Speeditwin Dimension/Clearance Help

[gearhead4]

Still working on my Model U Speeditwin project and am trying some other cylinders I had with the pistons that came out of the engine. My book is saying .004-.006" clearance cylinder to piston. Does this mean all together or each side (180 from each other)? The clearance I’m getting measuring the bores and piston diameter is .011-.012" and I just want to know whether this would be acceptable. I’m just dying to put this engine back together and paint the cylinders up before hand!

Thanks for the help in advance!

[garry-in-michigan]

The machine used for turning the pistons gives three thousandths clearance on each side of the crown and two thousandths clearance on each side of the skirt. The top of the piston gets hotter and swells more than the skirt.

[Mumbles]

quote gearhead4:

My book is saying .004-.006″ clearance cylinder to piston. Does this mean all together or each side (180 from each other)?

The given clearance is the total clearance between the piston and the bore it is running in. Cylinders are bored and honed to their exact given size at the factory while the pistons are machined to give the desired clearance.

To find your piston clearance, the cylinders bore must first be measured, in several places for a used motor and then compared to the pistons diameter, which is usually measured a quarter inch up from the bottom of the skirt and at ninety degrees to the pin. The difference between these two numbers will give you your cold piston clearance.

Some pistons are ‘cam ground’ and are actually egg shaped a bit to allow for heat expansion to make them round again when they come up to operating temperatures. This is more common on automotive type engines.

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