OM617 Piston/liner size - PeachParts Mercedes-Benz Forum

sag · #1 02-08-2015, 11:30 PM

So I am in the process of rebuilding my OM617 that came from a 1985 300SD for my 1997 Jeep wrangler.

I got the engine out of the jeep tonight and pulled the cylinder head and found some numbers on the pistons.

Cylinder# Size in mm? ???
# 1 90.86 1
# 2 90.85 0
# 3 90.86 1
# 4 90.85 0
# 5 90.86 1

are these the sizes of the pistons?

I was looking and was thinking the OM617's used pistons in the 90.90 range as the std. size or is that bore size?

I want to order my liners and rings but want to make sure I am getting the correct size......

Thanks every one
Shawn

Stretch · #2 02-09-2015, 04:21 AM

Quote:

Originally Posted by sag

So I am in the process of rebuilding my OM617 that came from a 1985 300SD for my 1997 Jeep wrangler.

I got the engine out of the jeep tonight and pulled the cylinder head and found some numbers on the pistons.

Cylinder# Size in mm? ???
# 1 90.86 1
# 2 90.85 0
# 3 90.86 1
# 4 90.85 0
# 5 90.86 1

are these the sizes of the pistons?

I was looking and was thinking the OM617's used pistons in the 90.90 range as the std. size or is that bore size?

I want to order my liners and rings but want to make sure I am getting the correct size......

Thanks every one
Shawn

Those are indeed the sizes of the pistons. The space after the size with a number (in your case a 0 or a 1) indicates the repair group. On a freshly built engine from Mercedes you should have a set of group 0 pistons. The next sizes up are in groups 1 and 2

I don't think it is great engine building practice to mix the repair groups within an engine. I assume "someone has been there before"

DeliveryValve · #3 02-09-2015, 10:45 AM

Quote:

Originally Posted by Stretch

Those are indeed the sizes of the pistons. The space after the size with a number (in your case a 0 or a 1) indicates the repair group. On a freshly built engine from Mercedes you should have a set of group 0 pistons. The next sizes up are in groups 1 and 2

I don't think it is great engine building practice to mix the repair groups within an engine. I assume "someone has been there before"

You sure about this? I understand it differently....

Mercedes tries to machine all the bores the same. No difference between cylinders, but what happens after the machining process there are slight variances in the bore. The factory would then choose a group of pistons out of three groups that best matches the cylinder. This is not a repair group. But straight group selection from the factory.

617.95x info..
Group 0 = piston dia. 90.845-90.855 mm = cyinder dia. 90.898-90.908 mm

Group 1 = piston dia. 90.855-90.865 mm = cyinder dia. 90.908-90.918 mm

Group 2 = piston dia. 90.865-90.875 mm = cyinder dia. 90.918-90.928 mm

p/n 6160111210 Replacement Cylinder liners are rough honed to 89.9 mm.

Stretch · #4 02-09-2015, 01:04 PM

Quote:

Originally Posted by DeliveryValve

You sure about this? I understand it differently....

Mercedes tries to machine all the bores the same. No difference between cylinders, but what happens after the machining process there are slight variances in the bore. The factory would then choose a group of pistons out of three groups that best matches the cylinder. This is not a repair group. But straight group selection from the factory.

617.95x info..
Group 0 = piston dia. 90.845-90.855 mm = cyinder dia. 90.898-90.908 mm

Group 1 = piston dia. 90.855-90.865 mm = cyinder dia. 90.908-90.918 mm

Group 2 = piston dia. 90.865-90.875 mm = cyinder dia. 90.918-90.928 mm

p/n 6160111210 Replacement Cylinder liners are rough honed to 89.9 mm.

I think that was what I was told by the machine shop when I got my block done.

From a machining point of view I can't see how such a great change in bore would occur from one cylinder to the other. From what i understand the casting of pistons is more problematic than boring holes. That's the reason for the grading of pistons.

leathermang · #5 02-09-2015, 01:32 PM

No matter what the factory did to start with..
what the FSM says about rebuilding is that one measures the piston GOING INTO THAT BORE....
and bores AND hones that one to the specs in the FSM for clearance...
You have go stop the boring ... if you need to bore, in anticipation of the amount to be taken off during the final honing so the surface is of the proper size AND ' roughness' .....
typically a 60 degree crosshatch...

t walgamuth · #6 02-09-2015, 03:24 PM

Quote:

Originally Posted by leathermang

No matter what the factory did to start with..
what the FSM says about rebuilding is that one measures the piston GOING INTO THAT BORE....
and bores AND hones that one to the specs in the FSM for clearance...
You have go stop the boring ... if you need to bore, in anticipation of the amount to be taken off during the final honing so the surface is of the proper size AND ' roughness' .....
typically a 60 degree crosshatch...

This. You don't need to worry about the sleeves, just buy them from a Mercedes parts place and they'll be what you need. New pistons or used, you hone it to fit the pistons.