Painful Question about Pistons - PeachParts Mercedes-Benz Forum

retmil46 · #1 04-08-2012, 04:56 PM

Uhhhh, guys?

I've got a question that I have a feeling I'm not going to like the answer to....

If you've got the injector and prechamber removed from the head, and are looking down the hole at the top of the piston....

What's the top of the piston supposed to look like?

Pictorial responses or links would suffice as well.

I may be doing an engine swap sooner rather than later.

Vehicle is an 87 300D Turbo, OM 603.960, build date 7/86.

Aquaticedge · #2 04-08-2012, 05:21 PM

Piston & Connecting Rod Mercedes 300d OM603.690 6 cylinder turbo diesel M603 | eBay pictures

retmil46 · #3 04-08-2012, 07:34 PM

Well, best I can tell looking from the pics......

I'm screwed.

Looking down thru the holes with the prechambers removed, you can see a nice flat surface on pistons 2 thru 5.

On # 1, there are two big divots, almost the size of a quarter and somewhere between 1/16 to 1/8 inch deep, missing out of the top of the piston. When you first look straight down the hole, it's like looking at the center section of an hourglass, and if you look to either side of the "hourglass", you can see the two big irregular-shaped divots missing.

And no, the missing chunks are not inside the cylinder.

That would explain why the intake manifold, intake valves, injectors, and prechambers were so bloody clean on an engine with 215K on it - sometime previously, someone lunched #1 piston, pulled the head to get the chunks out, and then slapped it back together.

layback40 · #4 04-08-2012, 08:35 PM

Quote:

Originally Posted by retmil46

Well, best I can tell looking from the pics......

I'm screwed.

Looking down thru the holes with the prechambers removed, you can see a nice flat surface on pistons 2 thru 5.

On # 1, there are two big divots, almost the size of a quarter and somewhere between 1/16 to 1/8 inch deep, missing out of the top of the piston. When you first look straight down the hole, it's like looking at the center section of an hourglass, and if you look to either side of the "hourglass", you can see the two big irregular-shaped divots missing.

And no, the missing chunks are not inside the cylinder.

That would explain why the intake manifold, intake valves, injectors, and prechambers were so bloody clean on an engine with 215K on it - sometime previously, someone lunched #1 piston, pulled the head to get the chunks out, and then slapped it back together.

I would not panic just yet.
What was the compression like before you did your investigation?
How much blow by?
Did it start on 5 & then get going on all 6 after a while?
was there bad piston knock?
Can you see the bore through the hole with the piston right down? Any damage?
Normally bits of piston are spat out the exhaust & mess with the turbo.
It may have dropped a valve at some stage & been repaired. If only the crown of the piston was marked, it could be ok.

Diesel911 · #5 04-08-2012, 09:44 PM

I cannot find the Thread but someone within the last to weeks posted some comments on a Borescope.
One of our member bought one on Ebay that used the USB port on your Computer for about $18.

I decided that was a good deal and bought one; don't know if it was from the same seller or not.
Mini USB Waterproof Endoscope Borescope Snake Inspection Camera 2M (It has a 2 meter length to it). It has not been delivered yet.

I questioned the Member about it and He said it works OK on windows XP (did not say what version).

vstech · #6 04-08-2012, 10:07 PM

I gotta come look at your motor. gimme a call this week.

whunter · #7 04-08-2012, 11:03 PM

Quote:

Originally Posted by Diesel911

I cannot find the Thread but someone within the last to weeks posted some comments on a Borescope.
One of our member bought one on Ebay that used the USB port on your Computer for about $18.

I decided that was a good deal and bought one; don't know if it was from the same seller or not.
Mini USB Waterproof Endoscope Borescope Snake Inspection Camera 2M (It has a 2 meter length to it). It has not been delivered yet.

I questioned the Member about it and He said it works OK on windows XP (did not say what version).

Here it is.

How many of you have used borescopes for diesel diagnosis?

.

layback40 · #8 04-08-2012, 11:14 PM

Quote:

Originally Posted by Diesel911

I cannot find the Thread but someone within the last to weeks posted some comments on a Borescope.
One of our member bought one on Ebay that used the USB port on your Computer for about $18.

I decided that was a good deal and bought one; don't know if it was from the same seller or not.
Mini USB Waterproof Endoscope Borescope Snake Inspection Camera 2M (It has a 2 meter length to it). It has not been delivered yet.

I questioned the Member about it and He said it works OK on windows XP (did not say what version).

X2
The motor definitely needs a endoscopy or is that colonoscopy!!!

If the bore is good, chances are the piston damage is only cosmetic.

retmil46 · #9 04-10-2012, 03:43 AM

Weeeellll, today I feel like the world's biggest dumba$$.

I shoulda checked the other cylinders before I inserted my foot in my mouth.

Two of the other cylinders that were near the top of thier stroke, I sprayed some WD-40 down on top of the piston, and then using a thin screwdriver and rag reaching thru the prechamber hole, wiped enough of the soot out of the way to really get a good look at the pistons.

Same exact markings.

As in "star pattern" or "moose antler" markings.

As in I shoulda checked the "modifications to pistons" section of the FSM before I went off the deep end.

What really threw me off was that I thought I was seeing the exact opposite of what the drawings in the FSM were showing - that the area INSIDE the star/moose antler patterns were supposed to be machined down/recessed, and the surrounding area still flat/raised up.

