7 replies to this topic

[DeadSquare]

I have been looking for a set of 68.25mm diameter pistons, with a 38.12mm  (1.5" ) compression height and a 15.88mm (5/8")  gudgeon pin.

 

I have found some 45 year old forged piston with a 15mm pin that I could bore out, a 40mm compression that I could shave the crown, but a 68.65mm bore.

 

I could turn 0.4mm off the piston diameter, it is only .015" , but what about the skirt profile ?

[Spider]

Pistons and in particular the skirts are an odd shape at room temperature so that when at running temp, of course, they will expand, but there's more expansion where there's more material, so areas like the bosses where there's more meat, they are 'smaller' than the thinner parts of the skirt. The shape can be calculated, but you need to also know the thermal expansion characteristics of the alloy they are made from. There's been a trend in recent years by some piston manufacturers to use a Hypereutectic Alloy which has a less thermal expansion but these pitons don't stand up well to pinking and detonation.

In days gone by, the shape was formed by Cam-grinding them. This isn't a machine for grinding Cams (though similar !) but one that can follow a template.

 

How much material can you remove from pistons ? Bit of a $64000 question that one. Pitons are generally made as light as they feel they can, especially with cast types, so there's seldom much you can safely remove from them.

 

[Shooter63]

I know pistons are oval and tapered so the chances of turning them is pretty small, you might want to try phoning Omega up and see what they say. I presume its a small bore block already heavily over bored ( imp pistons? )

Shooter

[DeadSquare]

Pistons and in particular the skirts are an odd shape at room temperature so that when at running temp, of course, they will expand, but there's more expansion where there's more material, so areas like the bosses where there's more meat, they are 'smaller' than the thinner parts of the skirt.

 

How much material can you remove from pistons ? Bit of a $64000 question that one. Pitons are generally made as light as they feel they can, especially with cast types, so there's seldom much you can safely remove from them.

 

The odd shape is the problem.

 

I was wondering, having taken and noted careful measurements of the skirt, if I could spin them in a lathe and remove .0075" from the skirt with emery tape.

 

I'm told that they are solid skirt, so I was also wondering if I could cut the sides away to make slipper piston.

 

 

 

 

 

[DeadSquare]

I know pistons are oval and tapered so the chances of turning them is pretty small, you might want to try phoning Omega up and see what they say. I presume its a small bore block already heavily over bored ( imp pistons? )

Shooter

Near, but no cigar.  Imp pistons have a very low compression height.

[Spider]

This is a bit of a guide to the shape you need to maintain

 

 

Given that piston to bore clearances are of the order of 0.001 to 0.003" -ish (depending on lots of factors) and that you are looking to take 0.015" off, you really need to have them cam ground. Given you are working from forged pistons, this is even more critical as they tend to have more meat in them. Also, the crown will also likely need material removed as that expands the most.

Some of the engine reconditioning shops still have cam grinders for doing pistons. It was once somewhat common practice to do this as part of an engine reconditioning process (generally pre-war), the old pistons would be 'expanded' and then cam ground back to shape.

You could try and do as you are thinking with emery, it's not impossible, but you'll need to be very patient and take lots & lots of time doing them.

[DeadSquare]

I'l try reducing an old plus .010" piston with emery, and see How I get on.

 

I knew someone who knurled the skirt to make a piston large enough fit his Atco lawn mower in Rhodesia, when there were sanctions in the 70s.

[Steve220]

This is a bit of a guide to the shape you need to maintain



Given that piston to bore clearances are of the order of 0.001 to 0.003" -ish (depending on lots of factors) and that you are looking to take 0.015" off, you really need to have them cam ground. Given you are working from forged pistons, this is even more critical as they tend to have more meat in them. Also, the crown will also likely need material removed as that expands the most.

Some of the engine reconditioning shops still have cam grinders for doing pistons. It was once somewhat common practice to do this as part of an engine reconditioning process (generally pre-war), the old pistons would be 'expanded' and then cam ground back to shape.

You could try and do as you are thinking with emery, it's not impossible, but you'll need to be very patient and take lots & lots of time doing them.

That's the best visual of a piston from new!

Modern pistons are CAD designed and are milled to utter precision so they are perfectly round when at operating temp. I wouldn't be tempted to alter the shape.