Piston to cylinder clearance on 3.4?

[David]

I've got Mahle 3.4 pistons and cylinders on my '79 930 from the PO's previous engine rebuild. The piston to cylinder clearance is 0.003". I think the spec on a 930 is about .002" but I found one turbo engine that called for 0.003". I've heard the turbo's should have a little more clearance but I don't know if this is excessive.

I sent them to EBS Racing for evaluation and honing, but I'd like some input from the board.

[garibaldi]

Where on the skirt are you measuring the diameter at, above, or below the pin boss?

[David]

If I remember correctly, it was 90 degrees to the piston just below the rings, but I checked the pistons in several locations to find the best reading. I've already sent off the pistons so I can't confirm.

[garibaldi]

Well, due to the geometry of the piston in tems of ovality and skirt taper, where you measure from to determine piston to wall is critical, most piston manufacturers will tell you where to measure from. You want to measure 90 degrees from the pin bore, but where on the skirt is equally as critical, since there can be different skirt tapers through the body of the piston.

Mahle uses a barrel taper on their skirt, so this means that if you mic the piston right below the oil ring land, and continue in a straight line down the skirt, it will get progressively larger towards the pin boss by a few thou, peak at the centerline of the pin boss, and then gradually get smaller towards the tip of the skirt. It mimicks the shape of an old barrel.

[David]

I may not have gotten the correct piston diameter. Hopefully EBS will measure them correctly and give me the correct advice.

[David]

Just got a call from EBS, they said the pistons are good but the cylinders need replating. Oh well, add another $795 to the bill.