Rule of thumb re. cylinders and pistons

[jscarmozza]

I'm curious if there is a rule of thumb regarding the piston rings used in a bronze cylinder vs. a cast iron cylinder. I'm looking into building Kozo's Heisler in 1" scale. I was going to cast the cylinder in bronze and was thinking about the pros and cons of lining the cylinder and using cast iron rings, Kozo recommends using asbestos graphite yarn in unlined bronze cylinders, is this a good method? John

[Builder01]

O rings work well in bronze cylinders. I have bronze cylinders and switched from packing to O rings. (After I blew the packing out!).

[Bill Shields]

The yarn works well if you do it correctly... Brits been using it for decades

O-rings are easier

[Builder01]

I guess I don't know how to use the yarn! (Thanks Bill). Yes, O rings are easier, even I can use them.

[LVRR2095]

I guess I don't know how to use the yarn! (Thanks Bill). Yes, O rings are easier, even I can use them.

You REALLY have to pack the graphited string in tightly. I roll the pistons on a piece of plate glass to really cram it in there. It not only compresses the packing, but it gets an almost glazed appearance. The piston should be fairly tight fit when being installed, it will loosen up as you run in the engine.

Keith

[jscarmozza]

I have a Pacific project on the bench that had a metal ringed piston on one side and a yarn packed piston on the other (in cast iron sleeves), I was astounded by how much yarn was packed into those grooves! So yarn or o-rings are legitimate alternatives to metal piston rings. What type of O-ring material and would they be round or square cross section? I think I know the answer but I'm going to ask anyhow, would bronze or brass rings be ok or is it best to go with 'soft' rings on bronze cylinders? Thanks, John

[gwrdriver]

In my opinion a primary consideration for bronze cylinders, after the basic mechanical requirements are met, is preserving the bores so I would always want to use a soft (ie,non-metallic) ring. O-rings would be my initial choice, but I'd think one of the ever-evolving modern plastics would make excellent non-marring rings.

[Builder01]

I used Aflas rings from McMaster Carr. High end temp is a little bit higher than Viton. Supposed to be good up to 450f. My first set have been in for three years now.

[jscarmozza]

For o-rings, should I use the standard dimensions for the groove or do something different? John

[Bill Shields]

There are dimensions for static and dynamic.

I typically work to .003 squeeze

[Builder01]

The squeeze should be on the ID and OD. The side of the groove should provide a little clearance.

[Builder01]

The squeeze should be on the ID and OD. The side of the groove should provide a little clearance.