90.909-90.918 mm tolerance... are they serious??
 

The Merecedes and Haynes manuals list some very narrow clearances/tolerances for the cylinders and pistons. Two machine shops (including Diesel Auto Service, who are in the business of this type of work) say that no one works to those tolerances.:confused: :confused:

What say the experts?

That tight and there is still blow-by!? :confused:

"engineered like no other car in the world" was an advertising slogan once..

:)

BORING ( pun intended )
 

Yes, they mean it... shops just don't want to take the time to hold those kinds of dimensions.....
But if one is doing it at home you can do it because grinding ( honeing) takes out metal very slowly... it is the constant cleaning and measureing which takes the real time...but as you start out,,, if you time the honing each application and put the results onto paper you can anticipate when you are getting close and change to finer grit... just be sure to allow the amount they say for the crosshatching.

Cylinder to bore clearance is about 0.001". Usually, cylinders must be honed to match the piston intended for them, individually. Oil clearance on the main and rod bearings should be no more that 0.003", 0.001" is better. Volvo specifies 0.0005" for their diesel.

This is why you can put 500,000 miles on a 617or 603 (or any other Benz) engine without working on the bottom end. A good diesel shop will do this without asking, as all the large high power diesel engines are set up the same way today.

Typical shop that rebuilds mostly junk American iron from the 70's will want to leave large and variable clearances, and the engine will self-destruct in 50,000 miles.

Another thing to watch for is valve stem to guide clearance -- MB speicifies 0.00 to 0.01mm, NO more. This is TIGHT by American standards. Basically, if the oiled stem will fit into the guide by hand, and the spring can pull the valve closed, it's fine. Set up by American standards (0.002 to 0.005"), it's worn out by MB specs and will burn large quantities of oil.

Peter

Thanks for the replies. One shop said we'd have to take it to NASA to get that precise. :rolleyes: I don't quite understand how this close a clearance allows for heat expansion, since they should expand towards each other when hot, shouldn't they?

So we finally got the micrometer yesterday and did some measurements. It doesn't look good. :( We haven't measured the pistons yet, but the cylinders are out of round, and they are much larger at the top than at the bottom. What would cause that? Could it actually wear like that, or was it machined (by someone who didn't know what they were doing?)? There is a crosshatch pattern on cylinder one, but not on any of the others. It is the worst... 0.006in difference from top to bottom and 0.005 out of round.:confused: And the whole thing is LARGER than what the manual states, which makes me think the pistons are going to be way too small. :(

But being optomistic for a moment.. if we buy new cylinder sleeves, will they be sufficiently oversized that we can make them fit within 0.001in to the pistons using the technique Greg described? We tried to find out from a seller what the inner dimensions were, but he jsut kept saying it was 'standard'.:confused:

We're beginning to think there may have been a lot more miles on this car than the odometer indicated. Although the seats had very little wear, and many of the other components were in great shape too.

That is how they wear.

If you purchase new sleeves they will have to be bored/honed to provide the proper clearance with the pistons.

The dimensions listed above are for the size variations of pistons new....
The piston to bore clearance is more than that...
Nat, I thought you had a shop manual for the turbo engine (?)...
Piston clearance when new is supposed to be 0.043-0.063 with a wear limit of 0.12......
This is from page 03.8-316/1 F2
I wish I knew how to read the page numbers in these manuals....
there are three groups of piston sizes...
All the sleeves ( new bores) are supposed to be bored or honed out to fit the actual piston going into it..

Interesting, our manual does not have a 03.8 section. 03.4, then it goes to 05.4. This is the sheet we went by, and calculated the clearances ourselves. Sorry the headers are chopped off, I couldn't post the whole page- it was too big. It works out to 0.001in, just as psfred said. So that is for new? I went looking in Haynes after reading your post, and, ahem, I did find the piston to bore clearance, just as you described.:rolleyes:

Yes, I know the cylinders will have to be honed to match the pistons, but how do I know (before I buy) that they aren't already too big? Maybe I'm not asking this clearly....:confused:

I don't think they come in alot of different sizes:).

Normal procedure is to obtain the pistons and measure them (they will be marked by "group number" if OEM -- this is the size class) and THEN machine the cylinders to size. Clearances are tight on these engines -- the piston on the turbo's is cooled by an oil spray, so they don't grow as fast as the cylinders do.

Any good shop will be able to get them done correctly -- look for one that does a lot of dieesl work on modern engines, they have the same tolerances.

Piston size range is pretty small for each size group, but that isn't clearance, it's the manufacturing tolerance.

Peter

Yes to what Peter said....
Take out the pistons,rods, rod caps... and be sure to keep them marked or layed out so you know which bore they came out of.... and the direction they were in the engine...
After they are clean enough to eat off of... you can inspect and measure them....particularly the ring lands.... you will be doing a lot of reading in the manual if you are to do this correctly....