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MG Midget and Sprite Technical - Piston clearance. Is this ok?


I'm rebuilding my 1275 and got it sleeved back to STD. The ring gap looks good at 0.011" but not sure about the pistons. They fit snuggly in the bore but there's a big gap at the top and I can fit 0.032" worth of feeler gauges in between the piston and bore which seemed odd. So I measured the piston and they are 2.779" at the skirt but only 2.750" at the crown.

I don't have an inside micrometer to measure the bore but using my verniers I got about 2.780' inches so I think that's ok.

I know there will be a bit of expansion on the crown of the piston but is this correct with the top of the piston 0.029" smaller than the bottom?

Greg
Greg H

Greg,

always measure piston clearance at the skirt. Cold pistons are tapered and oval so when hot they should become round and straight. The figures don't look too bad to me

Mike
M J Pearson

That is ok Greg and as it should be. The rings do the sealing the skirt guides the piston. When the gas pressures build up behind the rings the outward force seals the rings against the bores. You should be looking for 2+thou clearance skirt to bore. We tend to run 2.5 for road and 3 for race.

Peter
P Burgess

Goodo. worried about nothing then. Just surprised it was so much smaller at the top. I didn't think the pistons would expand this much and if there was still a big gap when running I thought there would be more pressure on the rings and they'd have to work a bit harder.

1st ring was just square section but 2nd ring had chamfer at top which I know should get forced out. 3rd oil.

I shall continue on then.
Greg H

Hi Greg

You will be wanting to know all about crevice gas pressure next :)

Peter
P Burgess


No Peter I defintely do not want to know about how much gas is in your crevice!

and my engine just needs to be a nice runner. I'm actually detuning it and I'm even considering not putting the 45DCOE back on and going with the twin SU's. When I was younger I used to give it a hard time and 15 years later I'm still feeling guilty about it.
Greg H

Id re-check that .011 ring gap....

that seems a bit thin to me, im trying to remember back, anyone else share my opinion.

Prop
Prop

Hi Prop

11 thou or so gives around 4 thou per inch of bore which should be fine for a road engine. Many proprietory pistons will be pre-set to around 18 thou.


Peter
P Burgess

Quoted piston clearances for the 1275:

Bottom of skirt: .0015" - .0021"

Top of skirt: .0029" - .0037"



Fitted Ring gaps:

Top: .011" - .016"

Second & Third: .008" - .013"



Deborah Evans

This thread was discussed between 29/05/2011 and 30/05/2011

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