OM617 piston ring installation question - PeachParts Mercedes-Benz Forum

Stretch · #1 11-17-2010, 06:05 AM

G'day Folks,

I'm getting close to putting my non-turbo OM617 back together again. So far it has had a re-bore and I've bought new oversized pistons.

The pistons have been supplied with the piston rings fitted - which I guess is the best way to transport them.

The new lower piston ring looks different from the original as it has a little bit of yellow plastic in there. The new pistons and rings are made by Kolbenschmidt. See photo.

I'm using a combination of my Haynes manual and the FSM for assembly instructions. I'm weighted towards the FSM advice - however - there is a bit of a discrepancy between the two that I'd like to check with you.

FSM says to just fit new rings and pistons

Haynes says check gap of each piston ring by inserting each ring individually into the cylinder bore - square it off with the piston - measure gap in ring with feeler gauges.

Instinctively I would like to check everything but I don't want to mess stuff up!

Furthermore the end edges of these piston rings seem a little sharp. Is it a good idea to ever so slightly round them off?

Alastair · #2 11-17-2010, 06:16 AM

Its worth confirming the gap is within limits. --Maybe just on one piston

Normally new pistons have a number like 'Sp 0.125' or similar, that the machine-shop uses to gauge the clearance when honing the block on its final stages to ensure correct clearances...

The Piston Maker 'Kolbenshmitt' will have gapped the rings with this clearance in mind, so theoretically, you should be able to fit them without messing round.

The oil-ring has a plastic sleeve on its expander-ring, maybe to ensure its ends dont foul with the ends of the ring. The gaps should be placed at 180 degree positions between ring and expander...

layback40 · #3 11-17-2010, 06:29 AM

Be ever so careful removing the rings from pistons. They can jam or break easily. Did your machinist over size to a known size or use the pistons as a guide?
Interesting that the scraper rings are 1 piece, most replacements I have seen are 3 piece.

Stretch · #4 11-17-2010, 06:38 AM

Thanks for the quick replies!

The sp number on my pistons is SP 0.030

I will check to make sure that the machinist did hone to the correct size - he did after all supply the pistons.

What are your views on taking the edge off of a piston ring end?

layback40 · #5 11-17-2010, 06:54 AM

I would leave the edge be. If the machinist supplied the pistons, he will have bored to suit.

Alastair · #6 11-17-2010, 07:01 AM

No, Dont do any filing of the sides of the rings--Best left well alone. Only Ever file the end-gap if the clearance when checked is tight, and make sure its done Carefully and perfectly 'square' to the ring ends.....

Great care must be taken, Rings are very brittle and can Easily break.

If you file the sides, it will affect the sealing and they may never bed in properly....

The maker will have given them a tiny radius of around 1 degree on the face to ensure they bed prooerly.

Yes, SP.030 sound about right, for a 91mm bore This is the clearance value. Its the Piston Size, Plus this 0.30mm which they bore the block to....

Worth checking the Block has a chamfer at its top of 45 degrees, and around 1mm to allow ease of ring install. A sharp edge to the top of the bore can catch and break the thin edge off an oil ring....

layback40 · #7 11-17-2010, 07:25 AM

X2 with Alastair.
BTW the decimal point is in the wrong place ! ...Its the Piston Size, Plus this 0.30mm..
Should be .030 clearance I think.

make sure you use plenty of oil when you are putting it back together. It doesnt hurt to have the rings dripping wet with oil & the bores like wise.
Same goes for the bearings. At least 1/2 ltr of oil used is what I would expect. You cant have too much, it will just drip off anyway.
Tip a ltr into the the filter housing when you fill the motor, I know I am getting ahead of you now. When you come to starting it make sure the injector lines are cracked until the oil pressure has come up so there is no chance of it starting before everything is full of oil.
No doubt we will cover this again later.
Good luck getting the piston/rings in. A ring compressor is the only safe way. Try & install the pistons the right way so they dont need rotating to have the bearings line up. dont forget to align the gaps at 180deg. Could some one else comment on that as it has the gap in the scrapper lining up with the top ring gap. Maybe the 3 ring gaps should be done at 120deg locations. I have only worked with new 3 piece scraper rings that dont have gaps like that.

