How accurate should an internal cylinder diameter be?

[student123]

I'm on my first engine build, the 'brassy babe':

http://www.davegoodfellow.com/plans.pdf
steam wobbler, I'm using mostly aliminium.

When I measure the internal diameter of the cylinder, moving the digital calipers around the internal circumference , the readings vary from 5.0mm to 5.3mm. (I may also be tilting the calipers slightly
causing measuring distortion).

Assuming accurate measurements, is this cylinder likely to work , or do I need to try again?

Also any tips on getting the piston to fit snugly would be welcome.

Mike

 

[b.lindsey]

Mike, it could just be how you are measuring but if not its probably a bit too much out of round. Generally a goodo sharp reamer of the size bore you want will give you a nice round and smooth hole, then you can adjust the piston to fit the finished bore.

Bill

 

[Cedge]

123
You're probably introducing error with the caliper. It's a lousy tool for measuring interior bores. You want a "free running" fit rather than a tight one. You'll also want to hone the cylinder walls to get them smooth and straight. Making the piston to fit the cylinder is usually the easier way to get things to fit up nicely.

Steve

 

[Tin Falcon]

as Cedge mentioned calipers digital calipers are not the correct tool for measuring a bore.
You are doing well to measure at different points in the bore. You also want to measure both ends to check for taper.
a T gauge and micrometer works well.
a bore gauge works well also but pricey and a bore micrometer also works well but very pricey and limed in range to boot. .
the ideal cylinder bore will be round ,no-taper and smooth. Then make the piston to fit.
Digital electronic calipers are great in the shop. but not for high precision I expect them to be within a thousandth or two that is it. When working in 10 thousands use micrometers.
As as as the fit the piston should not move when the end of the cylinder is plugged and should move feely or with very little force when both ends are open.
Tin

 

[shred]

FWIW, Brassy Babe is very forgiving for piston/cylinder fit if you're running it on air; just add more air to compensate ;D. I'd see how the piston fits and go from there. As long as it's reasonably close, it'll work. Ideally you'd get a 'pop' when you pull it out of the cylinder fast.