How much damage is acceptable for a piston?

[Pazuzu]

1980 SC 9.3CR Mahle cylinders

2 weeks ago, I had a big intake backfire upon startup (big enough to pop the air filter cover off) at work. Made it as far as the end of the garage before I had a hellacious banging noise. Stopped the engine, checked things a bit, verified banging, got it towed home. Pulled everything apart, and found the head of a small screw embedded in the #5 piston.








So, question is, how much damage can a Mahle take? I understand hot spots and detonation from sharp edges, I'm talking about physical damage. The cylinder wall is untouched, the valves seem fine, clearly the shank of the screw got chewed up and spit out. I expect that it made contact a dozen times, maybe 2 dozen? Probably embedded itself immediately and I head the contact with the head over and over.


Bigger question is, where did it come from? I expect that the backfire cracked something in the airbox and the screw was there. There are very few sheet metal type screws in the car, but I know there are some in the airbox. I need to tear through it now to see.

Fortunately, I had 90% of an EFI conversion collected (including an airbox to cut up)...

[Tigerrat]

Mike, keeping things interesting I see. Hope you can get some answers here. And maybe find the shank outside of the engine. Ken

[AlfonsoR]

You might want to measure the thickness of the screw head. That would be helpful, maybe.

Components that possibly could have been affected are the piston, pin, rod, rod bearing and crank.

The piston I would have checked for cracks. The pin, check for roundness by rolling on a flat surface such as a granite measuring table, visual inspection for irregular indications, may be better off just re-bushing and buying new pin. Rod bearing, mic at the 11, 12, and 1 o'clock positions, split being at 3 and 9, compare to one or two other rods, visually inspect both the soft side and back side of the bearing. The Rod have dimensionally inspected for center to center length, compare to one or two others. The crank, dimensionally inspect the rod journal in several spots, especially near where you would expect the journal to be at TDC, looking for flat spots.

[timmy2]

Well, I guess that explains the "I'm dying a miserable engine death" sound you described in an email.....
Take a look at the 5 screws on the inside of your airbox where the filter sits. I'm betting one of them popped out from the backfire and was sucked into the intake somehow. Maybe through the pop off, if you have one.
I just hope it didn't do too much damage and you can get it back together soon.

Dennis

[Pazuzu]

Originally Posted by Tigerrat

Mike, keeping things interesting I see. Hope you can get some answers here. And maybe find the shank outside of the engine. Ken

It's long gone, but there was no traces of it in the head, nothing rattling around in the headers, and the muffler is too big to shake effectively
Hey, it gave me the chance to finish some stuff that I ignored a year ago when I tore it apart, and it was going to get an engine drop for the EFI soon anyways.

Originally Posted by timmy2

Well, I guess that explains the "I'm dying a miserable engine death" sound you described in an email.....
Take a look at the 5 screws on the inside of your airbox where the filter sits. I'm betting one of them popped out from the backfire and was sucked into the intake somehow. Maybe through the pop off, if you have one.
I just hope it didn't do too much damage and you can get it back together soon.
Dennis

I actually immediately went over to the airbox and checked those screws in the main filter plenum, they're all there. I assume there's some further in the housing, I'll have to dig into there to see.

As for piston/head interference, I could turn the engine by hand when it came out of the car and I got no noise (I also had no noise when cranking with the starter and the fuel pump turned off, so 400ish RPM).

Only when I had removed all of the intake and exhaust, all 12 rockers arms, all of the spark plugs, everything that puts a "load" on it when you're trying to turn the crank by hand, then I could feel the slightest binding/pressure when rotating the engine by hand. The binding was once per crank rotation (which put the problem in the core of the engine, not the valvetrain, which would have been every other crank rotation). The binding was exactly at 120 degrees BTDC, which led me to either piston #2 or #4, which are the two in extreme positions at 120 degrees BTDC. I thought the noise was on the right side when I was running the engine, which led me to $4. also, #4 is at the top of it's range there, it made sense that it would be the problem child. I was making several assumptions when I removed the heads, but I was right. Yay me! I only had to remove one cam and will only have to re-time one cam, as well as only having to re-seal one side of the engine!

[kodioneill]

Mike you should remove the valves and check the seats and stems of the valves for any distortions.

