Thanks Walt. It's interesting, both the D400 and D600 series show the same numbers for ring gap.

The F series engine spec shows they went with a closer ring gap of .007" to .017" for the top ring though the same .015-.025" for the bottom ring. As I delve into that, I find there is controversy over this practice. Some 2 stroke manufacturers went the other way in recent years specifying a closer gap for the bottom ring.

Well shoot. I just checked these new rings, the ring gap exceeds .025" by a large margin. Closer to .036"! The old rings in comparison are just over .015" in the same bore. They were advertised as replacement rings for LawnBoy, and they would "work", but.

Any recommendations for a good brand or source for rings?

With a new cylinder, get a set of original lawn boy rings (e-bay). The gap should be very close to .015. The gap could be less, but this is how they were designed and manufactured.

Well I ended up already ordering a set of Stens brand the other day. I saw a couple sets of LawnBoy on eBay, they sure are proud of them, they are kind of hard to find.

Should any kind of sealant be used on the gaskets? (cylinder, reedplate, magneto plate, all to crankcase)

I used Permatex copper on the muffler just to make sure there aren't any leaks. The manual specs thread-loc on the magneto plate screws and the screws securing the reed to the reed plate. I have Loctite 271 on hand but I'm guessing they mean a less than permanent thread-loc.

Here we see the top ring in the picture, an old, worn out ring, in a new cylinder still showing approx. 015" end gap. The lower ring exceeds .025", closer to .028" and is therefore, out of LawnBoys specification of .015" to .025", I brought this to the sellers attention, and they wanted to argue a little bit "your cylinder is worn" now they don't say anything.

I suppose these rings would "work" but they cannot be considered acceptable as such? Whaddaya think?

The Stens rings are within spec but on the outside range. Except, the Lawnboy manual is confusing, depending on what specification is adhered to, even these are way out -

https://lookup3.toro.com/ttcGateway/acrobat/manuals/lball64.pdf 

I didn't notice at first there are two .PDF pages on specs. The first table indicates .015"-.025" for piston ring gap.

Scroll down to the 2nd page and it indicates .005" to .015" for a number of engines, including D600 series. Why the discrepancy?

Unless the ring manufacturer furnishes oversized rings there is no way to effect a .005" ring gap. Were OEM or OMC replacement ring sets always "file to fit"?

I don't remember ever having to file OEM or OMC rings, it's the aftermarket ones that I always have to file. The latest rebuild I did was an F series and yeah straight out of the box there was no gap once they were placed down in the cylinder. That set was made by Rotary...I think.