My build sheet indicates a 4.030 bore and 4.026 pistons obviously leaving a .004 piston clearance using Ross 90755 pistons. Right now the pistons are pretty noisy indicating they are running pretty loose.

If I where to purchase new pistons and coat them, should I buy .040 pistons w/ rebore or does the friction coating add enough to warrant more .030 pistons and a hone to fit??

Also, does the friction coating wear off quickly enough as to not make it a viable option for a street driven car. And (last but not least)…. How much does thermal coating add to the piston height effecting deck clearance?

 

The coatings are thin. Swain indicated that the skirt coating was ~0.0010", if I remember right. You still measure according to the uncoated size though. Here's a quote from the Federal-Mogul catalog that goes along with what Swain said:

"The "CP" suffix on Sealed Power pistons indicates that the piston skirts have been coated with our anti-friction, anti-scuffing, protective coating. The piston skirt size prior to coating remains unchanged. However, the diameter of the piston is increased approximately .0010" due to the thickness of the applied coating. The 'Minimum Clearance' as listed on the package label applies to the piston before coating. Do not measure the skirt and compensate for the increase in diameter due to the skirt coating as this could result in excessive clearance and possible piston failure. Refer to the recommended 'Bore Size Range' listed on the carton label. Piston to bore clearance has been affected by the addition of the skirt coating. Finishing the engine bores to the bore range indicated will result in the proper piston to cylinder wall clearance.

With thermal coating of the crowns, the coating seems to me to be too thin to worry about as far as the deck goes. However, the piston temperature will be lower and the skirt will expand less. It's reasonable to me to run a slightly tighter piston to bore clearance with coated crown than you would otherwise run.

The coating on the crowns was completely intact when we tore my motor down this winter. The skirts were a bit scuffed.

You might call Swain (or whoever you are using) to be sure.

Rich Krause

 

Guess the jest is to go ahead and purchase the .040 pistons as the coating has minimal effect upon sizing and from the forged piston engine noise, (not the build sheet) I’d say it was time to bump up.

While you bring up names……. What does it cost and whom does anyone recommend?

 

I have used Swain Tech (Rochester NY) a number of times. They are on the Web. Always satisfied. Cost is ~$25/piston for the crowns (most important as far as I am concerned) and if I remember right it was ~$15/piston for the skirts.

Rich Krause

 

Swain or Calico are both good names.

From what I can tell Calico is cheaper than Swain.

Here is a sample of Calico's price list for pistons:
PISTONS
Top & Skirt $26.00 each
Top, skirt, & ringland $35.00
Top only $15.00 each
Skirt only $15.00 each
Piston Rings $3.00 each

You can also get some pistons from the piston companies with the coating done already, that's usually more.

The Ross pistons you had were a -4cc dish (for valve reliefs) and weighed in at 490g's

Ross has a 4.040 x 3.75 x 5.700" -6cc dish piston that weights in at 479g's for around $500

They also have a 4.035 x 3.75 x 5.700" -4cc that is 493g's too

There are some SRP pistons both .035 over and .040 over that are around 480g's and are around $450 a set.

I would go with the lightest pistons that fit your motor.

Diamond has some 4.040 x 3.75" x 5.70" pistons that weight in at 433g for around $500. With coating from Calico you are looking at $120 to $215 extra.

The Diamonds seem to be one of the lightest off the shelf pistons out there that are a comparable strength to what you are using now. The Ross lightweight ones are about the same weight but are not recomended for alot of abuse from N2O or blowers.

What is the motor used for?

Bret

 

Very good info and thank everyone so very much.

Funny - I just spent the evening trying to find a cost effective light piston and all I really needed to do is just ask this list. Your basic live and learn.

The car is a street/occasional race seeing about 5-8k miles per year. No nitrous, nothing crazy, nothing special. Your basic somewhat quick street car with a stroker motor, gears, long tubes, etc.

Refreshing the combo by redoing the bottom end, swapping my "stage II" heads for some pretty good 210 AFR's and slightly larger cam along the lines of a GTP 6 cam. Figured I’d have the pistons coated while I was putting the rest of the pieces together.

As usual…… great input.

 

Guess there is no sense in being dumb my whole life.... While I've got a couple of good ears, I might as well get my monies worth. What rings does everyone recommend for my application??

I had ring flutter in a 398 combo so I'm a little leery of the best ring selection. Thoughts?

 

Depends upon your budget and what you are going to do with the motor.

Hey, no problem on the help. I look at this stuff all day so I know pretty quickly where to find what you are looking for.

Bret

 

Well, I can guarantee you that your piston/bore clearance is NOT .004, or else the noise you hear is not piston slap. Our last race motor was around .007 and didn't slap after a minute or two, and .0045 is the minimum clearance I'd run on any performance motor with forged pistons and 4" bore. What's more likely is that somebody did some sloppy machine work and your bore is oversized.

As for the coating, it shouldn't be calculated into the equation at all. Within seconds of starting the engine, the coating is worn to a thickness you won't be able to measure with a standard micrometer set. However, the fact that it's worn off does not affect it's effectiveness. The question about deck clearance is a good one, I've always decked the block after coating, so I can't help there.

If it were me I'd have the bore checked, and probably take it to another machine shop to have the final few thousandths honed out. Also, before you reassemble this thing set up the ring end gaps correctly.\

Best of luck!