Piston to Cylinder clearance

[david.g.chapman]

I have just received some new +20 thou pistons from PM, with QED markings. They look OK, but I am a bit disappointed with the skirt diameter, which is exactly the same as my old worn pistons, to +/-0.01mm. There was a fair amount of visual wear on the old piston skirts as well.

The old pistons have the smaller valve cut outs.

I had measured a bore to piston clearance of 6 thou max with the old pistons, anticipating a better result with the new ones. Just my luck!

With the old setup, there was no smoke and good oil consumption. There is also no wear ridge on the the bores that I can detect.

Does anyone know of a value for bore clearances where nasty things start to happen i.e piston slap, using oil etc.

My Brooklands manual specifies 3.6 thou new clearance, plus an extra 3 thou wear where oil control rings can be used - so maybe I am OK?

Dave Chapman.

[rgh0]

Normal design clearance for replacement pistons is 4 thou / 0.1 mm measured at right angles to the gudgeon pin at gudgeon pin height on the piston. Original Ford / Lotus specification clearance tolerances were a little tighter at around 3 thou.

What are the actaul bore and piston measurements?

A 6 thou clearance is unusual for a +20 piston in a + 20 bore but just possible if piston at the minimum end of range and bore at maximum end of range of normal tolerances of about .001 inch on each. I would check bore meaurement, piston measurements and if still 6 thou and bore not at maximum end and piston not at minium end go back to Matty's and ask why the piston is undersize versus specification.
regards
Rohan

[david.g.chapman]

Thanks for reponding, Rohan.

The piston measurements are 82.92, 82.92, 82.92 and 82.91mm across the widest part of the piston skirt at right angles to the pin. If they are measured as you say at pin height they would be even smaller (but my calipers cannot reach those points.

I believe they should be 82.978 to 83.008mm. Just take the Grade 1 to 4 standard piston dimensions and add 0.508 for 20 thou.

If they were in this range my 6 thou clearance would become 3-4 thou .

So they are going back!

Dave Chapman.