Wrist pin failure in built '99 motor
 

Hey everyone,

A few months ago I acquired a lightly used, built "supermiata" track motor. The guy I bought it from is a friend of a friend, and I saw it running well in his car before he pulled it out and sold it to me. According to the original owner (and many witnesses), the motor ran perfectly for two race weekends before the owner decided he needed a PTE points motor instead. It was pulled out, sold to me, and professionally installed and tuned in my car. Everything was great for a couple months of use (about five time attack events) until a couple of weeks ago at Laguna when I heard a loud knocking noise as I was approaching the corkscrew. I pulled off right away and towed the car home. My track buddy Joe Chang at Auto Logic just diagnosed my knocking issue as a wrist pin failure, it sounds like the wrist pin broke.

I'll be honest here, I love driving but engines/mechanics aren't my strong suit. I'd love advise on:

1. What could cause a wrist pin to fail like this
2. Advise on approaching the engine builder, and what my expectations should be. This is my first built motor, so this is my first rodeo here.

I'm really sad to have a broken toy after such light usage, and at a complete loss for how this happened. I haven't had any overheating or oiling issues, and I changed my oil right before the event the engine blew. Any advice would be helpful. Thanks in advance!

My guess is over reved or just a low quality piston and it failed from abuse. That's not a common failure on miata motors.

I agree, I'm having a hard time finding an example of another miata motor failing like this.

How could it have failed from abuse? We're not talking about an engine with hundreds of hours on it here, and I know my tuner set the rev limiter at 7500. I was told this build could safely rev pretty high.

Abuse could be oil starved or over reved. If it wasn't that, then I'd put my money on bad assembly, or bad part that just failed. You can rev stock pistons to 7,500 reliably, so if it never went over 7,500 then revs should not have caused a problem, I'd look to oil starvation on all the other parts. If no signs of oil starvation anywhere else, it was bad part or bad assembly would be my guess. Also I would never buy a used race engine, that just sounds like asking for trouble.

What kind of pistons were they? Supertechs?

I think the only Miata wrist pin failure I've seen posted is someone where the keeper came out and the wrist pin impacted the cylinder wall.

Do you have pics?

--Ian

Hard to say what the cause of failure is. Could be FOD in the oil, could have been overrevved, could have ingested something early in its life. Wrist pins or light knocking have happened to me after foreign object ingestion (throttle body screw) and overrev (8600rpm on stock bottom end, failed ~2hrs later).

For #2, don't expect any compensation. Assembly errors do not linger inside an engine for very long. If they knock or blow up on the first keyturn, then it's the builder's fault, but after two weekends of race time, the odds skew heavily towards abuse or bad luck. It's almost certainly not the builder's fault. The fact that you are not the original owner of the engine makes any compensation even less justified. This is the same answer I give to people who ask me about the "warranty" on my built engines.

The good news is that a wrist pin failure is annoying, but not catastrophic. Open up the engine, inspect the condition of all the parts, and evaluate what needs to be done to repair it. Best case, you need to replace a wrist pin, maybe a single piston, rehone/re-ring, and replace bearings for good measure. Worst case, metal got into the oil and you need to bore the engine.

It will be nothing but pure speculation until you tear it down and get a full idea of what has happened. Get some pics posted and the wealth of knowledge on here will be able to help.

Also, specs on the motor?

Tear it down and document your post with pictures.. speculation it is right now

I know the feeling, my built engine died after 5 hours. Oil pump destruction.. this engine is still on my workbench !