Viton O Ring for Steam Engine

[petep]

Hi everyone, I'm currently building a Bolton No 7 horizontal steam engine.
The plan calls for graphite yarn packing but I'd like to replace that with a Viton- O-Ring which, according to several posts, is quite satisfactory for a steam engine.
So, my questions are:
1. I'm presuming I require a Viton-O-Ring with and OD of 1.5 inches?
2. I presume that I need this O-Ring should have a CS of approximately 1/8th inch.
3. Assuming that this O-Ring has a CS of 1/8th Inch, I presume the groove width would ever so slightly wider and ever so slightly sallower.
Am I on the right track here.
Thanks guys.
Pete

 

[Brian Rupnow]

Don't make the piston o-ring grooves until you have the o-ring "in your hands". The diameter they list is a "nominal" value. I use 1/16" Viton O-rings on my model i.c. engines, and in reality they are 0.070" diameter. I machine my piston ring grooves 0.058" deep, which means that the ring "crushes" about .012" radially, and I make the slot for that 1/16" o-ring .094" wide, and it works fine.---Brian

 

[petep]

Thank you Brian. I was hoping to hear from you. Sensible advice.

 

[GailInNM]

I agree with Brian. It is very important the the width of the groove is much wider than the cross section diameter of the ring. This gives the rubber a place to flow when the ring is compressed. It also allows the ring to "roll" when the piston movement reverses and this reduces the stiction effect as well as trapping some additional lube under the ring .For an 1/8 cross section ring a minium would be 5/32 wide groove and I would go wider than that up to 3/16 wide.
Gail in NM

 

[petep]

Thanks Gail in NM. Have ordered the Viton O Ring today and will definitely take your advice. Much appreciated. Take care.
Pete

 

[vederstein]

The size of a numbered oring is not a "real" size. It's actually the depth of the oring groove for approximately 10% compression:

A 000 series oring has a width of .070" and a groove depth of .060"
A 100 series oring has a width of .103" and a groove depth of .094"
A 200 series oring has a width of .139" and a groove depth of .125"
A 300 series oring has a width of .203" and groove depth of .188"
I forget the width of the 400 series oringm but it has a groove depth of .250"

...Ved.