Piston bore clearance
Turbo Mini Forums

Hi this is my Turbo build which is almost finished. I have a nagging doubt over the piston to bore clearance at the top of the pistons. I can press my thumbs on top of the pistons and they rock from side to side a little. Using two sets of feeler gauges, I can get a nine thou feeler down the left side of the piston and nine thou down the opposite side as far down as the top ring. I know this ain't the best way of measuring, but it's all I've got.

Fresh+20 block and omega +20 forged piston supplied by MED.
Bore to piston clearance = 4 thou measured near piston skirt
Ring gap= 12 thou
Bore size = 2.8 inch

Bore to piston clearance measured at the top of the piston is 9 thou. Is this normal? I realise pistons are not supposed to be perfectly round. I've asked med about this, they said it would be fine but I would like a second opinion
Cheers
Daz

What you see and measure at the crown can mean nothing.

Clearance can only be measured at the skirt, as pistons are generally tapered towards the crown.

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

A Stevie says, clearance at the crown isn't really meaningfull as some pistons have masses of clearance above the top ring.

However, 4 thou at the skirts seems rather large for a road engine, new build.

Who bored the block, and did they have a new piston to measure accurately, and did you specify the clearance you wanted (any decent shop would ask for a piston for them to measure themselves and ask you what clearance you wanted) ???

Schrödinger's cat - so which one am I ???

So, good news then regarding the clearance at the top of the pistons.
MED bored the block and supplied the pistons all together. Iirc when I measured the clearance at the skirt, 4 thou was on the upper limit for a forged piston.

Another double post, bloody ipad

Edited by dazibee on 20th Jan, 2013.

piston clearance should be measured at the bottom as in the pic below.... if its mesured under the gudgeon pin it will be different,

9 though at the top isn't suprising, in fact thats tighter than a lot i've seen.


a lot of people leave forged pistons baggy as they are suposed to expand a lot with heat, modern forged pistons are pretty good so this isnt the case so much these days,

Edited by Joe C on 20th Jan, 2013.

Daz if MED machined the bores and supplied the pistons as they are with mine then your good to go mate.

It actually was a nine thou blade down the top left and another down the top right, so is that nine thou clearance or 18?

On 20th Jan, 2013 Joe C said:
piston clearance should be measured at the bottom as in the pic below.... if its mesured under the gudgeon pin it will be different,

9 though at the top isn't suprising, in fact thats tighter than a lot i've seen.


a lot of people leave forged pistons baggy as they are suposed to expand a lot with heat, modern forged pistons are pretty good so this isnt the case so much these days,

thats 18, but tbh its not unexpected,

clearances at the top can be hooooge

4 thou sounds good to me, if thats the clearance measured the way joe has shown, and not at the crown of the piston.

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1/4mile in 13.2sec @ 111 terminal on 15psi

I put the Accralites in at 0.002" clearnace as the bore grows as it 'breaks in' but as Joe says, modern materials and manufacture of forged pistons don't require such large clearancies as they used to.

One thing appears to be apparent with MED, they like to run their engines with a baggy fit when their cranks bearing clearancies are sized near double as well!

Edited by Sprocket on 21st Jan, 2013.

So.. Now happy with the clearance at the top of my pistons, tis normal. Regarding the actual skirt clearance, I spoke to a guy at Omega the other day who said for my pistons I need a bore size of 71.12 mm / 2.800 in. Bought myself a dial bore gauge and measured my bores to be 71.1238 mm or 2.80015 in, 1.5 thou larger than Omega specified. Hope this will be alright.

Edited by dazibee on 27th Jan, 2013.

That'll be absolutely fine. +or- 20/30 degrees C would see a difference of a thou in bore.

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1/4mile in 13.2sec @ 111 terminal on 15psi

0.15 thou?

Medusa + injection = too much torque for the dyno ..https://youtu.be/qg5o0_tJxYM

1.5 tho as its .80015 the last digit is going into 10ths.

Edited by matty on 28th Jan, 2013.

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1/4mile in 13.2sec @ 111 terminal on 15psi

Isn't .80015 equal to 800.15 thou?

0.0 .......... Tens

0.00 ...... Hundreds

0.000 ....... Thousandths

0.0000 ....... Tenths of one thousandth

0.00000 ....... Hundredths of one thousandth


0.00015 is fifteen hundredths of one thousandth or 0.15 thousandth

that's less than a gnats dick

Edited by Sprocket on 28th Jan, 2013.

Edited by Brett on 28th Jan, 2013.

Yes i moved to the darkside

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I wrote this down wrong. My bore measured at 2.8 inch plus 1.5 thou. So this is 2.8015 inch or 71.16mm. Sorry for the confusion