'98 WR 400 Rebuild -- Specific Q's about the upgrade to 426

[rogueguineapig]

A friend of mine recently bought a '98 WR 400.

Here's what we found: it got hot, and burned the piston/cylinder (see pix)

Whilst searching for options, I came across a TT post here that alluded to using a 426 jug, a 450 piston, and the stock

400 bottom end and head. We're kicking options around and reading as much of the material on here that we can.

If we're asking redundant questions, we do apologize...we've searched for these and not found answers!

Here's the original inspirational post:

https://www.thumpertalk.com/topic/935821-400-to-426-conversion-the-easy-way/

- Can the 400 jug be bored out to 426 (we think that's 95mm or so, right?) safely, without over-thinning the cylinder walls?

We've heard of them being bored out to 97mm and that seems extreme. Is it better to just buy the 426 jug and be safe?

What's the max a 400 should be bored out to?

- What's the correct piston-to-cylinder-wall clearance? (If we bored out the 400 and used a 450 piston?)

- What's the deck height differences on the stock 400 setup vs the 450 piston/426 jug setup?

- What's the final static compression ratio with the 426 jug and 450 piston?

- What years of 426 jug and 450 piston will work? (thinking ebay parts maybe...maybe).

- What kind of power increase/delivery should be expected from the bump up to 426cc?

Because of the original post, we feel that an OEM Yam 426 jug and an OEM Yam 450 piston will not need extra calculations and theorizing...it will just need assembly, not figuring.

On the other hand, it's tempting to bore out the 400 cyl to save a few bucks, but then we need very exact data for the boring and then matching the piston, etc.

Anything else we haven't thought of?

Pix of the damage prompting the rebuild

thank you all!

[grayracer513]

- Can the 400 jug be bored out to 426 (we think that's 95mm or so, right?) safely, without over-thinning the cylinder walls?

We've heard of them being bored out to 97mm and that seems extreme. Is it better to just buy the 426 jug and be safe?

What's the max a 400 should be bored out to?

95mm

- What's the correct piston-to-cylinder-wall clearance? (If we bored out the 400 and used a 450 piston?)

.040-.065mm (.0016-.0026") whichever cylinder is used

- What's the deck height differences on the stock 400 setup vs the 450 piston/426 jug setup?

Never checked, should be very little

- What's the final static compression ratio with the 426 jug and 450 piston?

Nearly identical to stock, AFAIK

- What years of 426 jug and 450 piston will work? (thinking ebay parts maybe...maybe).

Any and all, as long as it's an '09 or earlier piston (5 valve)

- What kind of power increase/delivery should be expected from the bump up to 426cc?

Not much. It's a 6% size increase. Maybe 4% more power.

[rogueguineapig]

Awesome!! Thank you so much for the info!

In sourcing the parts, it seems like the 426 jug is insanely expensive...compared to everything else that we need for the project.

What's the favored place around here for parts like that?

[grayracer513]

Everybody has a favorite. Mine's the TT OEM Parts Store.

[rogueguineapig]

I actually looked there first...they don't have 426 jugs...

or I looked in the wrong spot...entirely possible...

[jojo1224]

I actually looked there first...they don't have 426 jugs...

or I looked in the wrong spot...entirely possible...

Strange, no yz426f results are coming up. Even when I punch in the yamaha part number for a 426 cylinder. I bought a jug from the TT OEM store late last year and it was $397. I don't think it would really be worth switching from 400 to 426, there wont be much gain for the cost.

[rogueguineapig]

well one of the ideas that popped up was to slightly bore out the old 400 cylinder, buy a Wiseco 94mm 12.5:1 piston,

and call it good.

The trouble is that our local bike expert and machinist said the boring would removed the Nikasil plating, and

thus the engine would wear a lot faster.

So then our question became, "what if we just bought another 400 piston and jug?" and the answer came back:

"why not a 426 jug / 450 piston for the same price?"

In the end, he just wants the bike to run and be reliable. It seems a new piston is a MUST, and because of wanting

the nikasil plating a new cylinder is a must too. We're just trying to figure out the best way to go about it, or see if there's options we haven't thought of.

[jojo1224]

You don't need a new cylinder, you can have the cylinder bored but then it will need to be replated. Check out USchrome or Millennium Technologies, they can bore and replate your old one for half the price of a new 426 jug. You can even goto a wiseco 94mm piston at the same time.

[rogueguineapig]

awesome! We'll look into it...thank you!!