Should Pistons be perfectly square in barrels?

[paulandchez]

I'm well on with my rebuild now, and got to the point where I wanted to measure the deck height so I can shim the barrels to get them all about the same. That in itself isn't the issue. However, when I've measured I've found the Pistons aren't square in the barrels. I ended up fitting a plate on top of the barrel and using plasticene between the Pistons and plate, rotating the engine by hand to squash the plasticine and then removing the plate to measure.

These are the results measuring both sides of the piston along the pin axis:

Flywheel side first, then fan end
No 1 - 1.61 1.58
No 2 - 1.62 1.67
No 3 - 1.64 1.76
No 4 - 1.48 1.52

As you can see they aren't square in the barrels. It doesn't look like all pistons are sloping the same way. With exception of No 3, they're all within 0.04mm and the odd one is 0.12mm.
So....what do you think? I could remeasure No 3 again, just in case it was an error, but I checked it twice. Am I expecting too much for all these to be square?

Thanks

Paul

[silverbullet]

I have no Aircooled experience but fwiw:
Pistons do rock as they go over TDC due to the expansion clearance on the piston. #3 does look a bit off though.

[paulandchez]

Thanks for your input. I understand they rock, but these measurements are along the pin axis, not top to bottom if you know what I mean

[PetenAli]

Not sure if this helps and you've probably done it anyway - but have you fitted the pistons the right way round? There should be an arrow on top of each piston pointing towards the flywheel.

[Smcknighty]

I have nothing useful to add other than I thought the pistons were meant to be round...

[scottbott]

was waiting for a reply like that,beat me to it

[937carrera]

Paul,

I don't know the answer to your question, but are you thinking that the difference is coming from the pistons, barrells, crank position or somewhere else ?

Just mentioning this because all of these items could create the difference you are seeing, perhaps by design.

[paulandchez]

Not sure if this helps and you've probably done it anyway - but have you fitted the pistons the right way round? There should be an arrow on top of each piston pointing towards the flywheel.

Yep, all fitted right way maye

[paulandchez]

Paul,

I don't know the answer to your question, but are you thinking that the difference is coming from the pistons, barrells, crank position or somewhere else ?

Just mentioning this because all of these items could create the difference you are seeing, perhaps by design.

Not sure really. Crank has been reground, new bearings fitted and new Pistons and barrels. Wondered if it was case that was not square, or just a build up of manufacturing tolerances and therefore nothing to worry about

[937carrera]

New pistons & barells eh What make ?

I would measure gudgeon pin to top of piston as a first check. If they are the same, then the difference is coming from somewhere else. If not then it's either by design or a manufacturing / tooling error.

[silverbullet]

I have nothing useful to add other than I thought the pistons were meant to be round...

Pistons are actually subtly oval, to allow for the differential expansion across the pin axis compared to that measured at 90 degrees to the same.

[paulandchez]

New pistons & barells eh What make ?

I would measure gudgeon pin to top of piston as a first check. If they are the same, then the difference is coming from somewhere else. If not then it's either by design or a manufacturing / tooling error.

They're AA ones. Came from Coolair and are from before issues were found with the coatings on some of the pistons