Machinist Balking at MB piston clearance specs
 

I set out to rebuild my om617.95 engine last year, and its been to 3 machine shops now over the span of about 14 months. The first one got the head done but finally gave up on machining the block, the second one got nowhere and the place I have it at now is finally making progress, but the machinist is balking at the piston clearance of 0.043-0.063 mm.

It seems pretty clear from the 1992 FSM 03.8-316/1 that the piston to cylinder wall clearance is indeed 0.043 to 0.063 with the nominal clearance from the table being 0.053 mm (group 1 pistons fyi).

I did a quick search for ford powerstroke cylinder clearance is which is 0.11 mm (0.0045"), and since that is what the machinist drives, I can see his hesitation.

Am I reading the FSM correctly???

If memory serves I mic'd the pistons at about 90.1 mm, so it seems like the readily available 89.9 mm cylinder sleeves should be able to be bored out to accommodate them (which I hope is the case since they have already been installed:eek:.)

Any feedback on this is appreciated!!!

If it is what it is, tell him you'll take your block elsewhere if they're not going to follow the FSM. Simple ;)

South of the wall is Mexico i'm guessing right?

If you're not too far away from the border, it might be worthwhile investigating US based machine shops.

FSM states 1.7 to 2.5 mil. That sounds reasonable to me, given my limited experience rebuilding engines (some re-ring of U.S. gas engines, started on my OM617). You could compare with specs for U.S. engines, say the common Chevy small-block which has similar diameter pistons. Powerstroke pistons are likely larger diameter. I recall that forged pistons generally require more clearance because they expand more with heat. They tend to give slight piston slap when cold. Most factory pistons are cast aluminum. I suspect the OM617.952 pistons are forged aluminum, as are most turbo's and diesels. For sure don't let the machinist apply his "common sense", since there is no such thing for piston clearances.

You are reading the FSM correctly; .053mm = .0021"
.002" is an appropriate clearance for that bore w/ hypereutectic cast pistons, i.e., MB/Mahle, etc.

Sounds like it might be a good idea to take it somewhere that has worked on MB diesels before.

If you live close to San Diago CA there used to be a place there that machined and rebuilt high end european Cars.

The typical Boring Bar machine that you see here in the US mounts on top of the Block and the part that clamps it goes down through the adjacent cylinder or the machine is held on magnetically. The problem with them is that you have to shift the boring bar around by hand to center it (centered with a dial indicator I assume on the unworn part af the cylinder at the bottom).

If the to surface of the block is not level the new cut is going to be at a slight angle.

I went to that shop to by Parts from but also took my Volvo Diesel Cylinder Block to be rebored to fit new oversized pistons.

They had the equipment that you likely need. It was like a huge milling machine. The Block is swung into the machine lowered onto a rod that is the size of the main bearing Bores and the Bearing Caps are out on fromt there. Bore alinment has nothing to do with the top surface of the block.

I did not get to watch them do the centering in the bore but the Block sitting on that Rod but all of the adusments on the Machine were similar to a milling machine. Meaning they had accurate adjusments.

Once a cylindeer was cut all they had to do was change the head on the Machine and they could hone the cylinder to a more precise side as well as doing the cross hatch pattern.

The Machine looked big enough to do a Cummins Big Rig Truck Engine.

That was back in 1992 and I don't remember the name of the place nor know if it is still in business.

if you're close to Los Angeles bring it to Metric Motors, they know what they are doing as Mercedes is pretty much all they touch.

For people who do not have the MB FSM...
each bore is supposed to be bored and honed to fit the particular piston which is going into it...

I'm going on old memory brain cells here but isn't one cylinder a little different clearance than the other. I'd ask him to please build it per FSM.

Even if it costs more. Automotive machine shops are not what they used to be in my experience.

I only trust one now that is 600 miles away. There is another one much closer I would trust. They do not take in work off the street though.

Guy has probably never had to hold .0008" on a bore before. That's the tolerance in inches between .043 and .063mm. He likely just lines up within .001 and pokes a hole. Definitely hard to find a decent machine shop now. Most everybody just does hotrod engines, which are really not that precise and don't have to live very long.

This is all necessary because the block (at least on an inline engine) twists with use. To do the job correctly the top of the block therefore needs to be decked so that the deck is parallel to the crank as it was from the factory. Once it is parallel again it can be bored and the bores will be perpendicular to the crank. I suppose if you don't do all this all your bearings will not be even in their clearance across the face so it will be like an engine with high mileage even though all the wearing parts are new.

Thanks for the sanity check everyone :)

Phoenix is the closest big city (5 or 6 hours) and Denver after that (10 hours) anybody know of any good machine shops there???

I think this guy I have now is the best I'm going to do around here...

I will ask him if he can check the block for twist

FWIW, I had a transmission shipped across country and the core shipped back for $250. It was strapped to a pallet and wrapped in plastic film. It would be worth it for a correct job the 1st time. Next choice would be to source a good used engine.