Opinion Wanted On Piston Damage

Dave-R · Post by **Dave-R** » Sun May 22, 05 3:53 pm

I was just polishing the tops of my pistons (as you do) when I discovered #8 had these marks on the top.

I have cleaned off #7 and #6 and they are fine. As far as I can see all the others look OK. I would have to clean the carbon off all of them to be completely sure but I think the rest are OK.

Can I have some opinions as to the cause?

Pinking? Foreign object? Contact with the head?

What do you think?

Holly · Post by **Holly** » Sun May 22, 05 3:59 pm

If it was contact wouldn't all be in the same place?

I'm guessing foreign objects (from the fact they look like dents) - how does the corresponding valve look?

Pete · Post by **Pete** » Sun May 22, 05 4:39 pm

Can't see the pictures Dave....OOps, now I can, er sorry....

Pete · Post by **Pete** » Sun May 22, 05 4:58 pm

Definately looks like impact damage to me. did you install the pistones originally?

It does not look like detonation or over-heating damage to me.

I would not worry.

Now get yer kit off.....

Dave-R · Post by **Dave-R** » Sun May 22, 05 5:45 pm

There are no witness marks on the head or the valves that I can see and the marks on the piston were under the carbon because I didn't notice them until I started cleaning them off.

Plus thinking about it since. Some of those marks are in the valve reliefs. They would not contact the head. The valves are not marked and they are concave anyway.

But if it was pinking you would expect it on more than one cylinder wouldn't you?

I will inspect the heads again. I have never cc'ed these chambers as I didn't port the heads. Being a complete fanny about these things I was thinking of cc'ing and polishing the chambers anyway.

I have polished pistons #s 5, 6, 7, and 8 and the other three do not have any marks on them.

I didn't fit the pistons. Geof Hauser did. He also fitted the heads (he had to to cut the pushrods to length) but as soon as I got the engine back I removed them to check a few things. I found nothing out of order.

When I checked piston to head clearence (it has to be 40 thou in the quench area with these pistons) I only checked one piston on each side of the engine. I didn't check cylinder #8 for sure.

Pete · Post by **Pete** » Sun May 22, 05 5:49 pm

Neat pictures though, very "Dark side of the Moon".

MrNorm · Post by **MrNorm** » Sun May 22, 05 6:07 pm

Hmmm, tough one!!

I'm no expert for sure, but the thoughts I had....
1) You could theoretically get witness marks on a piston but not on a head or valve since they are much harder than the piston
2) The marks are pretty small diameter- so whatever made them (if impact damage) would also be pretty small. I'm guessing to make a mark they would need to be physically squashed between head/valve and piston (i.e. not just bouncing around). So.....I'd only expect to see marks where the quench pad is and not the piston crown. i.e. I'm assuming that your piston crown has a lot more than 40 thou clearance?? Probably not explaing this well (and maybe I'm talking rubbish!) but I'm saying that anything small enough to only scuff the surface at 40 thou is too small to make an impression at much bigger gap???
3) I've never seen detonation damage, but i'd guess since there are shock waves etc it might leave lots of small marks rather than a few big ones. If the det occurred at the quench pad, which does have some logic (area of highest compression), then it wouldn't be surprising to see most of the damage in that region and less elsewhere.
4) I'd say it's entirely conceivable that you COULD have det in one cylinder only - cylinder to cylinder variation in compression ratio, fuel distribution, etc could cause that.

sorry I don't have an answer, hopefully someone who recognises the marks from experience will chime in. I'd say at first glance it does look like (visually) like impact damage but I'm not sure that holds together...??

Dave-R · Post by **Dave-R** » Sun May 22, 05 6:12 pm

Because these are 'wedge' heads the area opposite the quench area (where the spark pug goes) on the piston is about a half inch below the top of that side of the chamber (even if the quench area Bananarama!) So you would not get damage at all on that side of the piston.

It must be damage due to pinking. We did have trouble with pinking when we first put the car on the road. But with the headers blowing so bad all the time (I eventually had to straighten them out) it was not really audible.

It has to be that.

Dave-R · Post by **Dave-R** » Sun May 22, 05 6:20 pm

I just realised you nipped in with an answer while I was composing Gavin. I agree with everything you say there.

Anonymous · Post by **Anonymous** » Sun May 22, 05 6:42 pm

That piston hasn't hit the head as you've got raised small areas around some of the impact holes, if a piston hits a head and if it's been running ok before then it can only be either the big ends or the small ends OR the rods stretching but you've got to have some very close tolerances for the rods to cause the problem.

That cylinder has swallowed something and IF there's no marks on the head/valves then i'd say it wasn't steel.... has that cylinder ever really killed a plug in the ceramic area not the steel strap??

Look for something missing that isn't steel, I.E baked gasket, carbon (maybe) ceramic, piston etc. Pete hit the nail on the head " surface of the moon" and we all know what causes that

A piston moves that fast that impact like this can be just bits that are in there it does not need to hit the head to leave marks like this.

Detonation could be the cause BUT there has to be an area of that cylinder that is missing some alumiuim and from the pictures it doesn't look like there is. AND it would be on more than one for sure..

And the good news is yes something has been in there... but if you had to clean the carbon off before you could see it, IT'S long gone Dave, give it a bit of a polish and everything will be fine

Anonymous · Post by **Anonymous** » Sun May 22, 05 6:46 pm

OH i forgot

just check the valve seats on that cylinder to make sure nothing has been trapped and buggered up the seat area, if so will need a lap and possibly a face

Blue · Post by **Blue** » Sun May 22, 05 6:57 pm

I'm with Wil, I think that damage is from some sort of debris that is long gone out the exhaust port.

Alex · Post by **Alex** » Sun May 22, 05 7:43 pm

That is not detonation damage, the cylinder ate something at some point.

Anonymous · Post by **Anonymous** » Sun May 22, 05 9:02 pm

I have seen that before,although not on a mopar lump. it turned out to be one of the small screws that hold the butterflies in the carb had come out and worked its way out of the ex. port but left very similar markings. the screws are also not always made of steel aswell.

Dave-R · Post by **Dave-R** » Sun May 22, 05 9:12 pm

Mmmm.

I will check the carbs etc and see if anything is missing. There has been no bits come off a spark plug.

What I don't understand is how could anything small enough to make small dints in the quench area (which only has 40thou clearence remember) can make the same size holes on the other side of the head where there is probably more than a half inch clearence?

Having said that i do think that some of the marks on the quench dome look as if they have been made by something already squashed.