ400F

I cannot hardily recommend the porting. It does imporove overall power and engine response. But as to the cost vs performance benefit I not convinced or sold. If it was $200 then maybe a different response. Remember it is a flow improvement and is most responsive at higher RPMS and you do not ride at higher RPMS the majority of the time. If you want better low end go for more cc and higher compression. If you want top end go for porting if you want both do both. However the improvement due to higher compression and more cc always works. The down side is realiability, possible head gasket leaks (less sealingsurface, etc.


You ever notice how much of the WB stuff they use on their bikes?

I think it has something to do with MXA "wrecking crew" member Gary Jones being the head of research and development for White Bros.


'99 WR400

'92 GSXR 7/11

Visit my photo album AT YOUR OWN RISK!! My photo album

Anyone here a sportbike fan also? Then visit us here at www.insanespeed.com

I read once that the head or base gasket of the yz is thinner than the wr one (they have different part number). If the yz is thinner it should lead in slightly higher compression ratio and higher power through the powerband. I also know than they have the same compression ratio on the spec sheet. How to esplaint the different part number and why should the gasket be a different one?

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