Thumped!!!

[DesertDog329]

I bought a used 98 YZ400 a little over a year ago and became addicted to desert riding. I have kept up on my oil changes and filter cleanings ever since I bought the bike. About two months ago I replaced the rings and every thing was fine. This last weekend I changed the oil and the radiator fluid before a saturday ride. I rode for about two hours with no problems, bike was running great! On the way back to the truck to fill up the two stroker I was riding with, I was just starting to accelerate through a gradual turn and about mid throttle there was about three loud crashes coming from my engine and then everything stopped. EVERYTHING!!! ? I got a tow back to the truck and when I started to pull the engine apart things got really ugly!! My balancer rod was the first thing I noticed broken. Once I got the cylinder loose the piston came with it. My Rod was shatered and everything inside the innner casing was destroyed except my gears. The crank weights have huge scars along the out side and one side has a little heat discoloration. Needless to say this is going to be an expensive repair.

Has anyone ever heard of this happening? Anybody have a clue what might have caused it? The parts guy told me that Yamaha had problems with rods breaking in the very begining but fixed it quickly. I was also told that with the high compression in these engines the rods are bound to fail sooner or later. Are there any aftermarket suggestions for the rebuild. I plan to replace all the bearings while I have it apart but are there any aftermarket items I should put in now while it is completely apart? Does any body sell complete engines?

This event has shattered my weekend plans for a while, so any information\advise would be greatly appreciated.

Thanks

[Tim]

Man does that suck. Sorry to hear that. Hope all goes well on the rebuild. I guess when you get over a thousand bikes together on one spot on the net, we will hear about everything we can imagine.

[Hick]

I know the small end of the rod was enlarged in ’00, I think something else about the crank was strengthened in ’99. It sounds like your rod or wrist pin broke first and this led to balancer and crank damage, but I’m only guessing.

Did you replace the piston or wrist pin (and circlips) when you changed the rings?

A slight heat discoloration at the crank pin is normal, they heat this area when they press fit the crank, I had to by a new crank and it also had this discoloration.

You may want to replace all 9 bearings in the case, I would replace the main and balancer bearings at least. I have an oversize, high compression Wiseco piston and I love the diff but I ride at elevation on pump gas. 13.5:1 at lower elevations will mandate race gas, I’m sure.

As for the crank/rod you may want to get it rebuilt, although I don’t know who to send it to (if you find someone let me know as I have a slightly damaged ’00 crank). Or you can buy a 426 rod/crank assembly, I paid $250 for mine, and then cough up another $250 or so for a 426 cylinder. Voila! Instant 426!! (I’m assuming you need a new piston regardless).

BTW if you can change your rings, and have access to a bearing press, rebuilding the motor yourself is an option you should consider.

My two cents.

[MrMXer]

Outch! That kind of failure hurts. The 98 has a weak crank/rod assembly. I'd look into springing for the 426 crank and cylinder. I do not know if this can be done. Has anyone here tried this? If so, give this guy some info on how it's down and what the results were.

[Moto-Mike]

I do not know if they are interchangeable (400 to 426) but check this out in EBAY. Search under "426". There is just over 2 days left on the auction...

This here is a new yamaha yz 426 motor complete with carb and electrics NEW never ran!!!! This motor was bought to install in a 88 trx 250r quad but ran out of funds so the motor has to go remember it is new and comes with everything to make it run motor carb statr rotor cdi box coil and wiring harness clutch cable throttle cable and twist throttle complete package ready to install in your atv,gocart or your yz highest bidder to pay $40.00 shipping and handling to U.S states i have many other parts e mail or contact at (909)788-0554 thanks and good luck bidding!!!

[Hick]

Originally posted by Boit:

If you are looking for a good crankshaft shop, Falicon has one of the best reputations going.

Hey Boit,

I called them about my (slightly) bent ’00 crank and the woman who answered said they were no longer rebuilding singles. I didn’t ask why, maybe I just misunderstood her (or her me).

Is there a # for crank works in Cycle News?

[Taffy]

don't panic

i presume the barrel is sliced up, that the cylinder head missed all this. i also presume that besides the scar that your crank is still ok.

you've had a rod failure, that's all. the other bearings were doing fine.

because i'm always broke i would wash & listen to all the bearings in the cases.

i would buy balancer shaft bearings, an aftermarket con rod, piston & barrel of either 400 or 426. the crank assembly only needs balancing if the new piston & rod COMPLETE, are heavier. go to the butcher's shop, fresh paper on the scales & WEIGH THEM, complete, i mean pin, rings, circlips the lot.

if the con rod turns on the crank nicely & looks sweet when you strip it, it wasn't an oil failure, you might be lucky witth the crank. in splitting, if the big end of the crank is graunched; it needs machining & a regular engine rebuilder can sort this if they have the measurements (which i think are in the workshop manual).

the camchain may have taken a yank so replace this as a good precaution.

to check for bent valves, put the plug in, turn upside down & poor parrafin or white spirit into the cylinder head & fill the combusion chamber. look in the ports, it'll be laying in there if you have bent the valves.

at the end of this you will have a close guide to what to do & what it will cost you.

now make a pledge to yourself that you will let everyone know how you got on, because most people don't & that isn't nice is it?

Taffy

[Boit]

Hick: Crankworks has a website at www.crankworks.com. I couldn't find their ad in Cycle News and was begining to doubt my geezer memory. By sheer determination I found their web site. Alzheimer's is delayed for a bit longer!

I didn't surf the site for very long since it was taking too much time to load, but it appeared that they may specialize in 2-stroke cranks only.

I knew Falicon specialized in multi-cylinder engines but I thought they also dabbled in singles since they make the cam sprocket adaptor kits for the YZF's. Perhaps it just so happens that those adaptors are interchangeable with some of the street bikes.

Eric Gorr has a forum at Dirtbike.net and will answer questions FOR FREE concerning problems such as DesertDog's. I recommend this as a starting point.

If it were me in this predicament, I would not only look to have the engine repaired, but to have it repaired in such a way as to make it bulletproof......fix and update any weak points. Those weak points have surfaced in this forum.

[Boit]

If you are looking for a good crankshaft shop, Falicon has one of the best reputations going. They don't do engine tear downs or rebuilds, you send them your crank with what you want how it is going to be used and they will make recommendations. I especially like the idea of having them balance the crank when they're done. They also can supply you with some dandy connecting rods such as their "Knife" rods. I don't have their number because I'm at work now. Cycle News has a good classified section for specialty shops that do this type or work. OH, another one tha just popped into my head is Crankworks. I don't know much about them though. If you would like a quality aftermarket shop to rebuild your engine for you, check with Eric Gorr.

[DesertDog329]

Thanks for all the info and advice. right now I am looking into a new engine. if all else fails I will be rebuilding my engine. And yes I will change out anything that has been discovered as a chink in the yz400's armor. I have loved this bike ever since I bought and will still reccomend it to any of my friends who are interested in riding.

Thanks again for all the advice. Sooner or later I'll be thumpin across the desert again. I'll keep you posted

Late......

[F-Pilot]

Did you check out that motor on e-bay? It was going for about $1000 and was brand new.

[DesertDog329]

Well I went onto ebay and looked at the 426 motor. I did't have the cash in hand. it will take me few weeks to put together 2G. Sweet looking setup. the guy who got it sniped it at the last second. I'm still going over my options. Any suggestions would be appreciated.

Great input so far thanks guys.