Piston-impact assessment; how-bad-is-too-bad…?

[Paul Smith]

Hi – I just suffered an e30 “interference situation”; not the usual belt breakage – but the tensioner-idler shaft shearing…same effect of valve/piston collision.

My question: Beyond the usual bent valves (all intake ) and broken rockers (all six intakes ), I have one badly nicked piston top, of which a photo is attached.

I’d say that the depth is as much as a 1/16″ in a few locations. If I grind/polish out the high spots, is there much of a chance of “burning through” the injured piston? Thanks for your opinions.

Paul

Attachments:

• DCFC0058.JPG

Topic

[college man]

In my honest opinion that piston is toast.

[Erin]

You might be able to still run the engine if you replaced all the other bad parts and left that piston in there.
The problem is that if things hit each other THAT hard, how much damage was done to other stuff? That head does not look good at all.

Anything CAN be fixed but you might need to assess if it’s worth it.

[Lorrin Barth]

If the piston is cracked or holed it is toast. Not having a nice smooth piston crown but instead having a small metal projection can create a hot spot and and cause preignition. I suggest trying to smooth the surface.

[Walt jr]

You absolutely cannot run that piston, that one is now a paperweight. Under microscopic magnification you would likely see the beginnings of cracks that will finish off your engine if that piston stays in use for a few more heat cycles.

[Lorrin Barth]

With so many interference engines on the road, there’s quite a few running with marked up pistons. I’ve seen everything from barely marked to all valves broken and the cylinder full off aluminum jello.

The marking in this photograph doesn’t look like valve impact to me, it looks like a foreign object passed through the cylinder.

[Bill]

I agree with barneyb. Looks like a foreign object has been floating around in there. I don’t think I would trust that piston anyway.

[Paul Smith]

All, three years after burnishing down the modest crown/craters on the piston tops (and replacing a dozen valves) the 3L Bimmer is still running strong. It was an interesting top-end rebuild exercise, and I was fortunate to find a local source of used valves to keep the spend down. I did a quick lap and liquid-leakdown check before re-assembly, and ALL-GOOD. The message is; no matter what the P/O says, before you buy a used interference car…replace belt and tensionsers!!

[Author]

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