Piston valve pocket radial clearance
Turbo Mini Forums

Specialist components apparently specify 2mm radial clearance, valve to piston pocket.

I'm surprised at 2mm radial clearance. I came up with the 1.3mm clearance from an engine blueprinting book based around Amercian V8's and that suggested 0.05-0.06" (1.27 - 1.52mm).

Given that they are running less well controlled push rod valvetrains and large bore sizes (more piston angular slop across the larger bore) I'm surprised the little A series with OHC head would require more than 1.52mm.

Can anyone provide any insight/advice based on experience?

Edited by adcyork on 3rd Oct, 2018.

I recall I gave it 2mm clearance on dia on my new project mini as I was looking to the compression up due to small swept volume.

carl

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Personally, I feel like it depends on the accuracy of what you’re building.
I pin punched the pistons thru the valve guides, then (with piston rocked at an angle) picked up on the dimple for my reference - see below for more detail - and used a 2-flute cutter (with edge radiused).
I used “inch” size cutters (in 1/8” increments) - I forget now what size the valves are, but I’m in good shape as it’s run well.

As far as picking up the dimple; the inconsistent positioning across the pistons of the valve position meant I decided to actually make my cut-outs the same across all - if only to make them all the same CC.
I measured the minimum distance position from the centre of the position to each dimple, and noted the min and max position of all. I then made my cutouts “oval” by basically sinking in the cutter at the “min” position, then moving across to the “max” position. That kept everything the same and CC equal.

Doing the math, with 1” and 1-1/8” cutters, I have 1.03mm radial on inlet and 1.20mm radial on exhaust.

I forgot to say - I also did my “dimple” with no head gasket as that really is the worse case.

Edited by adcyork on 8th Oct, 2018.