pormgb

Hi All,

Is this block toast? You can feel the ridge in cylinder #1.

Ski

No, I've seen worse that have run. JMHO if it was mine and time was available, I would pop the pistons out, get the hand hone Sunnen tool with felts and paste and try to clean it up and install new rings, again, JMHO.

You're going to be able to feel the ridge; the tops of the pistons are smaller than the bottom, that's where the top of the top ring stops.

pormgb

The band you see below where the ring stops can be felt with my finger, I'm wondering if oil will get by the band.

Ski

The proper repair would be to have it machined and finished with the correct procedure, after getting new pistons.

If that's not in the budget, find a machine shop that knows how to finish Alusil engine bores. You can clean up the bores with the last finishing pass; they can use the last stones, oil, felts and the correct paste. It's not perfect but if you install new rings, it's better.

You can see some lines in this one. Cleaned up the engine, had a machine shop (mostly BMW work) do a finishing pass on all bores, we installed new rings - the ring gap went up a tiny amount but not by much, still within tolerance. Again, not perfect but it can work out if done right.


got it cleaned up


and a couple of finish passes

pormgb

What did the machine shop charge for the work?

Wouldn't it have been easier to look for another block?

Ski

It's really a judgement call, based on needs, availability of such, and budget. Tom M'Guinn has some pics around from his old engine, that ran three years IIRC, on much worse lines than yours - there in lies the decision process for you. Do it now, do it later, will this last, etc. Although Porsche did a very good quality control process with these engines and their internals, you might have #1 or #2 piston group or the block that you find may have had those, been machined for such and you'd be SOL.

Not sure of the cost just for that, as he had the head redone by them, total was $625, so I'd guess $125 or $150 for the cost. You need to make sure you find turbo rings, not NA rings.

There is a set of Mahle slipper skirt pistons, Ferroprint Iron coating, in 100.5mm in the classifieds. They may have been sold but worth a check as their a damn good price; comes with wrist pins, clips, and new rings - they're also matched within .1 of a gram.

Bri Bro

You can also get the block resleeved to stock size and use the original pistons. There are several shops up that do this that are closer to you, ask over on the 928 board. Here is the link to one source.
http://www.944online.com/cgi-bin/ASI...block-944turbo

pormgb

Thanks for this great information !!

I have been trying to understand what piston groups mean, how do piston groups relate to the size of the piston? I have a spare set of tolerance group #1 pistons and have been trying to find out what the group number means.

This block comes from a used motor I picked up cheap, the owner complained about his motor smoking and thought it was the valve guides, could the smoke be caused by the rings passing oil over the ridge on the 1st cylinder?

Ski

Basic piston = 99.980
group 1 = 99.990
group 2 = 100.00

and for the cylinder bore, add .020 to the piston measurement

pormgb

Thanks !!
Any comments on oil passing via the ridge into the combustion chamber?

Cory9584

can tighter fitting pistons and rings be used in an alusil bore to make a tighter seal

Ski

You bore the cylinders in relation to the piston measurement, 61mm down from the crown or about 5mm from the bottom for Mahle (others probably similar or the same ??), and you bore it based on the recommended clearance needed - which is based on the thermal expansion of the piston.

comment on the oil - maybe - maybe not - head could come into play, turbo seals if the engine was old and the turbo was too, could have been blow by and had excessive crank case pressure.