Piston to bore clearance How tight is too tight?

[Guest]

Clearance is ALWAYS measured at the bottom of the skirt in the "cross the block" - in direction of rotation plane.

Pistons are oval both around them and up them, so that is why machining forged pistons becomes such a skilled engineering job.
AE turned out a shed load of pistons for TR6 years ago, (90s) which had been machined "on the cock".

Result,- they used to seize up all the time and it took a smart engineer+lawyers in Germany to measure them, take them to the cleaners and sue the daylights out of them.
They paid!
60 grand I believe.

The workshop guys that got kicked in the teeth for months by making and re-fitting the engines that "went wrong cos it was their fault", never got much of an apology (I was there!), BUT THAT WAS THE WAY at Cargraphic.

Michael my colleague in the engine workshop was so p...ssd off you can't begin to imagine!

[trunt]

We are talking radial clearances here guys, yes?

Its the difference between the diameter of the piston skirt, and the diameter of the bore -- across the block as stated.


So for the stag (see grade F on ROM)
3.3853 (Min bore) -3.3833 (max skirt) = 2 thou
3.3858 (max bore)- 3.3828 (min skirt) = 3 thou

So I asked for 2.5 thou.

[Motorsport Micky]

Just for the record, at 3 thou it's right at the bottom of the clearance I'd be happy at running. With these size pistons in use you'll find the performance perfectly acceptable, better even when the clearance gets up to maybe 5 thou, bit noisier initially on start up but less heat and drag and better power as long as the piston rings are good.

Micky

[trunt]

Just for the record, at 3 thou it's right at the bottom of the clearance I'd be happy at running. With these size pistons in use you'll find the performance perfectly acceptable, better even when the clearance gets up to maybe 5 thou, bit noisier initially on start up but less heat and drag and better power as long as the piston rings are good.

Micky

Noted, I told them to push it up a bit

[Guest]

...better even when the clearance gets up to maybe 5 thou, bit noisier initially on start up but less heat and drag and better power as long as the piston rings are good.

These are CAST PISTONS with 12% silicon, Mick.
5 thou is FORGED PISTON territory.
You don't want to go there, because the amount of blow by and oil contamination seriously starts to take root at those clearances.
The wider the rings the better alignment you must have, not the reverse.

My forged pistons are using silicon alloy and we can run them down to 2 thou no problem at all.
One engine was physically completely run out of water in OZ for 2-3 laps because it holed a cylinder head.
Water was pouring out of an exhaust port....

It then later had to be stripped cos the crank snapped.
The pistons had zero scuffing and were reused.