Strange piston damage

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I am rebuilding a 3.2 Carrera and also do the 3.4 upgrade using the original Mahle 3.4l piston and cylinder set. I had the top engine assembled but ran into problems with one side where the cam would not turn easily.



So I decided to take this side apart again and reassemble it completely. When I looked at the pistons I noticed two dents on the side of the piston crown. It appears to various degrees on all 3 pistons in the same position. Please see below and notice the dents at the outer perimeter of the piston crown. One at about 150 degrees and the next in symmetric position at about 210 degrees:





I have no idea what is causing this. Anyone who can offer a suggestion what I need to look for?

[JohnJL]

did you measure your "squish" clearance between the piston and the head?

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I measured the deck height using the solder method. I am following the instructions in the Wayne Dempsey How to rebuild book.

[syncroid]

I had a similar problem when I did my 3.4 liter conversion. My pistons didn't get dented but they were very slightly touching the heads. I had the heads chamferred (sp) and that took care of the problem.
Here are a couple pics of what mine looked like.
It looked like someone took a pencil and very lightly drew a line all the way around the edge of the piston.

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I was thinking about this too. But what is really strange here is the two symmetrical dents and there is no area that would match the dents in the heads. If the pistons touches the head I would assume there is a broader range of marks, like in your case a line or a more elongated impression. Those two dents look really weird and are hard to explain I think. The interesting part here is the symmetry. This is puzzling. Your case makes much more sense to me. Those two dents don't match up with anything on the head.

[syncroid]

Maybe Steve from RennSport or Henry Schmidt will chime in with a better answer. Good luck.

[renders]

Since you did the same thing did you run into any issues with the cams are hard to turn after tightening the heads and cam housing? I assume you should be able to turn them relatively easily by hand.
I assume I can still use the pistons. I will just sand the rough edges a bit. However now I am worried about my other side. I hate to take it all apart again. Darned.

[syncroid]

I had no issues with my camshafts not turning. I would highly recommend pulling the galley plugs in your cam towers and making sure they are clean and the oil squirt holes are clear.
Dumb question, but did you torque all the heads on first and "then" put the cam towers on? If so, try loosening all the head nuts and then retorqueing them again. You should be able to turn the cams by hand.
Your pistons should be okay so long as the ring lands are not crushed.

[renders]

I followed the exact direction that is given in the book. The procedure if I recall correctly is to tighten the head nuts hand tight and then torque down the cam housing and finally torque the heads. I guess it is a matter of the tolerance in your parts. I will check the plugs. Thanks for the tip.

[syncroid]

Originally Posted by renders

I followed the exact direction that is given in the book. The procedure if I recall correctly is to tighten the head nuts hand tight and then torque down the cam housing and finally torque the heads. I guess it is a matter of the tolerance in your parts. I will check the plugs. Thanks for the tip.

No prob. I always assemble the heads to the cam towers as a unit and then torque that entire assembly to the cylinders.

[cgarr]

Were the sealing surfaces of your heads machined at all? The cam not turning free is of concern.

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Yes. The heads did undergo the machine shop and a complete overhaul.

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How easy should the cams turn? I assume that they have a quite tight fit and will turn with a little of force meaning you can turn them by hand with some minor force. Is that an accurate assessment? I reassembled the other top end twice and on the second time around I can turn the cam with my hand with a little force applied. However when I move it slightly out it gets stuck. So it does not slide in and out easily which bothers me but maybe it is just a matter of tolerance and tight fit. I assume it cannot sit too loosely and also I have applied any lubrication. Thank you for thoughts and feedback on this subject.

[E Sully]

The photo shows what an exhaust valve hitting the piston looks like. Nothing like your dents. Perhaps from how you installed piston in cylinder? My cam was binding initially on the right side after installation. The paper gasket between the chain housing and cam towers had folded slightly and this was enough to bind the cam. New gasket installed carefully and it turned well, just some light resistance from the assembly lube.

[carrera turbo]

did these get nicked or dented on the head studs on assembly and your chasing a ghost?

look at your second or third pic at there relation to the head studs?

just a thought.

thanks ed

[renders]

Thank you. I thought that too and I think this is what happened. I am surprised the surface is so soft because all I can think of is that I pulled them against the studs when removing the clips. The dents are exactly on the side where I removed/installed the circliips. So I am pretty sure that is it. Amazing how easy it is to damage something. So next time I will put a a rag around the piston when I work on the circlips. Apparently I am still learning. I never thought the stuff is so soft.

[carrera turbo]

little pieces of hose in the studs works wonders.

good luck

cheers ed