Oscillating engine leaking air during testing

[student123]

My novice build of the littlemachineshop oscillating engine continues…

http://littlemachineshop.com/Projects/OscillatingEngine.php

piston brass,all else aluminium testing on compressed air, no long term plans for steam running. Here's a pic (prev posted) all assembled:

Having , with help from you guys, stopped air leaking through a gasket, things have progressed.
The cylinder moves slightly (maybe 5% of turn on the crank) but soon stops with air escaping between the cylinder & the upright.

Presumably I need to spend some time lapping the cyl to the upright?

I did spend a little time smoothing both surface with oiled wet & dry., but I did them separately.
Should I be rubbing them together with some toothpaste in between ??
if yes approx how long should it take (I understand t depends on the existing surface quality , but typically lap for 10 minutes ? an hour ?)
Or do I smooth each out surface separately with wet & dry?

Would a stronger pivot spring help?


I should add that occasionally , when turning the engine by hand , I get some resistance. Maybe 1 in 20 times. If I turn it back a little then go again things seem ok. I just mention that symptom in case it indicates a prob i haven't figured out yet.

Mike

 

[tel]

Yo Mike, your spring looks plenty beefy enough to me, in fact it could probably stand backing off a bit. You problem sounds to me like either the trunnion pin or the hole for it in the stand is not quite square to their respective surfaces. You could try easing the hole out by a few thou and seeing if that lets things seat a bit better.

 

[bearcar1]

I've really enjoyed watching your progress Mike and am also glad to hear you have at least one of your snafus overcome. One method that will give you a visual indication of the mating surfaces of the cylinder and upright is to apply a product called Prussian Blue, a Blue dye that is widely used to fit bearings and such in industry. Dykem layout fluid can be used as well and if you do not have any of that around, a felt tipped marker has been known to be used for such things. : The idea is to apply a colored contrast coating to the offending pieces and then to rub them together. Gently. Then separate the pieces and any high spots will have been abraded off and more work is required to bring the surfaces to flat. I'm with Tel though, and unless you are certain the sliding surfaces are not sealing well, have a better inspection at the trunnion pivot to be sure it is not causing the cylinder to lift. Keep after it, it will loosen up and be running in a jiff.

BC1
Jim

 

[student123]

Jim & Tel,

Thanks, I'll have a go on those lines.


Mike