What is it, an oil return?
Not sure on material, you can get cheap natural rubber rings, or silicone,
or viton. The cheap rubber ones are better with oils than silicone types,
silicone swells with oils but it isn't a problem on any of my locos.
http://www.marcorubber.com/material_chart.htm
As for size of seating, a 3/16" id ring will be ~5/16" od (its 0.070"
section). You are not going to need much nip, squeezing in callipers is an
easy way to find the nip you need, ~55thou depth is about right for 2 halves
meeting tight.
Simon
ps, John got his reply in as I was absorbed by marcorubber's site, quite
useful!

----- Original Message -----
From: "david_steam" <no_reply@...>
To: <halfinchlivesteamforum@...>
Sent: Thursday, May 28, 2009 10:24 PM
Subject: [Half inch live steam forum] Re: O ring query


> Ok I will come clean, its for a Royal Enfield 250 cc engine. The two
> crankcase halves have a 3/16 passage for (I think a low pressure return)
> are just butted together with a bit of sealing gunge and no gasget. So
> what type of material and what size and depth seating do I need?
> David