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Piston fit with o rings

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crueby:
For model pistons with rings, the compression of the ring that the charts give would be pretty snug, tight for an easy slide fit. Good starting point though!

Alex:
- When I finished my 3-1/2" gauge Tich back in the '90s, I put "O" rings on piston, and piston/valve rods.

For the valve rods, I made a "D" bit to get the OD just "whisper" tight, and used a simple flat plate with screw to hold them in place.

For the pistons, first try too tight, so I put graphite packing on. One day, when it would not make it up the hills at the local club, I re-did the O-ring depth on the pistons, and it transformed the locomotive.

I have not run it in a decade or so; maybe I should pull it out and check, but I'd expect it to roll just fine.

- O-rings on my Shay locomotive pistons, etc,, and, on my "will be finished sometime" Ivatt 2-6-0, o-rings on pistons, glands and piston-valves.

- when I started out, someone (Art Ellis, if anyone in North America remembers) gave me a ME handbook about 0-rings, but the writer used too high a squeeze factor; we are not running at 10,000 psi here...

Bobsmodels:
Hi
Just to add a bit to the suggestions.  I have used o rings in a number of engines.  You really want very low friction.  I have found that a difference between the diameter of the O-Ring and the Cylinder should be about .004" and .005".  That gives a nice seal and very little drag.   I would also recommend a 1/16 O-Ring rather than 1/8"  from a friction point of view.  For example if you have a perfect 1" bore  then you want the groove diameter to be 1.00" -( 2 x .070 (actual diameter of a 1/16 O-ring) - .005) = .865 diameter.  Note does not mater what diameter your piston is, the groove diameter is the same as it is based off of the cylinder bore.  Also minimal roll for width, I use .082 for a 1/16 ring.   Attached is a sketch showing arbitrary piston diameter could be any reasonable size, and PDF showing how I did it for my Gade.  Here is a link to a video showing low friction with my Gade, which had about 600 hours on it at the time with the same o-ring, vitron.

 http://www.youtube.com/watch?v=1QazAr4b3KY

Bob

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