[Wayne 962]

You can probably go two ways - either a full rebuild where you replace *everything*, or slap it together where you replace *nothing*. If it wasn't binding very much when you turned it over by hand, then you're probably pretty lucky. There might be some minor damage / wear on the bearings if they got compressed, etc on TDC. But, it looks like the aluminum of the head took most of the leeway there.

If you're going to keep the car for the next 30 years, then rebuild. If not, then clean it up and drive it. I would probably vacuum check the heads to make sure the valves are not bent.

-Wayne

[Pazuzu]

Originally Posted by Wayne at Pelican Parts

You can probably go two ways - either a full rebuild where you replace *everything*, or slap it together where you replace *nothing*. If it wasn't binding very much when you turned it over by hand, then you're probably pretty lucky. There might be some minor damage / wear on the bearings if they got compressed, etc on TDC. But, it looks like the aluminum of the head took most of the leeway there.

If you're going to keep the car for the next 30 years, then rebuild. If not, then clean it up and drive it. I would probably vacuum check the heads to make sure the valves are not bent.

-Wayne

12 months/7.000 miles ago, I did a full workover on the engine, cracked the case, all new bearings, new headstuds, new gaskets everywhere, new t-chains and all of the sprockets. I did not do any machine work, but i did manually clean up all of the various mating surfaces. Since I dropped the coin a year ago, I'm going to button things up and move on from here. I have a set of spare heads on the shelf if I find that a valve is bad from this, but they look good up close and sound good when I whack them with a rubber faced mallet.

One question is whether I take a file to the piston and try to knock down the sharp edges, or will I end up making things worth that way.

[Pazuzu]

Originally Posted by kodioneill

Mike you should remove the valves and check the seats and stems of the valves for any distortions.

I likely will do this, since it's pretty painless once the head is sitting on the workbench. I won't have someone mic it all up, but i will get someone who has a spring compressor to visually check things out.

[porsche moose]

hi it happen to my 3.0 if you take of the air box you will see 3 screws that a plate for cold start , my cam lose and went in the in take, when i started it up

[theiceman]

that is definitely an air box screw ..

[JohnJL]

Originally Posted by Pazuzu

Less than that.

Is that a piece embedded in the piston?

[kokenm]

wow I never thought I would see that again. I had the same thing happen to me and it has taken me a long time to recover. yours does not look near as bad as mine did. Mine scored the cylinder wall and put many impressions on the piston and the head. I was able to sand them out of the piston but I replaced the piston head and sleeve anyway. I still have them up above the work bench. I know this does not answer the question you were asking but I called DC Auto and Bruce took car of getting me a matched set that was used.
after going through many other life changing issues I just got everything working two days ago.

[afterburn 549]

I can see damage imprints all over the top of the piston?
I would be concerned about fractures out toward the edges...I think i would at least source 1 piston........seems doable on here

[NICE 69 S]

I would make sure the top ring is not binding in the ring groove. Wouldn't spend any time looking for the rest of the screw, you can see where the threads hit the piston and head before exiting the exhaust valve.
Bob B

[Pazuzu]

I've ignored this thread, and the recent talk about pop-off valves and such made me come back to it...

Almost have the engine back in the car. I did search around, and did find the missing screw (well, didn't find the screw, found the empty hole...), one of the three that holds the cold start tree in place. Damned Porsche engineers, there would be no need for that screw if they woulda solved the backfire issue!

I cleaned up the piston surface, replaced the rings, and crammed it all back together. I fixed the slight exhaust leak (missing nut one one of the exhaust studs, probably never even put in on...), fixed the OTHER exhaust leak (one headstud nut had backed off of the brand new headstuds, how can you get one of the barrel nuts to completely back off the stud??), and now I'm 95% done with the Bitz style EFI update with all of the bells and whistles (full AN style hose, custom engine harness, EDIS wiring, etc).

[nicfranc]

So do you think these screws need to be checked periodically. ?

It is interesting that there were two incidents on two different cars recently.

I guess another reason to look into swapping out the CIS and its dreaded air box.

[Pazuzu]

Theoretically, yes, one should check them.
The one that came out of my airbox was the one that you couldn't tighten even if you wanted to. The other two you could get a screwdriver on, but not this one.

I think we just keep taking life one day at a time and if the screw falls out, it falls out, and we deal with it

[nicfranc]

I guess I have stayed up reading far too many disaster stories.
Weird how you are drawn to them.

The guy who's car burned to the ground really freaked me out !

[timmy2]

I want to see pictures of the engine you're building...