How common is 964 piston slap and how much of a concern
 

I resealed a used 964 engine that I bought from a less than honest person, so I can't claim to have much history on it. The pistons and cylinders visually looked brand new, but I had them measured by a machine shop (I gave them the specs from the factory manual), and they were declared to be fine. Now that the engine is completed I get a noise that starts about 30 seconds after initial start. It sounds like a "bad lifter" in a car with traditional lifters, but i have traced it to the number 6 cylinder, and its coming from the barrel, not the head area. once the car is fully warmed up the noise is 99% gone. The power is good, engine runs flawlessly, and there is no smoke. Is this a common thing for a 964 engine with miles on it? Is my machine shop just bad at their job? Ideally I would like to be able to put 10 to 15k miles on it and then build it into a 3.8.... My guess is I should be able to get that kind of mileage with a little slap during warm up, but I wanted to see what others have experienced.

What were the measurements?

Within middle of Spec is fine but if close to the wear limit then yeah, it will work but you will probably get slap. This is what Ollie's Machine Shop "declared" on mine. That is why I chose to get new P&C's ( well to be clear, my old cylinders bored out for the new larger CP pistons for my 3.4L build ) No, they did not do the boring & plating, EBS did mine.

Post a sound clip or video to confirm. Others with heck of allot more experience may reply.

Just a ?, but have you rechecked the head stud torque on that cyl? Sounds to me like the cylinder is moving ever so slightly, and hitting either the case spigot, or the head. Is the correct base shim or gasket under the cylinder in the spigot?

im 99.99% sure its the piston. using the stethoscope the case is silent as is the head, only the barrel of the cylinder makes noise. I did check the torque, as well as recheck all the valve lash clearances. I also started the engine with that valve cover off to make sure the cam spray bars were clear, and there was plenty of oil up top. noise is only audible at idle when warming up.

Cylinder 4 is also borderline noisy with the stethoscope... but the 1-3 bank is quiet. Do these pistons have the offset pin that would be more inclined to slap on one side of the engine?

I don't have the measurements with me now, I may still have them at home.

Yes, the piston pin bore is off centre by 0.9mm. It's vital that pistons are correctly oriented with the stamped 'E' on the intake side

Worn rockers sounds like piston slap.
The piston offset really seems possible.
Bruce

Bruce,
I definitely thought it was a rocker until I isolated the sound to just the cylinder.

The race engine I am building complained of piston slap when I bought the car form the PO.
The engine was in 4 rubbermaid tubs -- so I don't know what it sounded like.

The jugs were Mahle, and the pistons Ross -- The pistons had obvious scuffing on the skirts, the cylinders looked fine, but had ovality issues when mic'd. The clearances for the combination were
on the high side.

I suspected the thin wall of the slip-In 3.8's and the mismatch of aluminum alloy types was contributing.

I had the jugs refurbished and tossed the Ross pistons in favor of Mahle RSR 3.8's to match the alloy type.

I'm in the process of reassembly this summer. We'll see if the piston slap goes away.

Mike

I have no real opinion about the noise you're experiencing but the design of the 3.6 (100mm) piston is prone to piston slap. (very short skirt)
In an attempt to control this phenomenon, Mahle tapered the bore of the 3.6 production car cylinder.
They have a smaller bore at the top of the cylinder than at the base.
When new, the piston will not travel through the cylinder without binding.
This is an interesting solution to a complex challenge.

Henry,
If we were to (however dangerously) assume that I am right, and it is a piston slap noise... Do you think it will stay together for a few thousand miles, or does it need to come right back down? basically I just don't want to end up with a hole in the case

When I learned about aircraft recips, we were told the taper bore was to allow parallel cylinder walls at operating temps as the top of the barrel would expand more than the bottom obviously due to the combustion occurring near the top.

You could be right but I believe the differing cylinder expansion from top to bottom is controlled by the cooling fin lengths. It wasn't until the short skirts employed by the 3.6 (100mm) pistons that Mahle decided to taper the bore in production cylinders.
I know that the use of a very short skirt created a rattle in the RUFF 98 mm pistons and Porsche wasn't interested in warranty issues associated with engine noises in production units.