Can these pistons be re-used?

hobieboy · 11-10-2005, 05:08 PM

I'm rebuilding the engine from the ground up and ditching CIS with EFI as well. The current dilemma is: the set of pistons I have is marginal.

The side clearance is ~.004 inch/0.1mm - according to BA's book, this is the max tolerance. Wayne's book recommends chaning the rings (which I'm going to) but didn't have any guidance. While factory spec says 0.2mm is the wear limit, my mechanic says that the wear rate from 0.1 to 0.2 will be exponential.

So, decisions, decisions. Do I need to replace the pistons?

Comments? Experience? For background - this is a track car so the wear will likely be much quicker than street use.

Thanks.

cstreit · cstreit's Garage

Hmm...

You don't say what sort of pistons they are, so it's hard to tell their value. I was thinking to myself that a good coating treatment (such as nickasil, or a new ceramic lubtrication coating) might ad a few thousandths to the thickness and get you back into spec. Won't be cheap but certainly cheaper than new pistons. I'd check with the coaters to see how much this adds and if they think this is wise. (just recalling that these coatings may add less, but worth ooking into anyway)

hobieboy · 11-11-2005, 05:30 AM

Hi Chris,
These are stock Mahle pistons for the 3.3. My debate is I'm at the top end of Porsche's spec for new installation but BA & my mechanics both suggests this is the wear limit

cnavarro · 11-11-2005, 05:56 AM

You mention the pistons, but are the cylinders in or out of spec as well? I'm with Chris- you may get away with coating the skirts of the pistons, but as the anti-friction coatings are considered sacrificial, they won't last too long and you may end up with piston slap under heavy loads.

cnavarro · 06:03 AM

After seeing that they are 3.3 pistons, I'd say replace them. Are the cylinders the fully finned 3.3 or half finned? If they are the latter half-finned, I would suggest getting some new cylinders or overboring a set of 3.2 carrera cylinders and getting some 98mm JEs to go along with them. I've never seen a 3.3 half-finned cylinder that I have been comfortable with :-) Also, if you have any dilivar studs, throw them out the nearest window and put something else in there- 993 steel, Supertec, ARP, Performance Developments, etc- anything but a dilivar.

hobieboy · 11-11-2005, 06:24 AM

Yes, the cylinders are within spec and they are the original 3.3 half-finned ones.

Wrt JE 98mm pistons - I'd really like to get some feedback on durability & quality. The overboring/JE route seems quite a bit cheaper than a set of Mahle replacements (3.3 or 3.4).

And I'm replacing the head studs & rod bolts/nuts with ARP ones. Plus the usual rod rebuild.

So back to the original question - is this $$$ something I can likely "defer"?

cstreit · cstreit's Garage

If it's within the factory tolerance, sure you can defer... With the understanding that you'll get a shorter life out of them.

As Charles said, and he knows a thing or two about pistons and cylinders, you are likely to get piston slap after time at the high end of tolerance. Sometimes you can hear it, sometimes you can't... It really depends on how soon you want to do a rebuild again...

cnavarro · 11-11-2005, 06:54 AM

I had a customer purchase Nickies and he chose to provide his own mahle pistons, rather than use JEs. Within a few months and some 5,000 miles after the skirt coating had worn down, he had the engine apart because the piston slap was driving him bonkers. The mahle pistons themselves don't expand as much, and at the .0025-.003" clearance he ended up with, it might as well have been the grand canyon between the piston and cylinder in my book. We ended up using mahles again, but we tighted up the clearance to .001-.00125" total clearance to make them work with the Nickies.

That said, you may experience the same thing by re-using your existing pistons and cylinders. Does it matter? It all depends how long you are going to keep the car or run the engine before pulling it back apart again. You may get a season out of it by coating the skirts and just re-ringing, then save your pennies for a set of new 98s to replace everything when it comes back apart again.

hobieboy · 11-11-2005, 07:31 AM

So it sounds like I need to consider stretching my budget to come up with the $$$ for a set of P&C

Kinda can I spend the $$$ now or have a time bomb I guess?

For my ignorance - what does piston slap sound like?

Charles, would like to find out the price for a set of Nickies for my application. I checked the Pelican catalog but they dont list any for 930 applications.

Thanks...

Tom F2 · 11-11-2005, 07:35 AM

The side clearance is ~.004 inch/0.1mm - according to BA's book, this is the max tolerance. Wayne's book recommends chaning the rings (which I'm going to) but didn't have any guidance. While factory spec says 0.2mm is the wear limit, my mechanic says that the wear rate from 0.1 to 0.2 will be exponential.

Check the clearance with new rings.

cstreit · cstreit's Garage

I would guess so! 8 thou of clearance is a lot of movement!

Eagledriver · 11-11-2005, 08:50 AM

I've noticed on my engines that they seem to always have about .004 inches of clearance even when they look like they have no wear on them. I think that a better way to decide is to look at the wear patterns. I've found that usually the pistons still have the original machining grooves around them. The wear will show up as a smooth area. The only wear I've seen on mine was a small area near the crown on the top and bottom of the piston. I bet if you measured new P/Cs you'd find they measure close to .004 inches.

I also am not sure we are measuring correctly. When we measure we are pushing the piston to zero clearance on one side and measuring the other. So the question is what is the proper way to measure clearance. Is my measurement telling me .004 on one side or .002 all around? (same thing I know, but what is the proper number to use for the wear figure?) I have this same question when measuring bearing clearances.

-Andy

CBRacerX · CBRacerX's Garage

Indeed a proper procedure for measuring the piston to cylinder clearance would be very helpful!

Michel J · 03-29-2006, 07:07 PM

I would like to know the exact procedure also.

