MSM - Spark Plug Ground ate my Piston - ISO Advice on Comp / R&R OEM (on the cheap) - Miata Turbo Forum -Boost cars, acquire cats.

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Old 04-17-2014, 02:41 PM   #1
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Question MSM - Spark Plug Ground ate my Piston - ISO Advice on Comp / R&R OEM (on the cheap)

Hey guys, first post IIRC.

FWIW - I've already routed my exhaust cut-out directly into the interior compartment to fry my nuts off. Now that we've got that out of the way...

Spark plug ground broke off in my BP. Tons of pinging resulted in terrified drive back from Chipotle.

Sounded so bad that I thought the metal was still bouncing around in the cylinder; sleeves look OK, both head and piston are kind of FUBAR. No pictures ATM as I'm at work, but can get some later.

Comp test showed 160 160 150 160

#3 took longer to build comp than the rest. Leakdown seemed solid, nothing was hissing too bad.

Both the combustion chamber & the piston have smaller-than-eraser sized chunks taken out of them, not to mention the aluminum dust floating in the chamber. Walls look good enough if I can get clearance I'm not even inclined to hone.

That said... I'm definitely in the market for a piston. I'm half inclined to just up and sell the MSM, buy an NB with the proceeds and turbo it as I'd wanted the car to come from the factory. Don't know if I'd rather do this, or sooner just turbo the CRX and fund a LeMons NA.

Question is - what is my best bet for low cost piston replacement?

It seems like I'd almost be better off grabbing a set of 96+ 9:1 comps instead of replacing the 9.5:1 I've got, but that'd mean a whole set. I've heard some very mixed advice on using used pistons...

tl;dr - I need another 9:1 or 9.5:1 comp piston to go back to stock on my Miata; I'm doing this job with handtools in a driveway and would prefer avoiding removal of the crank if I can lazy my way into pulling the rod (not possible? never had bottom end apart on a BP). I've read from the MSM forums that 99-00 pistons are the same cast comp ratio... is this an option for a single piston?

Finally, any good way to check if my rod is bent from the detonation caused by heat spots on the piston?

TIA, I did do some searching, promise
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Old 04-17-2014, 10:52 PM   #2
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Is the head off already?
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Old 04-18-2014, 09:09 AM   #3
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is the head off? take pictures. Probably just able to sand down the sharp spots and put it back together. Then never think of it again.
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