1st/2nd Repair

[Harry]

As I have reported in prior posts, I am rebuilding my 1966 230SL engine.  I have priced 1st rebuild pistons and 2nd rebuild pistons for this car (25mm wrist pin).
The price difference is huge, with the 1st repair substantially more expensive.  I’m considering going straight to 2nd repair, figuring this will very likely be the last engine rebuild for this car anyway.
What are your thoughts?
Harry

[Benz Dr.]

That depends on what size you have right now. If you had standard pistons in your engine it will depend on how much wear is in your cylinders. First oversize is 82.50 mm or .0020'' more than standard. This would be the piston you would use if you had only minimal wear. If you can feel the ridge wear with your fingernail, you will need 83.00 mm pistons. You should let the amount of wear determine the size of piston to use. All things being equal I usually opt for the smallest oversize I can get away with.

[Harry]

Based on measurements, I could use the 1st repair - although I can clearly catch the ridge with my fingernail.
I think I’ll proceed with 2nd repair.  I expect that even with this final repair size, the engine is still sound.  I just didn’t want to put the engine at a borderline position on operating.

[Benz Dr.]

Your machine shop should be able to mic the cylinders and tell you how much wear you have. I wouldn't order anything until you know what sizes you need and then I would give them the pistons so they can fit them to your cylinders. Don't get ahead of yourself.

[stickandrudderman]

And triple check the piston and rod combination that’s fitted. There are a few permutations and if the engine has already been built you should not rely on engine number alone to spec your requirements.

[Benz Dr.]

And triple check the piston and rod combination that’s fitted. There are a few permutations and if the engine has already been built you should not rely on engine number alone to spec your requirements.

I agree. I've changed rods and pistons on 230SL engines to earlier versions to obtain certain build requirements. Inspect carefully first and then order.

[Harry]

I did check the rod/piston as far as the wrist pin was concerned.  That was how I found that the 24 mm wrist pin with the 4 ring piston would not work with my 25 mm connecting rod.  Are there other fitment nuances that I should look for?

[stickandrudderman]

yes, comp height. The distance from the centre of the wrist pin to the top of the piston.

[Benz Dr.]

I've found that in most cases the manufacturer reduced the height of replacement pistons to account for decking the block. This is why you will see pistons that won't even come up to the surface level of the block if you just fit them without making any changes. I fixed that problem on my engine by milling the top of my block by about .020'' or .50 mm because the pistons can protrude a small amount and still be well within spec.

[stickandrudderman]

The difference is 10mm...........

[Benz Dr.]

10 mm?

That must be a typo. More like 1 mm. 

[stickandrudderman]

No mistake Dan. 10mm.

[Benz Dr.]

 I have a set of original third version pistons from a 230S that are exactly 40 mm from the top in the piston pin hole to the crown of the piston. The replacement pistons are stepped, because that's what MB supplies for this application, and they're 39.2 mm is one hell of a long ways away from 10 mm lower.  The lower portion of the step maybe getting closer to 10 mm but that's not where you should be measuring from.

Are these MB pistons you have or something else?

[stickandrudderman]

There are two different rod lengths used in the 230 and one is 10mm shorter than the other so it follows that the comp height of the pistons will differ by 10mm too.

[Benz Dr.]

The piston pin hole may differ in location from one piston version to the other but that's not what I'm talking about. I'm talking about the height of the piston as fitted in the cylinder. Original pistons either meet the surface level of the block or they sometimes protrude a small amount.
The top of replacement pistons, regardless of which version you have, will be slightly lower by roughly 1 mm to allow for milling the top of the block. Reducing the height of the piston by 10 mm might well give you as much compression as the worn out engine before you rebuilt it. Maybe we're actually talking about the same thing only from different directions.