Happy New Year!

I just cleaned up the pistons for my rebuild and found some pitting on one of them after all the carbon was removed.

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This piston was at #1 cylinder. Is it unusable? I'm certain they are NPR 96mm pistons from a late-80's rebuild.

I already bought new rings!

Any opinions would be appreciated.

Its gone.

That piston appears to be victim of detonation. May have been running too high of compression ratio.

Time to move on. Carefully check compression ratio on the rebuild.

Thanks for that confirmation, Superhawk996. I'm now the proud owner of four 497 g paperweights.

The heads were flycut pretty deep when I took the engine apart. I didn't bother to calculate the CR at that point, but detonation was my first guess, too.

Now I have to decide between JE and Keith Black...

This slope is slipperier than I thought!

Care to elaborate on the diagnosis? I've seen folks post pics of pistons nicked by dropped seats that others said were good to run. Curious what makes this damage different. I'm not challenging the opinion. Just hoping to learn a little something.

@bbrock

The irregular round pits that look like craters from mini localized explosions are what I'm keying off of. Not consistent with foreign body damage.

Also if you look at the edge of the piston you can see it is beginning to erode away consistent with detonation and localized overheating. You'll also see the small craters forming around the tangent to the piston dome in a place where foreign objects would be hard to occur.

I'm a chassis engineer by trade but I've been around more than a few dyno teardowns of race engines in my career and what I've seen labeled there as detonation looks to be similar to my eye. I'd certainly defer to someone like Jake though or a real powertrain engineer.

Personally, I'd never run a piston damaged like that. The sharp cratered and eroded edges that wrap around the piston dome will always present the opportunity for localized hot spotting that just encourages detonation to start. No way to fill them in or smooth them to my satisfaction.

Could you . . . . sure, if I had no $$ and no other alternative and IF you backed the CR down it would certainly run for a while. The problem is would I go to all the time and effort to rebuild, only to put a damaged piston back in? For me, no way.

If I was going to reuse, I would 1st check the skirt diameters to see if there is any evidence of collaspe. Collapse skirst would be a no-go. If the skirts checked out, I'd then do dye penetrant on the piston ring lands just to make sure there is no cracking there. Then I'd have to try to eliminate any of the shap edges around the crater rims, and then try to do something to smooth that damage at the dome edge. All of this is way too much work in my opinion to end up with . . . a piston that is still damaged. I've never understood the desire to reuse damaged pistons as you allude to, though I've certainly seen people do it. Personally, I don't get it and not worth the risk to me unless it's the difference between driving and walking. I've been there too but would still try to find at least a better used piston.

I'm currently stuggling with this right now on my 2.4L pistons from the six. Recall the valves met the pistons. The ethical question in my mind -- should I offer them up for sale cheap over on the Bird with full disclosure (and witness marks that pistons hit valves)? I personally would not reuse them but maybe someone in a pickle might want to? I haven't made up my mind - not sure I want to offer something to someone that I wouldn't personally reuse unless I was in a bind.

That logic was at the core of my assessment here. Would I resuse it? No I wouldn't, and that is where my answer came from.

Perfect! I was hoping to learn a little and wound up learning a lot! Thanks for the detailed reply.

Yes. sell them cheap. People use engine parts for all kinds of projects or mock ups. It might not be worth the effort to pack and ship but that's up to you.

But I agree with the idea that you never want to use below par parts for an engine build unless it's just practice.

If I were you (and you're glad I'm not) I'd clean those pistons up and run them for another 100k.

Your mileage may vary.

Sand it smooth, check the ring lands for free play and run it. You are not using it to launch a rocket to the moon. Your engine will run perfect. I have assembled lots of engines for drag racing and daily drivers. That is so minor. It will no affect on the life or performance of the engine.

I think I also see evidence of the piston touching the head on the circumference of the piston top. As long as the ring lands aren't collapsed, I would dress up the impact damage on the piston tops, install new rings, and use the pistons again. Check the heads for contact, and measure the deck height. Also check piston pins and rod bearings. Any excess clearance could allow the piston to travel a little farther at TDC and make contact.

Also, you say "rebuild" but you're re-using low compression pistons? I assume you mean cylinder hone, new rings and budget re-seal?

How deep are you going with this engine?

The main thing that concerned me was damage right on the edge. If it weren't for that, I would have sanded down the divots so they are flush with the surface and move on without another thought.

I have nothing to lose by spending a little time with some sandpaper. After all that time I took cleaning these pistons up, I would be doing myself a disservice by not trying.

Thanks for your input, gents.

I've built many engines with used jugs and slugs, if everything specs out I wouldn't have a problem using this piston.

Looks like FOD to me. I've seen far worse. Check that the rings move freely, dress the raised areas with a file and reuse it. Years from now the engine will fail for some other reason.

I was wondering about that slight bevel on the edge, if that's what you mean by the evidence of touching the head. These pistons have about 40k miles on them and I wasn't sure how they looked when new or if that bevel is a sign of wear.

And to answer your last question, I'm willing to replace everything that's unusable or questionable. I had initially planned to follow the budget reseal route, but every step of the way I'm finding garbage in this engine. The heads are extensively flycut and would need a lot of work to reinstall (some cracks in chambers and a broken fin), the lifter faces are pitted, and now one of the pistons is damaged. Oh, and the pushrods were stock...paired with a reground cam and those heads.

I've already ordered new heads from Len Hoffman and he is waiting for me to measure the deck height to get the CR correct. I have a 9590 camshaft kit ready to install. Stock crank and rods are at a machine shop being checked. Based on my amateur measuring, the crank had already been turned down .0010, so I'm on the fence about reusing it. The case has been align bored and registers decked by EMW, so that is ready to go. I'm close to the reassembly phase, but have a couple of decisions left. My next parts order(s) will finalize the build.

Considering the dish in these pistons and that I'm starting with new heads, maybe it's best to get some new flat top units and make my life easier. And it wouldn't be right to put new pistons in old cylinders, and then why connect new pistons with old rods to an old crankshaft...? And that's how this whole process can get away from you.

I'll go to work with a file sometime over the next couple of days and then post more pictures. Thanks for your experienced opinions

I agree with the statement above about saving them for a craftsman. I'm too far to give you a few bucks for them, I carve faces into them, but if you have a local car club I bet you could get rid of them. And you'd make someone's day.

Can also ceramic coat them after cleaning them up, do it yourself kits or send them out not that expensive.

This is a good idea as cracks in the ring lands can be hard (or easy depending on how bad) to see.

If you are spending the money for Len Hoffman’s heads, then spend the money for new pistons and cylinders too. This is not the time to turn ghetto on the rebuild. It would be like spending $2000 on a stereo system for a $1000 car.

Easy now! This is called due diligence

It's all part of me getting rid of all the parts that don't stand up to tough scrutiny. So far, none of the parts I've removed are good enough, with the possible exception of cylinders and case bolts.

And some cars are simply transportation systems for a good stereo!

Original cylinders are very good metal and are often reused if in spec or bored out larger. Also very common to do on 911 engines.