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pscook

426 Engine guys inside, please

12 posts in this topic

I just picked up a 2002 YZ426 with a seized engine. The seller said that he changed the oil prior to the last ride, and lo and behold- criiick- the engine stops running after every last drop of oil runs out of the oil tank through the hole that used to have a plug in it. I confirmed that the engine is stuck by trying to depress the kick lever, and nothing happened. I will be attempting to turn the crank later today, but I know that I need to rebuild the engine so what's the hurry, right?

Right now every bearing and the oil pump are suspect and will be replaced. I assume that the piston, cylinder and cams need some attention, just because no oil= plenty of heat= friction welding of incompatible materials. The transmission and clutch seem to work, as I can shift into gear with the clutch lever pulled in and the bike will roll with minimal drag while in gear and lever pulled in. But I am not an expert (by any stretch of the imagination), so here are my questions:

Besides the above mentioned items, what else should I look at? Is there a "Kit" oil pump available? If so, is it worth it? Aftermarket bearings or OEM? What about gaskets? Any vendor out their better than OEM? If the piston and cylinder are toast, are the big bore kits any good? I have no problem going OEM for parts, I prefer that over taking my chances with aftermarket items if possible.

Since I don't know how many hours are on this engine, what else should I be looking at doing while the cases are open? I understand that I have Ti valves, should I replace? If I do replace, should I install steel valves?

Thanks for your attention on this, and sorry for the amount of questions first thing out of the gate. And, I'm in Seattle so if anyone knows of local shops to do work like replating the cylinder or doing some crank work, let me know.

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I would stick with OEM parts with the exception of the exhaust cam which I would replace with a Hot Cams auto-decompressor cam. I would order the OEM parts from the TT store...the prices are more than competitive. If you are not down with an online parts store then I would recommend Adventure Motorsports in Monroe...an excellent shop that is a huge supporter of off roading here is Washington.

If you need to replace the valves I would stick with the OEM Ti valves also. If you switch to SS valves you will need to get heavier valve springs also as the SS valves are about 1.6 times heavier than Ti.

I have had good luck with Millenium Technologies for cylinder replating. As far as I know there is nobody in Washington that does motorcycle cylinder replating.

If you need any machine work done I recommend C & D Machine in Kirkland. It is owned/operated by a motocrosser so he knows bikes, although he does ride Hondas.

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You can pull the bearings in the bottom end - take them to a bearing place and have them match them up - HUGE savings.

Gasket kit - you can get one at the TT store - Moose racing - about 100.00 for EVERYTHING - gaskets, seals, valve seals - you name it - it's in there.

Go with OEM if you want longevity.

The only difference in SS vs Ti Valves is that SS wears evenly/continually - Ti wears Nil at first, but once the coating is worn they wear fast.

In both cases, valves should always be checked frequently, Ti vlaves will let you know when it's time to do them again - based on shimming frequency and amount.

SS valves can give you "false hope" since you shim relatively evenly all the time, this can lead to going beyond their usable life if you do not keep good records.

I prefer SS valves but I don't run high revs - anyone running high revs wants to run Ti valves due to the weight savings and performance gains they get.

When it's open, check your gears/shift forks for wear - pretty self explanatory - easy to replace - not too bad on $$.

In all likelyhood, I would replace the tranny bearings as an added measure of assurance.

Good luck with the rebuild!

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Thanks for the advice so far, I appreciate it. Yes, EVERY bearing will be pulled and replaced, no question. I hope to have the top end apart this week, but I'm not in that big of a hurry since I don't have any deadlines (except for school and work, of course).

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Notes on the advice given:

> To expand a bit on the transmission inspection, pay particular attention to the condition of the lugs and slots on the gear sides. These lock the gears to each other when changing from one ratio to another. If the appear rounded excessively, they will cause the trans to skip out of gear under a load.

> When "matching up" bearings, be certain they match exactly, and not just in dimension. Check for end shields, or lack of same, and any other features. Many of them, the mains, for example, have a step on one side of their outer races to match up with the retainer plates.

> With a 426, you have the option of using OEM 2000 model SS valves. These are far less expensive, and the performance gain, if any, from using Ti instead is marginal at best. Be certain to use the springs from an '00 model along with them. Faction MX also makes one piece stainless valves and the springs to go with. Their valves are nitride coated as are the OEM Ti valves, so they wear in a very similar manner.

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Notes on the advice given:

> To expand a bit on the transmission inspection, pay particular attention to the condition of the lugs and slots on the gear sides. These lock the gears to each other when changing from one ratio to another. If the appear rounded excessively, they will cause the trans to skip out of gear under a load.

