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70 shore sounds like O-ring hard, harder than the red ones. Nitrile being industrial O-rings, but can't remember if that's FPM or EPDM. I'll try to remember to have a dig in my materials book tomorrow at work

My bulk o-rings arrived this morning ... and they are visibly bigger

Perhaps I should have done some more measuring to check - but it appears that the o-rings Tamiya call "3mm" are not 3mm... dimensions (checked by measuring lots of o-rings, lot of times by putting them in a set of electronic calipers & carefully opening the jaws until the o-ring falls out, noting the numbers & taking an average) appear to be 2.5mm ID, 6.5mm OD, and 2mm cross section...

My hopes of getting the correct size in bulk may have gone out the window - I found this o-ring size calculator which suggests the Tamiya "3mm" O-rings are not a standard size

On the plus side, the ones I've bought do seem to be the same size as used very occasionally on Tamiya chassis - on things like body posts, brass shock joints & so on.

Yeah O-ring sizes are one of those funny things that everybody has their own idea of what the size means. Some say that a 3mm ring measures 3mm, others say a 3mm ring is sized to seal on a 3mm shaft (So it would be smaller than 3mm). Others say 3mm & are referring to the cross section. Best thing to do would be to take an O-ring into a industrial supplies counter (eg Haley Engineering) & ask them to match it. You could try online suppliers with some measurements, but you're likely to find they only list "sizes" which could again turn out to be anything.
What I normally do is measure the shaft diameter & take a bit off to allow an interference fit on it. Measure the depth of the recess the ring sits in, & add a bit so it'll squish/seal when you put the bits together (This will be the cross-section size). Measure the O/D of the recess & you should find that it'll be an exact or slightly smaller than exact size - if you get an odd number, do a metric/imperial conversion to find out which it's supposed to be.

Of course, the whole thing's a farce though, as O-rings are "Static seals", ie they're only meant to be used for non-moving parts & aren't the right shape or meant to be used for rotating or sliding shafts. If size were not an issue, the thing to use would be linear lip seals, as used on hydraulic cylinders or motorbike fork seals etc

Another trick you could try, would be to invest in some small wad punches (Or make some by sharpening the inner edge of some thin-wall metal tubing of the right size) & punch some concentric rings out of neoprene foam rubber - inner diameter needs to grip the shaft, thickness needs to be more than the depth of the recess it's gonna sit it, O/D only needs to be the same or slightly smaller than the diameter of the recess. By neoprene foam rubber, I mean the stuff that's usually used as draught excluder, shoe insoles etc, the closed-cell boingy stuff. You might even get away with using several layers of old inner-tube or other soft rubber.

mr. T is trying to improve

Mmm... very interesting.

Tamiya USA description

Edou wrote:

Mmm... very interesting.

Tamiya USA description

Intriguiging ... but just what section are they? If you cut one down the middle would it look like x x or > < ? If it's the former, are two (presumably pointed) sealing edges on a shaft really any better than one rounded sealing face? Or is it just a way of achieving a 500% markup?

I also see they're described as "50 Durometer" ... that would be Shore Durometer then - the measurement of foam/rubber hardness - univerally abbreviated to "Shore"

The o-rings I bought are 70 Shore, which by feel must be about the same as the balck Tamiya o-rings ... I still don't know what the red ones are.

42214 x-ring 50 durometer for silicone oil
and
42215 x-ring 70 durometer for mineral oil

both made out of silicone and looks to be the same design.

cross section looks like this x||x

got a set, but havent tried them out yet, my basher has the standard yellow manta ray damper not much use waisting money there.

is very importan the lubrication, all pieces.