Hello, I need advice of peoples that already rebuilded an engine of 190e and more specificly 16vlv if possible.

A couple of days ago I removed the head and saw that the piston gap on the top of the piston was about 0.5mm (I tought that wasnt normal). So I deceided to remove pistons to measure everything.
The cylinders have an average diameter of 95.52±0.03 mm.
The pisonts have a diamater of 94.65mm and 94.44mm (on each axe) on the top of them and a diameter of 95.45mm on the botom and middle of them.
So I touth that the pistons where worn since their diameter at their top where 1mm under the one of the spec.

So I deceided to buy some new ones. And it seems that the new diameter is:
94.44 and 94.65mm (on the both axe) on the top of the piston, and 95.44mm on the middle and botom of the piston...
So the measure are exactly the same....!!!!

So it seems that it is normal that the upper part of the piston is 1mm smaller in diameter then the rest of the spec (95.45mm).
My question is: How come???
A gap of 0.5mm-1mm isnt too much to allow some fuel to be cought in that gap and disable it to burn during the explosion?? That doesnt seem logical to me...
Can someone explaine me why?
I can even see a bit of blow by on the old piston, isnt that an evidance that the gap is too big??

The other thing is that I have some unburn fuel that is passing trew the engine, and I cant imagine where it can escape the burst of fire during explosion?? I noticed that my rings are loose on my old piston if I compare to the new one, could it be the source of the problem??

I really need advice of specialized and experienced member please...

Thanks you!

No need for an expert here. Pistons are measured at the skirt and ring lands, not the very top. You'll notice the top of the cylinder probably has a lip on it, this where the rings don't touch. If the piston top was full diameter there would be a problem in that area. The only to eliminate that lip would be to have the ring on the very top, but then how would it attach to the piston. Make sense?

The 16V engine uses FORGED pistons & require almost twice the clearance of the normal cast pistons found in most MB engines.
Clearance is measured at the skirt, 20-25MM below the piston pin center line.

Sounds like you need to re-ring the original pistons! OR over-bore the engine either 0.50MM or 1.0mm

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The reason for more clearance at the top is that the head of the piston sees much more heat, thus it expands much more than the skirt or pin area.

Forged pistons expand MUCH MORE than cast or hypereutectic.

Good luck,

Thank you guys!

LarryBible: thanks! That was the type of explaination that I was searching for. I also maid that hypothesis, but I first didnt beleived in it 'cause I didnt tought that a thermal dilatation was such big as 0.5mm... But youre explaination (the piston material) seems good, and I am sure you are right.

M.BDoc: So you think that my problem (unburn fuel) can be du to bad rings? I have to mention that my oil was going black in less than 1000km's... And it was also smelling fuel...
I re-measured all the cylinders and they are really at 95.52mm. The spec are 95.50mm(New) I beileve... Is it bad or good??

Thanks again guys, I really apreciate youre advice!!

You need to measure to 0.001mm. Use micrometer and bore gauge. Pistons are available in different tolerance groups, marked on the crown. Same for cylinder bores, look for marks on the mating surface.
Here is a table of piston/bore sizes:

Attached Thumbnails

 

Thanks!
But those value are only for the 192.982/192.985 , at least that is what it is told in my manual.
The 190.983 piston are smaller: 95.44 (for grade "0").
That is normal since the piston play of the 192.985 is 0.01 to 0.03 (I guess). For the 192.983 it is more 0.04 to 0.07. I have 0.09.

The problem is that New the play is 0.04-0.07. But theyre is no where mention of a wear limit... Someone knows what it is in % or value??