98/99/00 WR400 Big bored to 444

Hi guys,

im intending to bore up to 444cc.most likely to go with the stock YZ426 stuffs , bore the cylinder to fit a 97mm piston.

can i know how many of you guys have actually done it, esp with the 98 models.

99 and 00 models may join in too.

Pros and cons , against high revving and overheating issues.

i would like to have the firm reliability after the big bore.

and to know the feelings the happy big boreRS, haha .

please share more experience with me, like fixing it up , maintaining etc etc.

what ever things to make me go for it.

Thanks guys ! cheers !

Thanks alot for showing me the thread.

actually i just wanna know how many have done the bore, and their personal reviews about it. what faults they encounter , jettings etc etc.

thanks guys, and hope to get more reviews. cheers

Can't talk about the 98-00 bikes, but I can share my experience on the 01 426's. If you go big bore, you want the 426 crank for the bigger small end bearing. The install is no more dificult that a stocker, and the cost is about the same if your stock cylinder needs to be weld repaired or re-plated. You can get 2 flavors from most big bore kits...12.5:1 and 13.5:1 compression. I love the bottom end and overall punch of the 13.5:1, but I have to run race gas to keep it from pinging. Not sure what the gas in singapore is like, but here, it's 91 Octane at best. Even int he 100+ degree (F) heat of the so Cal desert, cooling has not been an issue. I ride at about 2000 +/- 500 ft above sea level, with 60-90% humidity. My jetting specs are Red JD needle on the middle clip, 170 main jet, 42 pilot jet. Hope that helps...

thanks birdy426, nice white yamaha you have there.

gas wont be much of a problem in my country, temperature might be a pain.

1) any more 400s that has the bore kit?

i am most likely to get it done on my 98 wr400.

2) What are the maintenance like after the big bore, just oil change after every few rides, valve clearance etc etc. i wont wanna be opening up the top after every 1 or 2 rides to check any loose or burned parts. please advise

3) i need pointers to look out for. this is what i know i should get.

a) 444 bore kit from lukesracing

:devil: OEM 426 crank assembly

c) ????

d) ????

4) most important of all. is fixing up everything nicely easy?

will a normal mechanic do the job? i only know 1 guy who did the 444bore on his 426 in my country, bust his top after 3 mths. im not sure what went wrong. not fixed up by professioner?

i will wan the bike to be as reliable as before, so i wont save on the parts to get things right. i need pointers from you experts out there.

thanks guys !

i did the Big Bore 444 from lukesracing.com . Great guys, tons of help, worth every penny. Night and day. Bike rips now! I put in the Hot Cams too, im sure that helped. It was doggy before and now it keeps up to buddies KTM 525. Wheelie machine, lots of torque. Had it on some long rides, no overheating or issues. Jetting: I never touched it and it runs great. Ive got a 98 WR 400 and put the 426 crank and big bore 444 in it with the HC's. Stock YZF 426 pipe. No compplaints... do it... do it!

Oh ya, forgot to mention... timed to YZF specs

do it!

Great !! thanks Steade

so no one actually broke their top with this kit before?


not yet. just adjust the valves around 20 hours. otherwise, twist and go.

Putting the big bore together is no different than putting the stocker together. Same kinds of parts going together the same way. Any reasonable mechanic can do it. I have about 600 miles on my 444 kit from lukes, with absolutely no issues (other than having to run race gas...) I change the oil after every riding day (did that with the stocker, too), keep the air filter clean, and ride. I have Hot Cams as well, and assembled everyting with a new top end (valves, springs, keepers, valve job, etc) and the valves haven't even moved!

As far as your buddy's issues, maybe he needed to run race gas (hi-comp piston) and didn't...maybe he didn't keep his oil changed...maybe wore down the cam gear on the crank and the cam chain jumped a tooth...?

thanks again birdy , your comments and advises really tempt me and maybe lots of 400 riders.

but what i heard from a rider buddy saying that the yz426 crank assembly will not fit into my 98 wr400 , as the 98 have a smaller crankcase compared to 99 and above?

any 98 guys have problem encountering fitting in the yz426 crank assembly?

lots of nice and great feedbacks from you guys in TT.

but lots of negative from my locals , sadly.

dude, you don't need a 426 crank. Just put a 426 rod on your 400 crank and replace the counterbalancer drive key as a precaution.

I told you if you use the 426 crank the clutch and counterbalancer drive is different and you will have to replace all the other stuff too. It's not necessary.

Lukes will sell you everthing you need.

I think R Little is correct...the important thing is to get the larger small end bearing from the 426, as the big bore kit does stress that part a little bit higher, and the 400 part was marginal to begin with.

thanks guys, got a little confused. but now i got it.

i read thru some old threads abt the difference between the YZ 426 rod and WR 426 Rod. the WR ones can last longer and the YZ ones are more for racing... is that true?

so for the stock parts .. i should be getting WR parts?

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