Alastair · #8 11-17-2010, 07:46 AM

Quote:

Originally Posted by layback40

X2 with Alastair.
BTW the decimal point is in the wrong place ! ...Its the Piston Size, Plus this 0.30mm..
Should be .030 clearance I think.

make sure you use plenty of oil when you are putting it back together. It doesnt hurt to have the rings dripping wet with oil & the bores like wise.
Same goes for the bearings. At least 1/2 ltr of oil used is what I would expect. You cant have too much, it will just drip off anyway.
Tip a ltr into the the filter housing when you fill the motor, I know I am getting ahead of you now. When you come to starting it make sure the injector lines are cracked until the oil pressure has come up so there is no chance of it starting before everything is full of oil.
No doubt we will cover this again later.
Good luck getting the piston/rings in. A ring compressor is the only safe way. Try & install the pistons the right way so they dont need rotating to have the bearings line up. dont forget to align the gaps at 180deg. Could some one else comment on that as it has the gap in the scrapper lining up with the top ring gap. Maybe the 3 ring gaps should be done at 120deg locations. I have only worked with new 3 piece scraper rings that dont have gaps like that.

Aye, you're right--A slip of the keyboard I think!

LOTS of oil all over pistons/rings, you cant ever have too much!

A Proper ring-compressor is really the only way with these rings, the Oil-Scraper has a very high outward pressure, and a very thin edge that can easily be damaged if you use 'jubilee-clips' as 'Pains' (Haynes) recommend....

The Later, three-piece oil-rings are more robust as regards to assembly as the 'rails' are steel. the Two-Piece (Proper) rings are made of some exotic Cast-Iron alloy and are very brittle....

The WSM shows the correct placement of ring-gaps on the piston. Its quite specific as to where they should be placed....

When you get to that point, its worh running the oil-pump by removing the skew-gear and running it with a drill At Low Speed (500-800 ish RPM) to get the oil-pressure up.

My rebuilt engine would NOT prime just by the starter, It cranked too slow to prime it up. Took out skew-gear and turned briskly by hand with an improvised tool made from dowel...

Been perfect ever since...

Stretch · #9 11-17-2010, 07:53 AM

Thanks for the help again.

For the starting lube I've got some Millers oils competition assembly lube from the Dutch branch of this company

http://www.frost.co.uk/item_Detail.asp?productID=9423&frostProductName=Competition%20Assembly%20Lube

layback40 · #10 11-17-2010, 08:00 AM

I have found filling the oil filter housing fixes the problem of priming the lube system. It takes a while to fill the cooler & lines though. You can leave the top fitting on the cooler loose while you add oil to the filter housing to help get the air out.
That assembly lube is fine. Just slop plenty of it on everything that moves!!

vstech · #11 11-17-2010, 08:15 AM

ya can't fill the oil filter housing... it's open at the bottom.

layback40 · #12 11-17-2010, 08:21 AM

John,
Its open back to the pump. It will drain slowly to the sump but will hold some oil & the oil that goes to the pump will prime it.

Alastair · #13 11-17-2010, 08:40 AM

Quote:

Originally Posted by layback40

John,
Its open back to the pump. It will drain slowly to the sump but will hold some oil & the oil that goes to the pump will prime it.

Yup, And this is the way I tend to 'fill' after an oil-change--Bigger 'hole' to fill through and less chance of spilling everywhere...

Would it work with my re-built engine...?

Would it buggery!-

-I have NO idea Why,--It Shoulda!-- apart from the engine had stood for a year or so after assembly, so everything in pump must have drained fully back to the oil-pan...

It didnt take much spinning of the oil-pump to get it to flow, and I allowed it to half-fill the oil-filter before fitting lid, then continued till I got oil outta all the little squirters over the cam, Its possible to get around 1 bar just by turning the oil-pump by hand, when its cold....

Ever since though, oil-pressure appears within 1 sec of starting, Maybe I created an air-lock by filling oil-filter on stone-cold and with the re-built/stood for a year engine...

Strange!

rydzio · #14 05-04-2013, 07:17 AM

Quote:

Originally Posted by Alastair

The oil-ring has a plastic sleeve on its expander-ring, maybe to ensure its ends dont foul with the ends of the ring.

I'm sorry to dig out such an old thread but I can't find an answer to this anywhere. Am I supposed to install these oil rings with these plastic sleeves? Won't they melt?

Brian Carlton · #15 05-04-2013, 09:34 AM

Quote:

Originally Posted by vstech

ya can't fill the oil filter housing... it's open at the bottom.

Agreed.

Any oil poured into the oil filter housing drains almost instantly down into the pan.

There is no way to get the cap onto the housing fast enough to start it before it has all drained away.

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