miracle cylinder scratch - PeachParts Mercedes-Benz Forum

nikkondy · #1 04-16-2012, 11:36 PM

My friend bought a 1987 300dt with low compression. He removed the cylinder head and sent it off to get resurfaced. He then purchased a victor reinz head gasket rebuilt kit with oil valve seals. He got the cylinder head back and reassembled it with all new gaskets. He tested the compression and all cylinder cylinders tested out to be 400. He then ran the engine for a bit. However he noticed that the victor reinz oil valve seals did not seal well and started to leak oil. He removed them and replaced them with original mercedes ones. As he was reassembling the head for the second time he noticed a hairline stratch on the fifth cylinder. Attached pictures show the stratch. The question is what could have caused this? Also, is this a major issue that he should be worried about and resurface the cylinder or just forget about it and reassemble.

Jeremy5848 · #2 04-16-2012, 11:42 PM

I don't know what might have caused that scratch but it looks deep enough to worry about. We need to hear from someone with more experience.

Jeremy

Aquaticedge · #3 04-17-2012, 12:04 AM

I would call that a moderate/severe Score.... Also not sure what would cause that, but it's a worry. nice to see the crosshatching on the rest of the cyl though

ROLLGUY · #4 04-17-2012, 12:15 AM

A broken ring will do that. I find it hard to imagine that the compression on that cyl was not less than the others. It is a lot of work to deal with a scratch that big (removal of engine), but if it does in fact have the same compression as the others, I would put it back together and run it. By the way, the head does not need to be removed to replace the valve seals. A spring compressor is available that will compress the spring from the outside (not the "C" clamp style). Just put the piston at the top, and remove the spring to replace the seals.

Brian Carlton · #5 04-17-2012, 12:40 AM

Quote:

Originally Posted by ROLLGUY

A broken ring will do that.

If so, how does the broken ring manage to get above the height of the ring land..........all the way to the very top of the cylinder?

It almost appears if it was done while the piston was being removed, but the score mark is too deep to be created by a single pass of a broken ring.

ROLLGUY · #6 04-17-2012, 12:53 AM

Quote:

Originally Posted by Brian Carlton

If so, how does the broken ring manage to get above the height of the ring land..........all the way to the very top of the cylinder?

It almost appears if it was done while the piston was being removed, but the score mark is too deep to be created by a single pass of a broken ring.

You have a point there. Unless a piece of the ring got stuck near the top of the piston, the score would not be all the way to the top.
It almost appears like it could have been there from the factory (if the engine has not been rebuilt).

Brian Carlton · #7 04-17-2012, 01:07 AM

Quote:

Originally Posted by ROLLGUY

It almost appears like it could have been there from the factory (if the engine has not been rebuilt).

............nothing surprises me anymore..........

LNGfish · #8 04-17-2012, 05:56 AM

Slap it back together!

ConnClark · #9 04-17-2012, 02:40 PM

Foreign object ingestion can do it too if it gets pinched between the edge of the piston crown and the wall.

vstech · #10 04-17-2012, 04:13 PM

... wow. the daylight pic looks like the scratch has been there for quite some time. the burn marks from the combustion are uniform in the scratch, no shiny parts to it.
I'd have to agree something got stuck on the crown of the piston, and scratched the wall during a run.

junqueyardjim · #11 04-17-2012, 09:03 PM

Well they had the head off recently and didn't notice the scratch. Could be the piston had it covered, (mostly) and they didn't see it. Could be that a little bit of foreign metal got in there like a wee small corner broke off a gasket scraper. He stated they drove it for a while. Long enough to make a real good scratch. I'd be surprised if the next compression check doesn't show it. And then to go through all the work of R&R that mean head to replace the valve seals. Sounds like he didn't read a service manual on it.

Brian Carlton · #12 04-17-2012, 09:27 PM

Quote:

Originally Posted by junqueyardjim

I'd be surprised if the next compression check doesn't show it.

I'm not convinced.

The depth appears to be somewhat on the order of .005" or so, IMHO. The width is a bit more...........maybe .010.

This would be quite a bit less than a ring gap and the ring gap is too small to noticeably affect compression.

There probably is insufficient time for the air to pass such a gap to affect the compression number.

Mad_Matt · #13 04-17-2012, 09:33 PM

Well if something was ingested the turbo would be toast as well. Might have been there since assembly.

Don't ask me how I know.

Beagle · #14 04-18-2012, 05:56 AM

That deep score mark was caused by the boring bar tool. The idiot machinist did not wind back the tool before lifting out the boring bar. The depth of the score mark is an indication of just how blunt the tool was and how far it was pushing the bar away. A properly sharpened tool would have scarcely left a mark at all even if he hadn't wound back! A score that deep is bad news as it allows hot gasses to escape that can eventually burn a vertical groove in the piston. SUE THEM!

t walgamuth · #15 04-18-2012, 06:07 AM

Quote:

Originally Posted by Beagle

That deep score mark was caused by the boring bar tool. The idiot machinist did not wind back the tool before lifting out the boring bar. The depth of the score mark is an indication of just how blunt the tool was and how far it was pushing the bar away. A properly sharpened tool would have scarcely left a mark at all even if he hadn't wound back! A score that deep is bad news as it allows hot gasses to escape that can eventually burn a vertical groove in the piston. SUE THEM!

I am sorry to say this is the only comment that has traction in my mind. But wouldn't there be more than one score from more than one cutting tool to a boring bar?