Thanks,

Michel

snowman · 07:56 PM

Measure bore, about 3/4 inch down from the top of cylinder. Measure piston diamenter as specified by mfg, usually on the skirt side, just below the wrist pin, ie about 3/4'" up from the bottom of the skirt.

Bore dia- piston dia = piston clearence =spec number or 0,004" max.

Thats only 0.002" slop per side, not very much to worry about. Remember the piston grows when hot and fills in a lot of the gap, so its even smaller than this when running. Rings will not effect this one way or the other.

Its mostly the cylinder that wears here, not the piston. The reason is that the rings are the wear surface on the piston.
Note the cylinder dia must be measured at several angles to catch the max wear as well as to tell how out of round it has become. also it may be desirable to measure several places vertically to make sure you have found the max wear point. a pointer here. If you cannot detect a ridge near the top of the cylinder you propably do not any siginficant cylinder wear.

What you need to worry about is ring SIDE clearence. Thats where you hold the ring in its groove and push a feeler guage into the slot. The piston wears, not the ring here. If its good, ok if its not, new pistons.

PS note: Find someone with a bore guage to make this measurement. Its just to difficult and time consumng to do it with a snap guage, unless your an expert, and it can be done just about as well as you can with the bore guage. But if you that good with a snap guage you wouldn't be asking questions on this forum.

Henry Schmidt · Henry Schmidt's Garage

With air cooled cylinders you must also check for cylinder taper and roundness.
It is common for Porsche cylinders to have very little taper and be out of round as much a .004.
It is important to remember that from 1989 on with the C2 cylinder, Mahle actually built a taper into the cylinder to reduce piston slap.
Measuring these cylinder from top to bottom they actually get larger as you move down the cylinder.
In the case of a worn cylinder the taper is from large at the top to smaller as you move down the cylinder.

Terence Melvin · 04-28-2006, 12:22 AM

As others have said the p+c must be measured independently and the numbers compared to get the clearance. Feeler gauges are not admissable. The most economical tools to do this are a 100mm/4inch external micrometer and a telescopic bore gauge which is "t" shaped and spring loaded that then locks at the point of bore measurement. When removed the gauge is then measured with the external micrometer and the piston measurement AT THE FACTORY SPECIFIED MEASURING POINTS subtracted from the bore measurement. Bingo!

The advantage of this method is that the same micrometer is used for both measuremnts and so no micrometer calibration errors are possible. This is VERY important. In fact the calibration of the micrometer can be way out but you still have dead accurate clearance numbers as only the difference matters and these are the set in stone numbers defined by the measuring head of the micrometer and cannot change. The absolute measurements are totally unimportant for this specific purpose. If you use two separate micrometers/gauges for this YOU MUST CALIBRATE THE GAUGES AGAINST A PROPER STANDARD PRIOR TO USE. Otherwise the measurements are likely to be meaningless. And then your decision to replace or not is a lottery.

All the books tell you where the measuring points are, but you need to have the bore gauge square in the bore. I do this by placing a suitably sized object ,such as a camping gas cylinder, in the bore to sit the gauge on an. Then I do a number of measurements in each position, as many as it takes to get repeatability, until it comes out the same each time. Same goes for piston diameter. Make a table and write ALL the measurements down so that you can see the actually numbers and how the repeatability comes in as your technique improves. Go slowly and methodically as technique is all in this, but it is actually quite easy once you get the hang of it. Oh, and if the micrometer has a ratchet head try it using the ratchet and also tightening, very gently, without. Sometimes one works better than the other. But all your final numbers have to be obtained by the same method or they will be garbage.

It is a good idea to also to tabulate the piston ring gaps alongside the bore/piston measurements. I have found these nicasil items can have sustantial differences in wear depending on cylinder location. On one motor both ring wear and p/c clearances being substantially greater on cylinders 3+6 than on 1+4. I think this was due to differential cooling due to poor cooling air distribution and maybe some oil leakage around the crankcase engine breather which also prejudiced cooling in that area. Porsche used horizontal fans on competition engines to equalise cooling.

I reckon any p+c set which has 4 thou clearance probably should be replaced one way or another. The precise solution depends on how they measure up. A good source of what can be done with cylinders would be JE. The forged cylinders they sell are made here in the UK by their subsidiary PerfectBore who do top class work for the F1 mob et al. They will rebore and recoat and hone your existing Mahle cylinders at about half the new cylinder price, but it may not be quite so economic with double transatlantic US Post airfreight added. You do not need a courier service for this hardware.

Incidentally, I think that a production engineer at Mahle would tell you that the different cylinder/piston size groups they have are more to do with economy, not trashing out of spec components, than any god given desire for a tight tolerance. The JE cylinders are bored to fit the standard tight piston tolerances and meet the Porsche specs precisely on every one I have measured.

shbop · 04-28-2006, 07:30 AM

While we're on the subject: What clearence is everyone using for new JE's. Considering new pistons, new bore? .0015--.002" ??

cnavarro · 04-28-2006, 07:49 AM

With replated mahle cylinders, the spec is .002" normally aspirated, say at a 98mm bore.

Lukesportsman · 04-28-2006, 11:03 AM

Has anyone seen pistons that were out of round? No one has mentioned in what plane this measurement should occur. I had assumed that the pistons were round, but the ones in front of me are not. They spec great across the pin (where skirt is longest) and are "small" 90 degrees from this.

Are these normal or junk? They are 3.0 Mahles (CIS) and seem to be consistent from one to the other. Any help??? Nothing shows "wear" otherthan a slight shine at the skirt just below the pin.

How would've the pistons have worn if the rings were doing their job?