Jump to content

Project Big Bore (2001 WR426F)

Recommended Posts

I know most of you guys dont get over to the Supermoto forum so I thought I would share this here also since this is where my roots are with ThumperTalk.com

This is a cut and paste from the original post, and the reply I made last night.

Ok folks, I have started the Team Scream Project big bore, I am going to take the bike to a dyno and get base line info, and will do the same post big bore.

The starter bike is a 2001 WR426F-SM.

The Parts:

Weisco 97mm (12.5:1) piston

New Stock YZ Wrist Pin (supposedly 15-20 grams lighter than the Weisco pin, will weigh it and post).

Cometic Gasket kit.

Brand New Yamaha YZ Cylinder.

Brand New Stock valve springs and seals.

The Work:

US Chrome bore and plate.

Hiroshi (Henry) Yokota competition valve job and port blue printing/clean up (Hiroshi is Matt Mladin's current engine builder for Yoshimura R&D, he also built my ultra fast 2002 GSX-R750 racebike which I won 4th overall in 750 Superbike on).


Not one person who's opinions I valued would commit to any significant gains in power by "porting" the stock YZ/WR head, they all said these heads are as good as they get, they all agreed that the engine would benefit from cleaning up the port and intake/exhaust areas and by matching the exhaust/pipe area fitment as well as matching the intake horn to the head. Beyond that they said I would be throwing away my money....I really appreciate that kind of honesty in a builder/mechanic.


The cylinder and piston are on there way to US Chrome with an estimated 5 day turn around upon arrival there.

The head will take one day once Hiroshi gets his hands on it which will be sometime next week (soon as he gets back from Elkhart Lake WI with Yoshimura)

If everything go's well the bike will be ready by the 18th for the practice day at Perris, but certainly ready for the next SuperTT event and absolutely by Laguna Seca for the opener which Team Scream is registered and licensed to compete in.

Link to comment
Share on other sites

Ok here we go, I get a call from Hiroshi today (wasnt expecting him to call until Wednesday) he states that he has to make 2 spare heads for Matt's (Mladin) bikes for Laguna Seca and his schedule has changed, bottom line? I need to get my cylinder head to him......today.

Oh jeeses, I wasnt ready for this phone call, but I needed to be....so...I took the wife to the market to get the husbandly duties out of the way and she turned me loose to my shrine (the garage) where I lit an incense, turned on the radio and swept the floor, put my bike on its stand, laid the carpet underneath it and proceeded to prep my tools, for those that dont know me personally, I like to lay all my tools out on the workbench prior to beginning any work, kinda like they do at the doctors office...not sure why....but it makes me feel better..so I do it, it was 7PM Saturday and I was just comming off a natural high unlike any I have had in a while (see "perfect day" post in the supermoto forum).

The place smells good, looks clean and the music is on, wife is happy, kids are screaming and laughing...they are set, I am in "the zone".

Below are the pics of my handywork, this is pretty easy stuff, even if you are a little apprehensive about brain surgery, you can handle this...make it so.


Tank off, seat off, carb off, stuff moved aside, no dirt, valve cover removed.


Top Dead Center,Intake Cam & cap removed,Cam chain tensioner removed, Cam chain secure.


Cams out, all parts accounted for, clean rag.


Head removed, front chain guide had to be removed to get the head off, it pulls right out.


Cylinder removed, chain secure, notice the tape covering the holes where the oil lines went, keeps "scoodge" from getting in.....clean is good....dirt is ....well...not as good.


Piston removed, parts laid out, everything inspected, all looks good, no chain guide wear, no funky aluminum chunks hanging onto the threads of the studs which could indicate thread failure, all of the studs had a generous amount of moly grease on the threads indicating the guy was not hung over when he built the motor.....so far.


The head, buckets looked good, no galling, the cams looked good, the guides looked good, the pin and rod show signs of light galling beginning, nothing serious but you can tell this is the weak link in the chain, the design looks a little questionable to me, 1 small oil hole per side lubes the piston pin, the piston pin was a light (not pressed) fit, afer removing the snap ring on one side, I used the rubber coated handle of a small pair of channel locks to push the pin out,...piece of cake really.

The head is now at Hiroshi's, it will be ready Tuesday, my cylinder and piston should be back from US Chrome Monday or Tuesday, I hope my valve shims come in on time, if not it will be Thursday before it is all back together.

More to follow.

P.S. it is amazing that it appears my bike was running rich all over yet I felt like it was running really strong, which only means that it could have been better, it will be on the dyno getting completely dialed a day or 2 after I get it built.....rich will be a thing of the past....I promise.

Link to comment
Share on other sites

Did not get a baseline run, I was planning to do that Monday since Hiroshi was going to take the head on Wednesday, but since he called me Saturday and said "its today or AFTER Laguna Seca" I was pinned.

I am sticking with stock valves for now since Hiroshi said they were fine after looking and them last night.

Project goals are to break 50 RWHP, anything over that is bonus time, and I would like to gain some engine braking, (more is better for what I do).

I am pretty dissapointed that I didnt get baseline power output, but after seeing the piston and head pipe, it was clear that I was off significantly in my jetting, so it would have been an all day afair at the dyno, none the less I would have done it, and it would have been really cool to post the results of both.

I Need to talk to you SFO, I sent you a PM.

Link to comment
Share on other sites

Create an account or sign in to comment

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

Reply with:

  • Create New...