Peter,

On some engines, there is an oil "groove" right at the top of the piston. It is of a very slightly reduced diameter compared to the main body of the piston. The purpose is to improve oiling of the cylinder. The fuel charge wets this area and because it is trapped on the upstroke it is forced into the clearance more aggressively. At least, that is my thought.

When new (or very clean) it is not so noticeable. Once run, especially using castor, it takes on a darker color since carbon builds up there, while the area right below it stays shiny.

If you look thru the exhaust port with good lighting, you will see that there are no corresponding wear marks on the land area. That pretty much proves it is recessed without taking it apart.

The OS .30S I have in my hand right now is like this. Since it has been run a bit, the groove is darker, just as you describe. It is also possible that it had a surface treatment, but I doubt it. To be honest, I don't think I've ever had a brand new OS S-series engine in my hand. I have been cleaning up the grandfathers and putting them back to work. Either the .30S or .35S does a nice job with a Super Combat Streak for example. The shortcoming of that setup is the aerodynamics of the plane, not the limitations of the engine.

Since we are talking about pistons, the best of them are not pure cylinders or even shallow cones--they are barrel-shaped. The biggest diameter is just slightly below the crown. It should stay shiny. That's the wear band. That's where most of the gas seal comes from. Above that it rolls in a tiny bit. Helps with oiling. Below the wear band it also rolls in a bit, which reduces drag. Since the area below the seal is where most of the length of the piston is, it is pretty easy to measure the taper. In a way, this makes the piston tolerant of rocking during operation. We need piston skirts in order to get the port timing we need, otherwise you'd see more "skeleton" pistons and cutouts. Not too important to get every last gram out of sport motor piston, but at high rpm it becomes a big deal.

Dave

Incidently , That 30S is the exact same as the H-29 R assy . And the ROD & wrist pin . some are listed as such. on the packet . So can be reved like snot . ONCE RUN IN .

brings us to . ..  Piston Crown . Was said the piston was like a dust bin . when Heated the CROWN expanded . So running in , was till it'd stopped . Forever , whereas, if
you chamfered the top edge , it'd run in quicker & wouldnt catch in / scour , at the port .

But translated to japanese , Chamfer ended up a cut away . Hence the cutaway at the top , so the expanded crown wont catch in the port. full whellie or runningin .

Dave,

Yes, the absence of vertical marks in the black band area is telling.  I'm now understanding the piston is turned down slightly smaller near the top.

Just to be sure I understand, how does the NOS set look?  No blackness, as you suspected?

thanks,

Peter

p.s. Attached two pics.  One is ringed, one not.  Pretending the caging is absent, you can still tell the two apart.

And....