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 6.9
 Starting rebuild #2 - need piston sample
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street

26 Posts

Posted - 07/14/2017 :  11:05:57  Show Profile  Reply with Quote
6,000 miles after rebuild #1, and the oil control rings won't seat. All the symptoms of oil ring failure are present: higher revs = higher consumption, good compression and leak down, no smoke on start up, no smoke on deceleration, etc. Did have a lot of fun during break in although it was a pain to add 2 qts oil with every fill up.

Can't find NOS +1 rings and pistons so we're going to get custom fabricated from JE, Carrillo, or Mahle. We are thinking of trying to copy mid compression Euro pistons, but we need a sample.

Anybody know where we can get a Euro piston sample? We have the specs for squish and distance from head at TDC, but we need the shape.

Alternatively, does anyone know the OEM supplier? There's a small chance they have the blue prints.

Thanks a ton.

'77 Silver 6.9

ctmaher

USA
666 Posts

Posted - 07/14/2017 :  13:29:09  Show Profile  Reply with Quote
I should have a sample of a standard Euro piston, let me look this weekend. As far as the OE supplier, should have been Mahle. They made pretty much all pistons used in Mercedes engines. They will be the only ones that will be able to supply an OE type piston with the steel band cast into the aluminum to prevent the piston from "growing" as it heats up. If you go with a company like JE Pistons, you will need to run more piston to wall clearance. Whoever you get them from aftermarket, will advise on what clearance to use.

Chris

Edited by - ctmaher on 07/14/2017 13:33:18
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Art Love

Australia
6150 Posts

Posted - 07/14/2017 :  20:57:50  Show Profile  Reply with Quote
John Tucker posted only yesterday here on this Forum saying that he got a full set of Euro 6.9 pistons from Mahle a year or two ago - "6.9 engine rebuild [video]" is the thread as I recall. Mahle were the OEM supplier as Chris said.
Art
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RANDYS

Australia
487 Posts

Posted - 07/14/2017 :  21:01:05  Show Profile  Reply with Quote
You are probably "over it" by now, but to my mind, this problem was always likely to be the oil control rings. By a process of elimination you have pretty well come to that conclusion yourself.

All the other characteristics seems to tick the box..eg holds compression..no smoke.here .no smoke there. Only wallops the oil when driven....etc etc

So I would be homing in on the oil control rings and examining them and their fit to the existing piston.
Something is not right here..........were they assembled correctly......? is the end gap correct?
What is the ring groove float like?

It doesn't make sense. If the piston ring grooves are good enuff to host the compression rings, then 99.999 percent the oil control groove is OK too.

Then we come to the bore. How true is that?

Until such time as you can pin point where the problem lies, I'd be hesitant to proceed with the labour/expense of pulling the motor, dis-assemembly, boring, replacing the pistons and all new rings etc : it is going to be a big deal
.
I'd be having one more roll of the dice with what I've got!

Of course a clean slate should solve all and we can see the attraction to that cour$$$$e of action.
Sometimes you just gotta call it "barleys..enuff is enuff !!! ".

Best of luck.
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street

26 Posts

Posted - 07/17/2017 :  11:51:52  Show Profile  Reply with Quote
Thanks ctmaher. Great input and if you should locate the Euro piston, greater still. We will need to check the PNs to determine if they are low, mid, or high compression versions.

We will be reaching out to Mahle first for custom pistons and inquire if they can cast in the steel bands. Since custom pistons are usually machined from blanks, steel bands may not be available.

RANDYS: Shop and I can't recall if we measured the entire length of the bore. Most likely didn't. In any case going plus one will solve taper issues. While we have everything apart, we can double check the oil control rings.

'77 Silver 6.9
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street

26 Posts

Posted - 07/24/2017 :  11:54:53  Show Profile  Reply with Quote
Thanks to all who provided Mahle as the OEM piston mfg. I contacted Mahle's custom shop and they got in touch with Germany. Bingo. Say they can make pistons in 3 months which is about how long it will take me and the shop to do the tear down.

Any advice on which pistons to get? High, Mid comp, or USA spec? Are the US OEM rod ends compatible with Euro pistons?

'77 Silver 6.9
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oversize

Australia
261 Posts

Posted - 07/24/2017 :  22:43:38  Show Profile  Reply with Quote
Id say all the rods are the same but you should check the epc. If fuel is good in your area I'd go with the hc pistons.

Mark


W116: Last of the steel Mercedes

1979 6.9 #5541 (Red Bull)
1979 6.9 #5398 (Blue Mallard) - Parts
1978 6.9 #4248 (Skye Blue) - Parts
1979 6.9 #3686 (Moby Dick)
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ctmaher

USA
666 Posts

Posted - 07/25/2017 :  01:36:30  Show Profile  Reply with Quote
I downloaded the latest Mahle catalog from their website. They still show the 6.9 pistons listed in std and 1st oversize. They only show one compression ratio...8.4:1. I am not sure if they will make a custom piston in higher CR but I guess it's worth a shot at this point.

Chris
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street

26 Posts

Posted - 07/25/2017 :  11:14:07  Show Profile  Reply with Quote
Do I ever love this forum! I found the European catalog on line (thank you ctm) and I can find the Euro spec low comp pistons (based on the crown cavity dimension of 3.9 mm) and rings. I'm crossing my fingers we've isolated the oil consumption to the oil control rings and that oversize pistons and rings do the trick.

While the car is apart, we'll rebuild the struts and re cover the drivers seat and then it will be like 1976 all over again.

You guys are great.

'77 Silver 6.9
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