What I finally realized that the piston drawings were showing, and confirmed by looking at my pistons (and poking at them with a screwdriver to confirm I wasn't seeing an optical illusion), is that area inside the star/moose antler pattern on the drawings is still flat/raised, and the surrounding area OUTSIDE the patterns is what has been machined down/recessed.

Making the latter assumption, then the tops of the 3 pistons that I cleaned off match the drawings in the FSM.

Now, it looks as though instead of having a trashed engine, I may have one that someone did an extensive rebuild on sometime in the past - the intake manifold, valves and passages, and prechambers were suprisingly clean - too bloody clean for an engine with 215K miles on it - as in the entire head had been pulled off - and instead of matching the star pattern on the top of the pistons, as you'd expect on a vehicle with a build date of 7/86, they seem to more closely match the "moose antler" pattern they switched over to in mid-88. Either that, or someone swapped in a newer engine at some point.

Comforting thought, but I still feel like a prize idiot over that initial mistake.

layback40 · #10 04-10-2012, 08:54 AM

Quote:

Originally Posted by retmil46

Weeeellll, today I feel like the world's biggest dumba$$.

I shoulda checked the other cylinders before I inserted my foot in my mouth.

Two of the other cylinders that were near the top of thier stroke, I sprayed some WD-40 down on top of the piston, and then using a thin screwdriver and rag reaching thru the prechamber hole, wiped enough of the soot out of the way to really get a good look at the pistons.

Same exact markings.

As in "star pattern" or "moose antler" markings.

As in I shoulda checked the "modifications to pistons" section of the FSM before I went off the deep end.

What really threw me off was that I thought I was seeing the exact opposite of what the drawings in the FSM were showing - that the area INSIDE the star/moose antler patterns were supposed to be machined down/recessed, and the surrounding area still flat/raised up.

What I finally realized that the piston drawings were showing, and confirmed by looking at my pistons (and poking at them with a screwdriver to confirm I wasn't seeing an optical illusion), is that area inside the star/moose antler pattern on the drawings is still flat/raised, and the surrounding area OUTSIDE the patterns is what has been machined down/recessed.

Making the latter assumption, then the tops of the 3 pistons that I cleaned off match the drawings in the FSM.

Now, it looks as though instead of having a trashed engine, I may have one that someone did an extensive rebuild on sometime in the past - the intake manifold, valves and passages, and prechambers were suprisingly clean - too bloody clean for an engine with 215K miles on it - as in the entire head had been pulled off - and instead of matching the star pattern on the top of the pistons, as you'd expect on a vehicle with a build date of 7/86, they seem to more closely match the "moose antler" pattern they switched over to in mid-88. Either that, or someone swapped in a newer engine at some point.

Comforting thought, but I still feel like a prize idiot over that initial mistake.

Not dumba$$, perfectionist !!

Simple rule ~ if it aint broke then dont try & fix it !!!

Best you get back to enjoying your car!!!

Its for driving, not pulling to pieces !!

Glad to see you had the brains not to jump in & pull the head off on a wild goose chase.

Stretch · #11 04-10-2012, 10:01 AM

Quote:

Originally Posted by layback40

...
Its for driving, not pulling to pieces !!

...

Now I totally disagree with that!

rs899 · #12 04-10-2012, 10:46 AM

Quote:

I cannot find the Thread but someone within the last to weeks posted some comments on a Borescope.
One of our member bought one on Ebay that used the USB port on your Computer for about $18.

That was me. This would be a perfect use for that borescope- looks like you don't need it though.

Mine took about 2 weeks to ship from China.

retmil46 · #13 04-10-2012, 01:03 PM

Quote:

Originally Posted by layback40

Not dumba$$, perfectionist !!

Simple rule ~ if it aint broke then dont try & fix it !!!

Best you get back to enjoying your car!!!

Its for driving, not pulling to pieces !!

Glad to see you had the brains not to jump in & pull the head off on a wild goose chase.

Yeah, that's the problem with having been a Navy nuke power machinist mate and a submariner for 20 years - it HAD to be right, your life might depend on it - and old habits are hard to break.

OTOH, when I'm done "refitting" a vehicle, it doesn't give me any major problems for quite some time afterwards.

I'm still thinking of getting the borescope and taking a look-see down inside the cylinders - might be the only time in the remaining life of the vehicle I get a chance to do so.

I picked up one of the Mityvac coolant system pressure test kits at NAPA for $54, and did a cold pressure test on the system to spot any internal leaks. Good news is inside the cylinders and the head everything stayed dry as a bone. Bad news is the gaskets for the thermostat housing and water pump were not so dry, and at the joint between the plastic and metal cap fitting on the coolant bottle is an air leak that was clearly audible anywhere above 10 psi. Now I know why the PO had a 13 psi cap on the bottle.

Already had a new coolant bottle to swap in, need to order new gaskets for the thermostat housing and water pump. Might as well get a new bypass/relief valve for the back side of the IP while I can get to it. Sending off the injectors in the next couple of days to get them checked out while I've got everything apart.

Diesel911 · #14 04-10-2012, 01:43 PM

Quote:

Originally Posted by whunter

Here it is.

How many of you have used borescopes for diesel diagnosis?

.

Thanks, I put the thread in my notes this time.