You got it. If I need to I'll send them down to APE to get the dogs addressed. Nothing like getting a well sorted undercut trans to play with.

> When "matching up" bearings, be certain they match exactly, and not just in dimension. Check for end shields, or lack of same, and any other features. Many of them, the mains, for example, have a step on one side of their outer races to match up with the retainer plates.

The nice thing about Yamaha (at least on older bikes) is that their bearing p/n's use the industry numbering convention. I discovered this on my road race bike when the bearing took 2 weeks to get in (and I needed it for a race in 8 days). Unfortunately OEM 's use very specific bearings, and sometimes it can be a challenge to find a needle bearing with an internal wiper and an alignment groove outside of the OEM supplier. I will keep your advice handy about the mains, as this is my first adventure into the bottom end of a single.

> With a 426, you have the option of using OEM 2000 model SS valves. These are far less expensive, and the performance gain, if any, from using Ti instead is marginal at best. Be certain to use the springs from an '00 model along with them. Faction MX also makes one piece stainless valves and the springs to go with. Their valves are nitride coated as are the OEM Ti valves, so they wear in a very similar manner.

That's good to know. Would the price difference of the SS Kit to Ti replacements make that much of a difference in life expectancy or performance loss/gain?

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In my opinion, for the majority of people riding these (and you haven't stated your skill level, or how you intend to use the engine), there really is not a lot to be gained one way or other by using Ti, or by using SS. But since that is true, why would you pay $80 and $88 per valve when you could pay $11 and $26?

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In my opinion, for the majority of people riding these (and you haven't stated your skill level, or how you intend to use the engine), there really is not a lot to be gained one way or other by using Ti, or by using SS. But since that is true, why would you pay $80 and $88 per valve when you could pay $11 and $26?

My intention is as a trail bike and Supermoto platform. I have a pretty fast road race bike, but there are a couple of guys that have a bunch of fun on their singles and I want to go and play. So, extended high revs for 1/4 mile or so with lots of flicking. I'm probably going to be looking for longevity mostly. If I can get the gearing right I shouldn't be up against the rev limiter too much (fingers crossed). I have never had to deal with Ti valves before, I know that being light that they would help with valve float (or maybe remove it) and put less stress on the cams and camchain. But yes, I'm broke (self induced) and cheap (genetic predisposition), so I might look at SS valves and springs and just pay attention to adjustment intervals and spring deformation. I looked on the Factor MX site but they didn't have a price for the 426 valves listed, only the 450/250 stuff. If it is cheaper to get SS valves and springs than just new Ti valves, I will go that route. But, I haven't cracked this thing open yet so who really knows what it looks like in there?

Should I look at getting a taller 5th gear, like the one from the WR? Is that a lot of trouble inthe YZ engine?

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The nice thing about Yamaha (at least on older bikes) is that their bearing p/n's use the industry numbering convention.

the not so nice thing, at least up here in canada, is that the guy at the bearing shop who spent 15 minutes on the phone with nsk and one other supplier said they are yamaha proprietary bearings and cannot be matched, he made sure to point out he could get me something close but may not be able to match the chamber on the inside of the main crank bearing or the exact lip on the outside, and the dealer up here wants 85 bucks a peace.

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the not so nice thing, at least up here in canada, is that the guy at the bearing shop who spent 15 minutes on the phone with nsk and one other supplier said they are yamaha proprietary bearings and cannot be matched, he made sure to point out he could get me something close but may not be able to match the chamber on the inside of the main crank bearing or the exact lip on the outside, and the dealer up here wants 85 bucks a peace.

That is exactly what happened to me with my bearing. "The numbers mean that there is an internal wiper, it's 25 mm long, 12 mm internal, 20mm external with a groove for a locating ring. Here's a picture of it. I can't get it for you. But that's what it is". Yippee.

So what about the WR 5th gear?

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That's a nice gesture, but yeah, a bit far for me. Surprisingly, I just finished tearing into the engine, and it is a melted piston that is the cause of the issue. The crankcase still had oil in it, and every bearing was wet and slippery. The oil pump had less than the maximum clearance with no excessive grooving or notchiness, so it's going back together tomorrow, except the top end, of course. I need to get it honed out before I decide if I want to go big bore or not, cuz if the bore is good, I'm going to get a new piston and get that sucker running.

Any recommendations on pistons? Is the big bore worth it and do I need to do anything else to the bottom end if I go up a couple of mm?

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