250sl piston rings,

[w113dude]

Hi to all,

I'm in a process of overhauling my engine, the markings on the piston says 82.47 I'm assuming this is first over size, or is it? If so, What would the part number be & Where can I get a complete set of rings?
I haven't had much luck with the regular suppliers. (including MB classics)

Thanks for your help,
Shaun

[ja17]

Hello Shawn,

Mercedes original rings are very hard.  They will usually out last the cylinder walls, which eventually wear over size and out of round. In addition, by the time this happens the top ring groove in the piston wears too wide and may cause even a new ring to twist and break.  

So if you put nice new round piston rings in out of round oversize cylinders you are asking for trouble. Not trying to discourage you but you must be aware of the pitfalls. The permissable wear in the cylinders is only .005" diameter, that is only a .0025" on each side!

Mercedes original rings are exteremely expensive if you can find them, there are seldom sold by themselves.  Most instances oversize piston and rings are used for repairs after the block is bored oversize to remove all wear and uneveness.

Normal ring life can be 150,000 ro 200,000 miles or more if a engine is taken care of.  Possibly your old rings are still good and just your cylinder head needs reconditioning?

Anywas, a lot of techs like "Deves" piston rings (Sweedish I believe) they are a bit softer than the originals so they are more apt to adapt to used cylinder walls.   They are a fraction of the cost of originals.  Hastings also makes rings for these cars.

Never had much luck re-ringing these engines after they get some wear on them. So I avoid it these days.

[w113dude]

Hi Joe,

I did measure the bores very carefully with a digital bore gauge, they are not oval and are @ 82.50mm the original bore size is 82.47.
I did get intouch with "Deves" & they seem to have the right set of rings.
Two of reasons I thought might be a good idea to change the rings, end gaps were about 1.mm or slightly over (compressed) & more importantly  I broke an oil seal ring while taking the pistons out
Any special way to hone these bores? Length of time or any particular type of honing device?

Thanks,
Shaun

[ja17]

Hello Shaun,

Make sure you move the tool up and down while operating so you get a nice cross hatch pattern.  I use the cylinder de-glazing tool that has the a cluster of abrasive balls on the end of the shaft. works great.  Knock all the gloss of the cylinders so it is a nice even crosshatch all over. The cross hatching will help the new rings seat.

[w113dude]

Joe,

Thanks for the tip. and advise.

Shaun

[Shvegel]

I could never get a good crosshatch until I watched a honing machine at a machine shop. Very slow rotation and alot of up and down motion. Finish quality(ie the grit of the stone) is dependent on the type of ring. Cast iron rings require a rougher finish than a moly faced ring. Lots of info on the internet I am sure.

[Shvegel]

Here's a video of a honing machine in action.


http://www.youtube.com/watch?v=HQfh6AZxL08

[Benz Dr.]

82.50 is the cylinder size which is first oversize in a 230 or 250 SL.
82.47 is the size of the piston measured at the bottom of the skirt. The difference between these to sizes .03 mm ( .001'' )  is the running clearance. This a standard clearance used on almost all older benz engines regardless of piston size.
The metal band below the piston pin is responsible for controlling thermal exspansion to the point that really small clearances can be obtained. By comparision, most US style engines from the same time period used pistons that would expand as the engine heated. A rough guide was about .001'' of clearance per inch of piston diameter.