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Home > Help Needed / General Tech Chat > Piston valve pocket radial clearance

adcyork

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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.


fastcarl

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leeds/wakefield.

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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TurboDave16V
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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.


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TurboDave16V
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Doing the math, with 1” and 1-1/8” cutters, I have 1.03mm radial on inlet and 1.20mm radial on exhaust.

On 17th Nov, 2014 Tom Fenton said:
Sorry to say My Herpes are no better


Who remembers this first time around: https://www.youtube.com/watch?v=JF0a5aoEGsw


adcyork

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On 3rd Oct, 2018 TurboDave16V said:
Doing the math, with 1” and 1-1/8” cutters, I have 1.03mm radial on inlet and 1.20mm radial on exhaust.


Thanks for the input guys, that's reassuring that you're running the clearance that low, Dave.

As for measuring up, I've done exactly as you have Dave, marked the valve centres on the crown @TDC with a silver steel punch down the guides.

The calcs say that max interference occurs @4deg ATDC crank position with cam timing swung to extreme case of peak lift @102deg ATDC. But that all occurs ATDC so radial clearance isn't a factor at that point.

I have free choice on cutter diameter because i'm having them fly cut so I'll go with 1.5mm I reckon. All of the clearances will be checked with the Plasticine method prior to final build too should be fine.


TurboDave16V
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I forgot to say - I also did my “dimple” with no head gasket as that really is the worse case.

On 17th Nov, 2014 Tom Fenton said:
Sorry to say My Herpes are no better


Who remembers this first time around: https://www.youtube.com/watch?v=JF0a5aoEGsw


adcyork

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On 4th Oct, 2018 TurboDave16V said:
I forgot to say - I also did my “dimple” with no head gasket as that really is the worse case.


Yea agreed.

If you used no head gasket for marking up then your indicated valve centres will actually be fractionally further outboard by 0.34mm (assuming 0.94mm bk450 compressed thickness).

In the built up condition with gasket your radial clearances then essentially become 1.37 and 1.54 so doing mine at 1.5mm (measured with a gasket) still sounds sensible. Cheers

Edited by adcyork on 8th Oct, 2018.

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