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'O' rings for i.c. engine pistons

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Hi Folks,

I would appreciate your advice on the size of the groove for an elastomer piston ring on the Farm Boy engine.  Jerry specified a 3/32" (0.094") section 1" OD ring in a 0.105" wide groove 0.110" deep.  I've drawn this up, together with the sizes recommended by Tubal Cain in his Model Engineer's Handbook:

Looking at the sketch it seems that Jerry's design is completely unconstrained by the groove in the piston.  The ring is 1" OD as is the bore of the cylinder.  In Tubal Cain's design the ring is slightly compressed (pinched) by being in a groove that is slightly shallower than the ring section and has room to expand sideways when squeezed by insertion into  the bore.

Clearly many Farm Boys have been succesfully made and run so: any comments?



All the ones I have fitted have been done to the charts in TC's ME handbook and they give good compression and minimal drag. I use a brake cylinder hone on the bores.

Though the table gives 0.125wide x 0.098 deep on a 3/32" nominal which is 0.103" actual

Brian Rupnow:
I use Viton o-rings with a "nominal" cross section diameter of 1/16". They actually measure 0.070". I make my groove on the piston 0.094" wide (Parting off tool) x 0.058" deep. Only one ring is required on the piston. I have many engines running this way, both i.c. and steam and they work fine.

I'm not a fan of O rings but I would go with Jerry Howell's idea. My reasoning is to let the O ring do the sealing without adding any pressure to it. If a 3/32 (nominal) ring is actually .101 (that's what my rings measure) then only going .092 deep would mean that the ring would be compressing .009 plus .001 (cylinder bore .998) or .010. To me that seems like an excessive amount of compression when only a few thousands will do the job. You have to understand that it's not just .010 at one point it's all the way around the 1.00 dia. bore. That's a lot of stiction!

I do have made an engine with an O-ring at the piston, only once; when cold this engine doesn't have any compression, nothing, and after a few minutes of startup, the compression comes back, and the engine starts. This behaviour is really confusing, all my other engines (about 10) have one or 2 cast iron segments, or no segment at all for smaller diameters.

this point suggested to me that the section of O-rings are not constant, and may grow up with fuel, oil and heat. 


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