Technical Jahns piston question

Question for the older engine builders. I have a set of Jahns pistons for a 324 with a 4.00 bore. My engine machinist contacted me because the pistons are .008 under 4.00 and would be real loose. Should I have him tighten the bore just a bit to fit the cast pistons and file fit the rings? Or run the 4.00 bore and be a little loose. Here is what the paperwork included with the pistons says.

 

Personally I’d run them at .008” and never think much about it. Yes you will hear some pistons slap until warm up but at 180* you won’t hear any so be easy when driving until it warms up. I use file to fit Total Seals on race engines and file fit cast iron on all street engines if they’re attainable.
If you haven’t bored the block yet you could easily have the final hone by your machinist tighten the bore a few thousands and file 4” rings.

 

You will be fine. General rule of thumb is 0.001" to 0.002" of RADIAL clearance per inch of bore. At 4.00" of bore, and having 0.008" of diametrical (0.004" radial) clearance, you are right in the ball park, standing near home plate.

 

I seem to recall that those old aftermarket pistons didn’t have much in the way of skirt profile technology, so they were always run lose.

 

Assuming it was even necessary, does anyone still knurl pistons?

 

I bought a "new" set (NOS) of Jahns pistons many years ago for a 409 (Chevy) that I built. As I recall, the machining recommendation sheet also ended up at .008". The machine shop that I used was a quality shop. I also checked the bores and piston diameters after getting the pieces back from them. All good, per the sheet anyway.

The piston slap sounded like a diesel engine. While I did get used to it, it was still LOUD, and DIDN't get any quieter at 180 degrees, coolant temp. !! I did a little drag racing, a fair amount of street driving.
I don't recall now how many miles that I put on it, but after a few years, I sold the car (61 Impala).
The new owner called me a month or so later, asking about the pistons. He said, that the engine broke the skirts on "two" pistons ! I went to the guys house. Yep, outer skirts had released themselves from the rest of the pistons.

Personally...I'd go a little tighter !! Maybe .0065" / .007" at the maximum.

Mike