Which wears out, the pistons or the cylinders?

[88911coupe]

As a follow up, which typically tends to wear out, the pistons or the cylinders? I am about to remove the heads on my 88 3.2 and was curious as to if its likely they will both be worn out vs. one or the other.

[304065]

Buck,

I started this as a new thread so more folks would see it.

My own view is that it depends-- some engines, like mine, had 41-year-old pistons and five broken compression rings because the PO who rebuilt the engine 25 years ago used new rings, but the ring grooves had worn beyond spec and they all broke probably within a few minutes of startup. Some pistons had scuffing on the skirts from overheating, and some of the cylinder liners had accelerated wear on the rubbing surface up near the top of the cylinder.

That said, the modern Nikasil cylinders have been shown to be very durable as long as there's not a catastrophic failure, whereas the pistons have the potential for wear on the skirt, and wear on the ring grooves. But of course you should check both.

[88911coupe]

Thanks, that's useful information. I hope my cylinders are in good shape!

[boxermania]

The original idea of the hard cylinder coatings came about as a result of the good thermal conductivity of aluminum along with the poor wear characteristics. This was specially of significance on air cooled engines as they, by design would, never achieve the uniform cooling/warming of the water cooled variety.

So by applying a hard coating on the cylinder the wear is taken up by the rings. The idea was brilliant as the rings are far easier and less expensive to replace than the cylinders. Of course one has to keep in mind the condition of the piston as well, specifically that of the ring groves.

Once the rings grooves enlarge, both in depth and more importantly in width the ring is no longer square to the bore and is allowed to rock slightly, this not only increases the wear and ultimate destruction of the rings and ring groves, but also reduces the efficiency of the engine (a lesser problem than broken rings) as the ring function is diminished.

So, the bottom line is that Nikasil cylinders will wear very little and a new set of rings, properly gapped will restore the engine to it's prior glory, as long as the piston dimensions and specially the ring grooves are within factory spec, as mentioned earlier by john_cramer.

[88911coupe]

Is it possible for the novice mechanic to accurately measure the cylinder roundness or ovality, assuming they have the correct equipment? I get the impression it takes some experience to do this properly.

[3.2 CAB]

It does take some experience and the proper testing gauges, to check the cylinder bore. If you have the tools and the know-how, you should not have a problem. If you don't you can still remove the parts and take to a shop that specializes in this and get them to check. The engine machine shops usually will have a small fee for this, but it is worth it if you don't have the experience or correct tools. My shop of choice, charged me $50.00 to spec out a 440-6pak MOPAR, it was well worth it to me. A lot of times, the shop, will absorb this cost if you have the machine work done there. Good luck!!

[boxermania]

88911coupe

If you are familiar with micrometers and have or can borrow some it is fairly easy to do.

The easiest way is with a bore mike, taking 3 readings ~ 120 degrees apart and ~ 1" down on the bore to insure you are on the area that the rings ride on.

The same can be accomplished with a snap gauge and outside mikes.

If what I mentioned makes little sense to you, pay to have it done.

[BK911]

Originally Posted by boxermania

88911coupe

If what I mentioned makes little sense to you, pay to have it done.

LOL!!

[davidppp]

Pistons.

Its rare to find top ring grooves in spec.

Do not even think about reringing them..

Kind regards
David

[BK911]

Originally Posted by davidppp

Pistons.

Its rare to find top ring grooves in spec.

Do not even think about reringing them..

Kind regards
David

What about machining the top ring land and fitting with thicker rings?

[304065]

Originally Posted by BK911

What about machining the top ring land and fitting with thicker rings?

Theoretically possible if you had the ultra-rare 4-cam carrera pistons out of the missing James Dean engine. However, the trend with rings is to make them thinner, not thicker: they seal better. My custom JE's have a ring package that's thinner than what Mahle used. You would be moving in the wrong direction from a performance standpoint and the cost of new, or good used, pistons isn't so high to justify this step.

[Eagledriver]

I've never had the top ring lands enlarged for new rings but I bought a used set of pistons that had been modified that way. I had to buy a custom size top ring from EBS for them. They seem to be working fine.

