Baja Mini-Bike Engine Failure - Pics

[DNVDMAX]

I repowered my nephews' minibike with a 79cc HF Predator engine.

The OEM engine had less than 100 hours and OCI was about every 25hrs with Rotella T 15w-40.

My father was building a twin-turbo 632ci BBC for a customer while I borrowed some workspace to swap the engine. I took the liberty of giving the HF engine a full teardown for inspection and re assembly. We gave the head a light port and polish job. And used some left over Cerakote coatings on the head, valves, and piston. The piston top, combustion chamber, and valve tops received V136 thermal barrier coating. The valve stems and piston skirt received C110 micro slick dry film ceramic coating. I also posted this in the Small Engine OC thread, but forgot to mention I polished the connecting rod.

Too bad I did not take pics of this build as my cell was dead. It was pretty cool.

I did take pics of the OEM engine. All other areas of the OEM engine were in great shape, like-new condition, including the piston, bore, bearings, and valvetrain.

My guess is it failed due to inferior materials used in the Chinese clone. Their machining looks great, but I think they need help in the metallurgy arena. After tearing down this engine I regret not shot-peening the Preadator's crankshaft.

Anyone else have ideas for this type of failure?

Intake Lobe:


Exhaust Lobe:


Lifters, Intake Left, Exhaust Right:


Crankshaft, PTO side:


Crankshaft, Flywheel Side:


Engine Label:

 

[morepwr]

I believe you diagnosed it right. In the shop we call stuff like that made of "chinesium" meaning you have no idea what they tossed into the mix making the materials!

 

[Fleetmon]

In the 5th picture, is that a discoloration on the broken crank end I see? If so, it looks like a classic case of stress-corrosion cracking.....in other words, chinisium.

How long did it take to fail?

 

[Pablo]

It does look like a metallurgical issue.

I have no experience with Fe alloys in China, but working knowledge of Cu and Al alloys in China 2000-2005. In a nutshell the problem stems from ignorance, spoken/written language issues and to some limited extent, pushing the limit of what they can get away with. Some people/factories just don't know, to communicate the elemental constituents the alphabet must be used, Chinese characters for an alloy are simply not specific enough and yes "close enough" is NOT good enough.

 

[Fleetmon]

Hardening of metal is a true science and some of the lessor foundries only go through the motions as far as hardening goes.....improper heat treatment does not allow for the elimination of hydrogen which leads to microscopic cracks which then allows corrosion which then leads to failure.

Unless the part failed within an hour I don't feel the failure is hydrogen embrittlement but I wouldn't be surprised if it wasn't caused by hydrogen-assisted stress corrosion cracking. The tell tales are discolored areas (darkened)in the broken field and sharp edges on the break "crater" which appears in the 11o'clock to 5 o'clock position in picture 4.

 

[DNVDMAX]

Fleetmon, the engine was in service for an estimated 85 hours. Thanks for the insight on the hydrogen. My father has magnaflux machine, would that have detected H2 cracks on the Chinisium parts?

Pablo, I agree. My only negative anecdotal experience with Chinese Cu is with customers bringing blown out "ebay" turbo bearings. They do not survive heat soaking very well, especially on the diesel Holset clones.

 

[Fleetmon]

I'm not sure magnaflux testing would reveal the cracks....although a lab does our testing, the cracks shown in microscope pictures are very small.

We are currently looking at some 12.9 wheel studs that broke and the cracks are not even close to be seen with the naked eye.....we can see the corrosion outline and shading but not actual cracks.

FWIW, some of these wheel studs have been in service for several years....however, new studs from stock show what appears to be corrosion (rust) at the underside of the hub-face of the stud and microscopic study shows very fine cracks throughout the corrosion affected areas.

 

[Cujet]

Highly stressed Chinese parts have been failing like crazy lately. I refuse to purchase much of that stuff. Especially if it's failure is likely to cost me time. Such as a motorcycle, lawn mower or outdoor equipment engine.

A $69 two stroke generator that can be thrown out if/when it fails is another story. As it's useful for times where you don't want an expensive unit to be stolen/abused/left behind etc.

And the term I use is "Ma dein chin a" (said as a single word)

Example: What brand is that? me: Oh, it's a "Madeinchina".

 

[SOHCman]

Sum Ting Wen Wong