I'm rebuilding my first engine. It's a 1981 300td (617.952)
One of my pistons was bad and I bought a used one from a member. Got it all put in the block but then realized that it is a different group number than my other pistons. I guess that means it is a little larger. Does anyone know if this is a huge problem? Do I need to take it back out and get a different one? My crankshaft seems to turn fine. All of my pistons are group number 1 except this one that is a #2.
Thanks for any help.

You have to be sure you have the proper clearances. The crank may turn now that it's cold, but then you fire it up and that piston expands seizing in its bore ruining your rebuild.

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So you think I should just pull it and look for another? Any tips on how to go about measuring the clearance?

To measure =>

Bore micrometer

How to Use a Dial Bore Gauge - HowStuffWorks

Compare your measurements with the data in the FSM

(Do you need a link?)

Realistically though it is unlikely the bore has worn to the next repair stage - still check piston diameter to bore size to see what is needed

You also need to have the correct piston rings fitted to the piston...

...if your bore is worn then you are (most likely to be) wasting your time.

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I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

I don't have a bore micrometer but I'm guessing it's not good anyway. I had new sleeves put in and bored to the specs of the original pistons.
I guess I should just pull it.
Thanks for the help.

Some odd bits to help others.

If sleeves are getting replaced, bored and old pistons being reused, have the machine shop measure and bore to the worn pistons.

Piston group numbers are generally slight variations from "standard" diameter. In the old days mass production machining was variable so parts were select fitted to keep everything closer to spec.

In the case of the engine in question, measuring is a better route but you could look at the specs for new parts and compare.

Congrats on trying a rebuild. Few do that. I started on my failed 1985 engine, but stopped after installing new sleeves (myself). I agree w/ post #6. The machine shop should BORE a bit under, then HONE the last ~3 mil to match each piston (and mark them) for specified clearance. That is standard procedure for any automotive engine, diesel or not.

The "group" thing in the FSM has been much discussed. As I recall, the #1 cyl spec'ed a slightly different "size bin", probably because it gets more cooling. As post #6, I think that applied only to original factory assembly and isn't relevant to rebuilds.

BTW, I assume the machine shop also "decked the block" when they machined off the protuding ends of the sleeves. They might take off ~2 mil to get a perfectly flat, clean surface. The head more commonly warps. I expect they must remove the prechambers to machine the head flat. In gas engines, shops typically bolt the head to a rubber surface to verify valve sealing, but I don't know how they would plug the pre-chamber holes or work around protruding pre-chambers. It would be slightly risky to wait until the head is bolted to the block to check valve sealing. You might at least flip the head up and fill each chamber w/ alcohol and see if it leaks past the valves under gravity. I haven't gotten to those issues yet in my rebuild.

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Thanks for all the help everyone. What I ended up doing was leaving all the pistons in and turning the crankshaft to get the skirts up towards the crankshaft end. Then I tried to slip some feeler gauges in-between piston and sleeve to check clearance. Mercedes manual specs piston clearance at 0.043-0.063 when new and a wear limit of 0.12mm.
I seem to be within specs on all pistons when checking on the part of the skirt that dips towards the piston pin but on the longer part of the skirt I can't get a feeler in on any piston.
That last piston with group no 2 is a bit tighter still.

Piston diameter is measured 90* to the pin and only in a certain area.

Pistons are not round and squared, they are oval ( cam ) with the high spot being 90* to the pin and bulged top to bottom ( Barrel ) with the high spot somewhere between the lowest ring and end of skirt.

While measuring the piston and bore is the best way, lets try some detective work first. What diameter were the bores taken to? What is the new piston diameter for each group you have?

Did you measure the ring gaps? I would think that would be a good time to check piston/bore clearance as you will be inserting the piston upside down from the top to square the ring up in the bore.

I did measure all the ring gaps. They were good.
I ended up taking the block back to the guy that bored it to have him check the bores again. Hopefully he can just hone that cylinder down a bit. I'll let you all know what he finds.

Did you send the pistons along so the bore can be adjusted to actual piston size and not a number in the manual?

Sure did. He has pistons and block.

Just heard back from the guy that bored my block. That cylinder is too small. I could have him re-hone it but then I'm going to need to clean it all out again and I've spent some serious time making it spotless and painting it. My other option is to find a used piston from a similar group number as the others. Group 1.
That number should be stamped on the crown of the piston along with a direction arrow and the number 10 which is the code number.
If anyone has one and can help me out I'd really appreciate it.
Thanks

Here's a link to a picture of the piston numbers
https://www.dropbox.com/s/evj59pvioyxwq64/piston.jpg?dl=0