M127 engine rebuild

[Kiwibenz]

M 127 rebuild. Piston issues.
During the course of my 230SL rebuild, I have finally got around to looking at the motor.
This 1st series 127 motor was taken out of service many years ago when the car was taken off the road. The car had recently been purchased and allegedly had had an engine rebuild. Some invoices referred. I find that it has been bored to 83 millimeters and that No. 5 cylinder has some water corrosion pitting, (storage?) and No.1 cylinder has 2 points of damage due to a broken oil control ring, One end of the ring was broken, and has excavated the bore at upper and lower stroke positions. Probably broken at assembly.  Both bores will need to be re-sleeved. 
But my purpose for initiating this discussion is in the hope that some of you more knowledgeable folk can assist with a piston ring inquiry.  Examination suggests that the motor has in fact had a recent (in Kms) rebuild as the crank has had a regrind and there is no ovality and nice bright journals with no circumferential lines. The pistons are of the four ring type, and appear to be newish, very clean and with no visual or measurable skirt wear and perfect gudgeon pin bearings. I was looking to replace the rings.  However, when checking the piston ring groove clearances, the top first compression ring (that scrawny chrome plated item!) has a groove clearance of 0.009in, whereas the ‘spec’ is 0.003in.  I am assuming that 0.003in is the ‘as new’ spec. and that some deviation as engines age, is permissible.  But how much?  I cannot believe that all those beautiful Pagodas out there are circulating the world with an exact or less than 0.003in top groove clearance!  I do note that there is noticeable wear on the top and bottom of the surface of the ring itself, but I don’t know the correct width of the ring groove in the piston to check for groove wear.  Anybody know this?  Obviously, with apparently near new (and expensive) pistons, I do not want to have to discard them for such a deviation.  I would be grateful for your valued comments and opinions. I cannot find any prior discussion on the forum.

[stickandrudderman]

A good machine shop or Mahle piston supplier should have a data book that contains such details.

[Kiwibenz]

Hi Stick.  Well, yes, you would think so.  I have just taken the block and pistons to the machine shop I use for engine work and which will sleeve the two bores. The owner - now over 70 - with many years experience, has done a few 127's over the years. He does most of the work for the local racing fraternity.  He had no idea what the top groove width should be.  He looked at the pistons, measured them in the bores, felt the fit of the rings in the grooves and said that he would not hesitate to re-fit the pistons again with correct new rings. So that's what I'll do. I also now see that the top ring is tapered on upper and lower faces. Nowhere in the 1996 Technical Data, Passenger Cars, (p.166 refers) is a tapered ring cited.  p.164 cites a 'rectangular ring, inside chamfered' for the top ring, 230SL.  It may be that a P.O. installed incorrect rings. I'll order a new set from SLS and measure carefully.   

[ja17]

The top ring on these 230SL engines is much thinner than the rings on the 250 and 280 series. If the top ring groove wears or becomes tapered the ring is much more likely to twist and break as a consequence. Top ring groove clearances are critical on these M127 engines. Here is some information from my 1963  Mercedes Technical Data Manual pages 125, 126, 127.

[Kiwibenz]

I do have the tech manual but I note that on P.165, only details for the 3 ring piston are given. The top ring I've just removed is a steel ring and chrome plated.  Nothing delicate about it and very tough.  I haven't been able to break it by hand. Only the 190C (on page 164) is listed as having a chrome plated top ring.  I conclude that a PO sourced the rings on my pistons from his local supply which is why they are a bad fit.  I take the point about the slender top ring, but the the 'end clearance' (not gap) cited on p.169 of 0.060 - 0.092 ( 2.3 thou to 3.6 thou in.) reads more like a manufacturing tolerance than a wear allowance, (1.3 thou??)  Haynes, using the same MB data suggests throwing your pistons away if there is more than 1.3 thou wear!   Do we think this is reasonable ?  Given the mileages that these engines do, I would be impressed if they maintain this tolerance. Does anyone know what they're like at scheduled overhauls, or do you always have to throw your pistons away??   

[ja17]

Some shops recondition used pistons. Not a procedure recommended by the factory, I am sure! However, technology is much better these days. For instance, with worn ring grooves, which have become too wide or tapered, they might be able to re-machine the grooves wider to fix the taper. They can then use a wider piston ring and solve the excessive clearance problem also. In some cases, they might re-machine the groove wider to square up the ring groove, then use a spacer along with a new original size ring, in the groove. Don't fool yourself into believing that you can just put new rings into badly worn ring grooves. Normally, by the time pistons and rings are worn out and failing, the cylinders have become too worn to pass inspection also.

[Kiwibenz]

I take the point - but the whole reason for my query was that the bores are near perfect, with no lip at all (just the damage to 1 and 5.) all the other rings are in spec., the pistons and gudgeons are excellent and 'new' clean - all good except these top rings.  It's clear that the engine has had a recent (Kms) overhaul and would have done less than 1000kms by the look of it. I've built a few engines in my time, it's just that this is a peculiar situation and I wondered if there is any history associated with this issue with the M127.  I noticed that MB moved quite quickly to a 3 ring piston.  I conclude from what you folk are saying that there are no known issues and that we are looking at an 'oddball'. I'll know more when new rings arrive. Thanks for your kind input.

[Kevkeller]

I didn’t know you could sleeve these engines. Is it just the 230 or can the 250 and 280 be sleeved also?

[Benz Dr.]

230 and 250 use the same size bore and there is plenty of room between cylinders.  280 has little to no room once you reach 87.5 mm maximum bore size. 

[Kiwibenz]

As a useful note, the owner of the shop I use told me that there would be no worries at all sleeving all six cylinders and re-boring/honing my M127,  BUT if sleeving only a damaged cylinder or two, as I am, unless great care is taken in arranging the interference fit of each new sleeve, excess pressure when pressing in the new sleeve, can run the risk of pushing adjacent untouched cylinders slightly out-of-round. If excessive, the result can be that all cylinders must then be sleeved. He tells me he thinks it will probably be ok with the M127 but that it can happen.........   He's put sleeves in many engines, fours, sixes and vee-eights, so I'm hopeful.  I will measure the results with him, when the job's done.