-Andy

[JeremyD]

I would take my pistons and cylinders to a tool and die maker/machine shop that deals with engines and pay someone to measure them correctly. They will have the proper tools - they will have the know how to measure effectively and correctly. In my humble opinion - worth the money to have someone else do it and provide you with a report.

All Porsche cylinders have some taper machined in - you add the piston wear - and you want to make sure you are well within spec - both for taper, and roundness.

Just my humble opinion - and xpensive lesson nevertheless.

[Steve@Rennsport]

Originally Posted by BK911

What about machining the top ring land and fitting with thicker rings?

Thats what MM does to "save" worn out pistons,........

[304065]

Ah yes the Taper. . . here are my taper measurements. As you can see, where you measure on the skirt is critical-- the right location means the difference between using it and junking it.



Buck, it's not easy to measure, particularly with those "t-gages" that everyone calls snap gages. Nor is the typical bore gage method that repeatable-- most folks set up the bore gage with an outside micrometer (held in a stand works best) and then measure the variance in the bore with the gage. The trouble is, the bore gage is VERY sensitive to alignment, if it's not held parallel to the cylinder walls the measurement will be off. This is evident when you pick one up and try it, however, so a patient amateur (like me) can learn the feel over time. Better would be to zero the bore gage on a setting ring, but these are expensive steel donuts.

If I were doing a lot of engines and I needed to repeatably, accurately and quickly measure cylinders, I'd probably use an Intrimik or a Holtest-- these are bore micrometers that are less sensitive to alignment. Of course the ultimate would be to use an Air gage such as the ones made by Mahr-federal-- these consist of a cylindrical gage very close to the measurement diameter with jets of air that shoot out the sides. The amount of air that escapes is recorded on a dial that corresponds very closely to the bore diameter. The advantage is that you don't have the hard carbide tips of the bore gage either scratching your cylinder wall or deforming it when the measurement is taken (but this level of precision is probably irrelevant to an aircooled cylinder, that looks like a banana at operating temperature and load).

See my thread "Measurement Instruments and Metrology" for more.

[88911coupe]

John, thanks for the information. Just mentally picturing how this is done the most difficult part would be making sure you are measruing perpindicular. I don't really plan to try this myself...I'll be sending them out to have a pro do it.

[efhughes3]

Buck- I'd pm'd or e-mailed you the other day thru Pelican, but never heard back. Any chance of borrowing my flywheel tool back for a bit? My buddy Brad is doing some rehab on his engine....

[tom1394racing]

Originally Posted by 88911coupe

John, thanks for the information. Just mentally picturing how this is done the most difficult part would be making sure you are measuring perpendicular. I don't really plan to try this myself...I'll be sending them out to have a pro do it.

The cylinder bore measurement is not that tough to do repeatably.

The key is to orient the bore gage approximately along the stoke axis and then to move the gage around that axis to obtain the maximum dial indicator deflection. The max deflection will occur when the gage is measuring the minimum distance which will be the cylinder bore diameter.

This same process should be used when zeroing the gage against the micrometer.

[88911coupe]

Ed, Sorry, I did not get the PM...
Which part do you need? the stand/yoke or just the flywheel thing that holds it in place? Let me know and I'll get it to you as soon as I can.

[Alan L]

Here is another option to measure bore taper - anyone tried it?
I did and it seemed to work.
Fit a ring to the bore near the top (preferably a new tighter one). Make sure it is square - use a vernier tail or whatever. Measure the ring gap. Then progressively slide the ring down the bore - again, keeping it square - once you get into the bore, you can use a de-ringed piston skirt to square it.
The ring gap will change in accordance with the wear in the bore.
A simple C =2 pie R calc gives you the wear in accordance with the variation in ring gap.
I tore my 250,000Km SC down about 18 mths ago. The bores were excellent (thankfully Nikasil). Had them professionally deglazed. The ring lands from memory were good enough to re ring. Same pistons, new rings. After about 15,000km, I changed the oil a few days ago (after ditching run in oil at 500km). I never topped it up in between. It never used enough to warrant it.
Alan