SpudFiles

MrCrowley wrote:On another note, anyone familiar with the practice of keeping a part that has epoxy drying on it in your room so you can rotate it throughout the night/day to stop epoxy dripping?

Poorly thought out casting plan ;P

jackssmirkingrevenge wrote:Poorly thought out casting plan ;P

I'm far too lazy to bother epoxying the second joint a few hours after the first

To give you an idea of what I've found to be a reliable setup, my piston uses 1.75" diameter washers to form the inner circumference of the groove, leaving a gap of (2.067-1.75)/2 = 0.16" for the ~0.19" thick o-ring to squeeze into. Even though the 2" diameter washers are somewhat smaller than the 2.067" diameter pipe, the seals have never slipped and never leaked.

Good thinking, I really should calculate the tolerances of my o-rings

I think you may be half right, I can't achieve the correct compression due to my large washers being slightly too small. Just need to think of something I can add to boost their diameter, epoxy is tempting but I don't think it has enough purchase on the 2.5mm thick washer edge to not get pushed off.

What about a bigger thicker o-ring? Or possibly a harder compound to the o-ring

Not a bad idea, I might stop by somewhere today if I get a chance and see if they have some better made o-rings. A thicker o-ring would probably work as long as its width did not increase, only the 'thickness'. If the width increases, it wont fit in the grooves any more because it'll be too wide.

So you're looking for an oring with an oval cross section? Why not widen the grooves?

Because then I'd have to destroy the piston and re-epoxy everything

I tried widening them a fraction yesterday with a dremel, it worked but only just enough. Would take for ever to widen them a significant amount.

Dude build a new one... This one looks like sh¡t.

It used to look new, but then it got used in a piston friggin hybrid. What do you expect?

Most of the crap is teflon tape, grease, broken o-ring and dirt (small pieces of teflon, dust, epoxy fragments, rubber, other crap) from rolling around on my bench. Except for grease smudges and soot, there's nothing different between this piston and a new one.

Not to mention making a new one costs $30 and takes time and patience to put together.

Nothing wrong with it if it works... (yes, it does work. The o-ring just needs replacing, I can't fix the large washer diameter issue as they only come in that one diameter from a local supplier).

Blah blah blah

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I've probably made the fatal mistake of epoxying the o-rings inside their grooves with a spacer (to get good compression) to try stop them from being torn out of their grooves. In my experience, araldite wont work well and will crack after a firing or two but I thought I'd give it a shot.

I was going to leave it to dry inside its housing but I thought it might dry with the o-rings compressed, it wouldn't be good to have them hold their shape. Would be better if they weren't compressed until the araldite dries, even if that breaks the bond.

Are you using any lubricant on the o-rings?

I was thinking (smell something burning did you) what if you where using squared o-rings like what's used in some 2 stroke cylinder heads for the mounting seals (head gasket). Only issue's I can think of is:
1. In the engine they are stationary, in your cannon they move.
2. In the engine they are a compressed fit more so the in your cannon. 3. Is changing o-ring configuration worth changing piston configuration on your cannon?

Being as how you seem to be getting a few shots off as it is now, I myself am not so sure I'd be willing to build a second piston set up to try a different o-ring style.

Seems as if I miscalculated how much the epoxy would expand when dry. Took bloody forever to get the piston inside its housing, I have to bang it against the seat with a hammer or heavy object and it moves back about 3mm when the cannon is fired as a pneumatic at 100PSI. How's that for o-ring compression, SpudBlaster?

I have three options:
1: Leave it, should work normally at high pressures.
2: Sand down the o-rings slightly
3: Un-do all the epoxy and take the o-rings out and start from scratch.

The first two options I don't mind so much

Start over!!!! Do it!!!

MrCrowley wrote:Seems as if I miscalculated how much the epoxy would expand when dry. Took bloody forever to get the piston inside its housing, I have to bang it against the seat with a hammer or heavy object and it moves back about 3mm when the cannon is fired as a pneumatic at 100PSI. How's that for o-ring compression, SpudBlaster?

I have three options:
1: Leave it, should work normally at high pressures.
2: Sand down the o-rings slightly
3: Un-do all the epoxy and take the o-rings out and start from scratch.

The first two options I don't mind so much

Is there no one in all of new zealand with a lathe? for the effort you're putting into getting this piston working, I feel like a machined one would be worth every penny