How thin above top ring?

[Nutball]

What do the experienced builders recommend as a minimum piston thickness above the top ring if cutting a popup?

 

[David Young]

.1 was the standard I’ve gone .085. This also is port shape dependent

 

[Nutball]

How is it port shape dependent? My main concern is the piston handling the force of the ring changing direction. I suppose a long stroke is easier on acceleration, worse on top speed, or worse on both vs short stroke?

 

[Agent Smith]

How is it port shape dependent? My main concern is the piston handling the force of the ring changing direction. I suppose a long stroke is easier on acceleration, worse on top speed, or worse on both vs short stroke?

I'd be more concerned about crown thickness being enough for heat dissipation long before it handling the ring changing direction

 

[Funky sawman]

How is it port shape dependent? My main concern is the piston handling the force of the ring changing direction. I suppose a long stroke is easier on acceleration, worse on top speed, or worse on both vs short stroke?

The rod length would also be a determining factor, as it controls dwell at top and bottom dead center

 

[Ronie]

How is it port shape dependent?

I think the flatter the port the more force that would be put on the edge of the piston when it pushes the ring back in, just a guess.

 

[huskihl]

The ring locating pin is the issue. When you get much below .090” there isn’t enough aluminum to sink the heat away fast enough and the pin will loosen

 

[Funky sawman]

The ring locating pin is the issue. When you get much below .090” there isn’t enough aluminum to sink the heat away fast enough and the pin will loosen

Has anyone tried gapless rings in a 2 stroke? Like totalseal rings for the automotive industry

 

[huskihl]

Has anyone tried gapless rings in a 2 stroke? Like totalseal rings for the automotive industry

Some older stuff had them. Nothing I’ve ever personally seen, just some old pics on AS. @jacob j. will know

 

[Woodslasher]

Some older stuff had them. Nothing I’ve ever personally seen, just some old pics on AS. @jacob j. will know

The old 1-4* or 1-5* Macs had them, I think a few 10-series had them, and a bunch of other random Macs that Jacob will know.

 

[David Young]

Exactly Ronie

@Nutball
What part of the port do you think would be most important?

does it matter if it’s a 1 or two ring piston?

what if you left the bottom r

 

[Nutball]

I was considering cutting a popup to raise the exhaust, but I already decided to cut .038 since I'm cutting the cylinder down .040. I'll just grind the port higher. I'm going to try an off center popup to match the combustion chamber.

 

[David Young]

Popup will keep your ex and trans where they are. Sometimes it’s good to not grind if you can’t improve the port.

wider the port the more important putting the ring back in is. it’s important on the top of the port and. Even more important on the bottom because the ring has less piston supporting it. Single ring piston have more meat above the ring than the 2 ring pistons. And the area between the rings is weak. For a popup I believe a single ring piston is much better.

 

[Ketchup]

I took a popup on a 2511 down to 0.040” above the ring pin. The pin was fine and no deformation of the remaining crown. It died after about 15hrs of use for other reasons so I don’t know how long it would have lasted. The small saws seem to tolerate smaller margins.