1st Repair Pistons

[Harry]

I just received my 1st repair pistons from the Classic Center and found that the compression height is about 0.020 ins (0.5mm) shorter than the original pistons.  I’ll call them later this morning but I was curious if others had found the same, and anything you might have done as a result?  The pistons also have a step in the top, i.e. about half of the piston is slightly recessed.  Any thoughts?

[Cees Klumper]

I recall a point made by, I believe, Dan Caron (Benz Dr), that this may be to compensate for the assumed skimming of the cylinder head when an engine is being rebuilt, so as to not result in too much compression. If your cylinder head (or engine block) is not going to be skimmed/decked then you could end up with too low compression?

[Harry]

Thanks Cees.  The block only had to be skimmed (about 0.1 mm) so that is not going to do much for it.  The head can offer further offset but I don't want to take anything off of the head more than is necessary to be sure it's flat.  Need to do some research I suppose.

Thanks again!
Harry

[Benz Dr.]

This is a normal procedure that MB employs on new pistons for our cars. I measured a set I'm ready to install on a 230S ( very similar to a late 230SL ) and found they were .92mm lower than the standard original pistons. This is not for planing the head and is rather for decking the block. Since these pistons are .036 '' lower, I asked the machine shop to remove around .020 - .025'' off of the top of the block. This still leaves plenty of material for subsequent machining many years into the future. Once your engine is assembled turn the crank to 5 degrees ATDC and then measure how far the intake valve can be pushed down before it hits the top of the piston. You should have at least .9 mm or you could run into trouble. Since you are working with stepped pistons ( as I am ) you will already have lowered compression and decking the block will bring those levels back up. Expect about 150 - 155 PSI compression when done.


In any good restoration you never want to do anything that can't be undone or corrected 50 years from now.   

[Harry]

Thanks Benz Dr.  How much an be removed from the block deck before you have to be concerned about slack in the timing chain?  Not an issue with  0.020 - 0.025?  Do you know what the combustion chamber volume is in the head?

[Shvegel]

My belief is it is to compensate for the additional bore size.  Nothing more.  I did the math once before so it should be here somewhere.  Bore area X stroke then Bore area with the .5mm overbore X stroke.  I bet you come up with a difference equal to oversize bore area X .92mm

[Benz Dr.]

If your pistons are only .50 mm shorter I wouldn't go more than a .005 - .010'' cut off of the top of your block. A  .010'' cut shouldn't be enough to cause any chain problems but I usually add a .015'' cam shim under each bearing to compensate for head and block milling as well as anything removed during a valve job. Cam shims do two jobs - they help to keep your chain tight, and they keep your rocker geometry where it should be.

[Harry]

Thanks Dan.

Do you make up your cam shims or are these available?

Harry

[Benz Dr.]

I buy them from Metrics in California.

[MikeSimon]

   Do you know what the combustion chamber volume is in the head?

Assuming you have a 230SL and that motor has a compression ratio of 9.3:1, with a displacement of 2306cc or 384.3cc per cylinder, based on bore x stroke of 82mm x 72.8mm, there would have to be a combustion chamber volume of 46.5cc.

[Harry]

Mike - thanks for the response.  The 46.5 you calculated will include the volume of the head gasket as well as the combustion chamber.  The head gasket I removed was about 0.075 ins thick.  The new head gasket measures to be about the same.  So that portion of the volume is about 10 cc.  This results in an actual combustion chamber volume of 36.4.

Dan - thinking this over, I see where cam tower shims would be helpful in recovering chain tension but they would actually change the rocker geometry - right?  Because you are raising the cam relative to the rocker/head?

Thanks all!
Harry

[Benz Dr.]

Mike - thanks for the response.  The 46.5 you calculated will include the volume of the head gasket as well as the combustion chamber.  The head gasket I removed was about 0.075 ins thick.  The new head gasket measures to be about the same.  So that portion of the volume is about 10 cc.  This results in an actual combustion chamber volume of 36.4.

Dan - thinking this over, I see where cam tower shims would be helpful in recovering chain tension but they would actually change the rocker geometry - right?  Because you are raising the cam relative to the rocker/head?

Thanks all!
Harry

Yes, that's the main reason I use them. Raising the cam shaft requires that you also raise the rocker arm at the ball stud. Since this is not a 1 to 1 ratio, a .015'' cam shim would be more than that at the raised ball stud. I don't have the exact dimension in front of me so maybe I'll do that measurement some time and report back with my findings.
 One of the problems with doing an accurate before and after assessment is parts changes and machining. Unless you were to measure everything before tear down, how would you be able to account for the differences in wear and material removed during a valve job? Keeping this in mind, cam shims will definitely cause you to raise the rocker arms up to the required .003 and 007'' respectively.   

[mbzse]

Assuming you have a 230SL and that motor has a compression ratio of 9.3:1.../...a combustion chamber volume of 46.5cc

M-B workshop Literature gives the volume of the combustion chamber for M127 (230SL) head to be 45,3 to 48cc, whereas M129 (250SE/SL) volume is 48,0 to 50,7cc.  Nominal compression is 9,5:1 for both engine types