Catastrophic Engine Failure Analysis

2005 CRF with nearly seven years of hard desert racing and riding on her. Have done two top ends since original, last one in late '09. This happened last weekend on mile 30 of a 100 mile race.



She lost power, I heard a clunking sound, then shut her down. No strange noises before that.

There was not a drop of coolant in the tank afterwards, but it was full before the race. Almost all of the engine oil was pushed through the valves, through the carb, and out the airbox.

My theories -

1. coolant system leak and damage caused by heat.

2. Never replaced rod and crank bearings, excess freeplay in piston, caused all damage, all rad fluid boiled off but not root cause.

3. Overdue for top end, damage caused by warped piston?

Any thoughts? Can anybody spare me a thousand bucks or so?

600 dollars will net you a complete big bore kit. get it done and don't look back before you change your mind and sell the thing.. trust me i did and now my bike is a beast.

what does the head look like?

what does the head look like?

Head is fine, will probably need a valve job tho. One of the rings got sucked into the exhaust port.

hmmmmmm, was the piston cast or forged?

hmmmmmm, was the piston cast or forged?

All 4-stroke mx bikes utilize forged pistons.

The piston skirt began to collapse due to wear. I have heard and seen it. As soon as you hear a little more engine noise and the bike has lost some power you have over run the life of the piston. The bottom 1/4" of the piston will show rounding. If you catch it before your situation you will see this. The piston tries to turn sideways in the bore blowing out the cylinder liner. I have also seen where it just plain bulged the cylinder. It also can put a ton of stress on the connecting rod and it should also be replaced .

Thanks for the reply Ron, I would agree with your diagnosis.

It was a wiseco piston, fwiw.

I would say you had a AMAZING run with that motor.. outstanding. That is a fair amount of hard racing (not easy on/off like woods riding). Im almost more convinced now than ever, that i replace stuff in my motor WAY too often. :thumbsup:

Out of curiousity, what types of oils do you run?


Out of curiousity, what types of oils do you run?

Rotella T 15W-40 both sides, changed every 5 hours.

After reading all of these posts on how long these engines last on Rotella im just going to use it on all of my vehicles to simplify my life. I just picked up a blue T6 gallon at Wally World.


Thanks for the reply Ron, I would agree with your diagnosis.

It was a wiseco piston, fwiw.

Ron hit this exactly on the head if you ask me.

How many hours were on that piston? And what was the C/R?

With that many hours on it, it needs a crank anyway (connecting rod)

i recon if those piston skirts were another 5mm longer, this kind of problem wouldn be an issue.

if the connecting rod were longer there would be less of an issue also if the piston were centered to the crank center this would help honda claimed a slight off set of there cylinder to the crank much like an off set piston i have found on yamahas that a centered rist pin produces a lot less piston rock with theses short pistons .The reduced piston skirt area is some of the performance gains we see in theses new engines .Im curios to see what type of piston life and performane gains we see with the new technology basket skirt designs that will be coming out soon.

A slightly taller deck-height would help a lot with durability I would imagine. You could run a longer rod which reduces piston side-thrust (reducing friction), and also reduces piston acceleration (reducing friction further), and then you could run slightly longer skirts since the piston is now further away from the crank, and with the reduction in friction from reduced acceleration and side thrust, you could afford a bit more from bigger skirts.

I know Honda (and everyone else) sank the cylinders as far down into the crankcase as possible to keep the center of gravity low, but would raising the deck height just by 5-10mm really make that much of a difference in weight and center of gravity? Perhaps manufacturers should start looking into using the frame-spars as part of the fuel tank to keep the weight down low, and use a shallower tank and use a slightly taller engine?

@Ron Hamp, basket skirt piston? Like this?


The short rod increases piston speed, and torque. The cylinder being kicked off to one side of the crank gives the rod better leverage on the crank on the power stroke increasing torque even further. Both these things are how they get so much grunt out of an engine that's bore/stroke ratio is so wildly over bored. Before these engines came out an over bore of 10-15% on a mass produced engine is about all you'd see. Sometimes you'd get to 20%. Even a 400 small block Chevy is only 10% over bored.

With these slipper style pistons, the tiny skirt has an issue in the up stroke side. The cylinder being cocked over to take advantage of the power stroke is at a serious angle with the short rod and the cylinder angle. So its trying to cock the piston over sideways when it fires and when it compresses. With that silly-assed tiny was never meant to last. Honda suggested 15 hours..which is pretty short, but honestly, if your racing the thing in the desert (WFO through sand etc) or a dune goon, 50 is probably the limit for safety.

longer crank means more weight changing direction... how can that help?

i figured a little bit more alloy on the piston would be sufficient. lighter too.

I didn't say longer crank, I said longer rod.

longer crank means more weight changing direction... how can that help?

i figured a little bit more alloy on the piston would be sufficient. lighter too.

The piston material really isnt the issue. Its the angles in operation and the shortness of the skirt itself.

The pistons are forged. Pretty damn good forgings too. But when a friggin half dollar has as much skirt as your piston, it doesnt take much wear to flip the thing over sideways in the bore.

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