Running engines in question.

[kwackers]

Slightly odd question.

I've finished a Stuart No8 and have run it on air for a couple of hours. All the mechanicals now turn over really nicely, no sticking or play and the engine will run slowly on a few pounds of air pressure.
However the piston is still 'tight' in the cylinder. It runs on a couple of cast iron rings which simply seem to be 'gripping' the bore pretty well.
I've been sending plenty of oil down with the air (in fact the entire engine has practically been sat in an oil bath).
(I've also whizzed the end cover off and the bore is fairly shiny where the piston has been travelling).

I've seen a number of engines that will spin on slightly when the flywheel is spun, that's the sort of free running feel I'm after.

Any hints or tips? (It probably doesn't really matter as the engine will simply live in a glass cabinet, but I feel it should feel 'free' before I can consider it finished...)

[Artie]

Hi kwackers, could you not make another piston that didnt use rings? Or use a packing rather than the cast iron ring?

The point is I guess that an internal combustion engine can use a loose ring as it relys upon combustion pressures to press the ring against the cylinder to make the seal... with a steam engine or air engine you need a bit of ring to bore tension to make that seal... it is never going to be really free, as it sounds you want... hence the suggestions... otherwise a LOT of running will help some...

Good luck mate...

Rob T

[John Hill]

I think if you have a shiney surface it will not wear very fast,  how about giving the shiney bits of the bore a once over with medium to fine abrasive paper  to take the shine off and repeat until the piston has the fit you want?

[doubleboost]

Hi
On petrol engine blocks that i have rebored (full size)
we use a cylinder hone to get a nice x hatch finish dull looking this gives the rings something to bite in to for the first run in period
John

[kwackers]

Thanks for the replies.

Having had a play it's not as tight as I remember although not as free as some I've seen but thinking about it I'll leave it alone. Seems like a lot of effort to fix given the engine runs well on a small amount of air.

Be nice if I could get it free enough to run on an aquarium pump - although I reckon that's asking way too much!

Anyone use any small and v.quiet compressors for 'demo-ing' engines?