-steves- wrote: ↑Thu May 30, 2019 5:35 pm
What do you think I should use as a piston ring? I can't seem to find it on the sheets, probably there somewhere I would think.
There are no o-ring details anywhere in the drawings, that I can find. The Idris model has those details in the article that was published with the drawings, but I don't think there is an article that goes with Wild Rose 3.
It looks like the piston rings are intended to be metric o-rings and a 7mm x 1.5mm o-ring seems to be suitable. Note metric o-rings are specified by ID and cross section for some reason, so the OD will be 10mm. Personally I always use Viton o-rings on pistons, as they are lower friction than silicone ones.
O-rings used as piston rings need a wider groove so they can roll a bit as the piston moves back and forth, so the groove in the piston as drawn needs to be widened to 2mm. The piston has to come out anyway to fit the ring, so the groove can be widened with a parting off tool and the piston rod is a convenient way to hold it in the chuck.
I'm not sure what sort of gland seal was intended and there's nothing on the drawings, but I'd use teflon tape instead of an o-ring in any case. Yes, teflon tape is the same as ptfe tape, as used by plumbers for thread sealing. If I asked for ptfe tape in the plumbing section of my local hardware store, I'd get a blank look........
Not sure how you'd go about packing the pistons with ptfe tape, as I've never tried it. Solid ptfe bar is available and machines easily, so it is possible to make pistons from it and both Merlin and Aster have used it that way. In the 'good old days' graphited asbestos string was used as piston and gland packing and there's a modern equivalent available. I've got some graphite packing made by Klinger, but have never gotten around to using it.
If your pistons are a very good fit in the cylinders, oil will provide some seal, that's what low pressure oscillators rely on......
Steam/air escaping up the exhaust usually indicates a sticking valve, so check the d-valves can slide freely on the valve rod and drive clamp. It could also be air getting past the pistons, but piston rings will fix that.
You'll be surprised how small a gap 40psi steam can leak through, so fit gaskets of some sort when you are reassembling the cylinders and valve chests. Modern liquid gaskets like Loctite 515, or one of the automotive ones, work well and are easy to use, but are a bit of a pain to dismantle. Oiled paper gaskets are a pain to make, but easy to dismantle. As with most things you pay your money and make your choice.......
Regards,
Graeme