Can I just fit these P's & C's?

[berni29]

Hi There

I am in the process of doing a top end on my 993. I had assumed that after 185k miles that the pistons and cylinders would be worn, so ahead of time bought a 36k mile set off a Rennlister here in the US and had them shipped to the UK. Picture below.

The part number on my Mahle pistons is 100P45 and on the "new" ones P100P47. Is there a difference? and also can I just fit them and not worry?

As it happens my existing pistons and cylinders look very good. I have not measured the bores and lands yet but they certainly look fine. I did have a broken ring one one, but there are no marks on the bores and you can still see the cross hatching.



Thanks

Berni

[Walt Fricke]

I think you already put your finger on the key: measure. Are your bores within spec? Are the ring lands within spec? Especially the one with the broken ring.

Did you do a leakdown before you took the motor apart? What did that show? Guessing you didn't, so what now?

Measure

[berni29]

Hi Walt

I have the motor in bits. I have the old P's and C's and the new 36k mile set. I am going to fit the new low mile set (pictured) because if there is no meaningful difference between the two sets then why not. As for the old set I will measure up when I get the chance. Sooner if it turns out my new ones are not compatible.

I did not leakdown test before removing the engine, but I did a compression test which showed low pressure on the pot with the (what turned out to be a) broken ring.

By the way, what are the bore and land specs?

Berni

[bh912e]

Measure the new cylinders to check if they are within specs for bore and taper. Measure piston ring gap. After that, measure all cylinders for height, I have seen people sell some mismatched cylinders that are way off height wise..... which would screw you up when installing the cam housing on the cylinder heads. Good luck and have fun !

[berni29]

Hi bh912e

Yes good point RE the heights. They do look like a matched set, but like you say you never know......

Berni

[JeremyD]

measure measure measure. - So flipping critical. If I was at your place in the build - I would probably find a local machine shop that can precisely measure everything for you - may cost you a $100 or so - Have them measure cylinder taper, bore, height, weigh and measure pistons, rings, - everything -

I can't emphasize this enough. From someone who bought P & C "fresh back from the plater" from a "reputable" company - that ended up being out of spec - bore and taper and height that caused blowby...

It was an expensive lesson

[350HP930]

With that many miles AND a broken ring I'd be willing to bet that you will find that your old cylinders are well beyond what you would want to put back in a fresh rebuild once you mic them.

[Walt Fricke]

Berni

Wayne's book gives the various specs you need for measurement. Explains how to do it also, as I recall.

If you don't have the tools (mainly a bore gauge), you might be money ahead to take the machine shop suggestion. Good machinists are awfully good at this kind of thing.

[berni29]

Hi

I will have to get Waynes book, also I shall have a measure up tomorrow with the digital gauge I borrowed, and if in any doubt will get a machine shop to do.

Thank you for the good advice

Berni