1 ring 2 ring ding a ling

[soonerbillz]

Buddy of mine overseas is always working on old shit.
Today he's telling me he is building a old YDS2 for vintage racing.
Say he's gonna run a 2 ring piston and run it with only 1 ring.
It's all piston port of course and pre mix.
Now I  know single ring pistons have been around for a while can't say I ever remember seeing a 2 ring piston run with 1 ring.
Hello???

[kramdua]

1 ring less friction but wear quicker. 2 rings more friction and longer wear.                                         
https://wiseco.com/blog/single-ring-vs.-two-ring-2-stroke-pistons-which-one-is-better

[soonerbillz]

I understand and know the 1 vs 2 argument.
My question is about running  1 ring in a 2 ring piston.

[m in sc]

i can honestly say ive never seen it done. but who knows. has he run it his way before? or is he experimenting?

[teazer]

I read an article years ago that IIRC concluded that 2 ring pistons made more power below something like 9,000 rpms and single ring made more power above that point. 

The idea of running a single ring in a two ring piston has been tried many times but no one that tried it, that I know of, has ever published the results.  I would imagine that if the piston has relatively thick rings, like one from that era, then ring flutter would be an issue and so would sealing if a ring is removed.  If the piston in question had thin rings - say 0.8mm to 1.0mm, then a single ring on a 2 ring piston would probably be OK as long as the ring land diameters were suitable.  I suspect that on a single ring piston they may be different.

Key thing in this instance, is that we are talking about a DS2 which won't rev very high before the clutch comes off and didn't make a huge amount of power, so why add to its stress and unreliability. If the OP is making a stronger crank, they could probably also find suitable 56mm pistons. 

Anyone remember how many rings in a TD1 piston? I suppose I could check the parts book if I could remember where I filed it.

Derek Pickard raced a DS2 Asama racer ie race kit back in the day in Australia but I have no idea if he's still around or what pistons he used.

[soonerbillz]

i can honestly say ive never seen it done. but who knows. has he run it his way before? or is he experimenting?

He's contemplating doing it and asked me I thought. 
So there's that.

[1976RD400C]

Think about this. Piston at TDC and fires, starts heading down. Top of piston clears the top of the exhaust port and exhaust gases start to flow. At the same time, the downward movement of the piston is pressurizing the crankcase. Now, when the top of the piston still isn't low enough to open the transfers, BUT the open lower ring groove, with no ring in there, is low enough and in the transfer port area and causing an open channel around the empty groove to the exhaust port. Kind of like a short circuit. All that being said, I tried running a piston designed for 2 rings with just 1 ring and didn't notice any difference other than the ring wore out quickly.

[kramdua]

i guess the next question is which groove to put the single ring in. in my opinion 2 rings would help keep the piston straight and that would give you less friction from the piston and cylinder.

[m in sc]

I just read a factory Yamaha gyt tech bulletin regarding the YL1,
and they recommend running just the top ring in the top groove in racing applications.  so, there's that 

[Td250]

Do not remove the botton ring.

If you do not fit the bottom ring: the land and below the top ring will beak away, and the top ring will not seal against the bore and will gouge the bore.