Scooterotica

Standard 66mm pistons seem to be supplied with 3 rings. I have been informed that it is permitted to use 2 rings, i.e. remove one ring (I presume the bottom ring of the 3).

As 71 mm pistons (Asso, etc) only use 2 rings, I can see the logic.

My Indian GP has a Mahle licensed 66mm piston with 3 rings, the top two are chrome and the 3rd ring is normal.

However the width of the rings is different on the two makes of piston, so is this why there are the differences in number of rings for each make of piston?

Any answers gents.

when i stripped a stage 4 ralph saxelby top end to replace a piston..he,,d only fitted the top two rings for wotever reason,so i left it out on the rebuild...no probs with it

Ive have posted a bit about this on the old Foum & on LCGB site
The thickness of rings have a major effect on what maximum RPM you can go to with out
doing terminal damage.
Basically the thinner the ring the more RPM they can take as ring flutter doesnt break them
Lots to talk about but the basics below.

Piston Acceleration is found using the formula:

G=0.000457 x RPM² x S" x (1+ (S/2L))

So using above and rpm below:

S= 2.28" (58mm Stroke)
L= 4.21" (107mm conrod)

RPM @ 6500 G= 56000 ft/s/s OK with 2mm rings and below
RPM @ 7000 G= 65000 ft/s/s getting towards the high end of 2mm rings
RPM @ 7500 G= 75600 ft/s/s Should now use 1.5 mm rings and below
RPM @ 8000 G= 84800 ft/s/s getting towards the high end of 1.5mm rings
RPM @ 8500 G= 95800 ft/s/s Should now use 1.0 mm rings and below


Where limits are
2.0mm Ring max piston acceleration = 65500 ft/s/s
1.5mm Ring max piston acceleration = 84700 ft/s/s
1.2mm Ring max piston acceleration = 105800 ft/s/s
1.0mm Ring max piston acceleration = 127000 ft/s/s
0.8mm Ring max piston acceleration = 158750 ft/s/s
0.63mm Ring max piston acceleration = 201600 ft/s/s

So from the above you get a good feeling that really 1mm ring should be used on 8500+ RPM motors.
Hence I was amazed when AF brought the RB22 out with 1.5mm ASSO piston
Also why I change my rings out and checked piston at 3K on my TS1's.
But now with the new 1mm ASSO AF piston this should help the RB & TS.

Also remember MBs race tour pistons are 1mm rings

I when doing ST4 Cast barell I try to persuade the owner to go for a two ring 1-1.5mm piston.
If not I too always take bottom ring off a 3 ring piston dont need it and can cause more probs

Smiffy

Thanks all. I will now take some time to digest all the info and probably remove the bottom ring from my piston