The problem with measurements on old O-rings is that they shrink or grow over time.
Also inner/outer diameter will never be the same as new.
My measurements are:
~19,3mm inner diameter
1,5mm in radial direction
1,7mm in axial direction
The only JIS size in this range is the S20, It`s also my opinion that this is what we need.
But also keep in mind, this part was designed in ~1980, there is also a possibility that the JIS has changed.
I have measured all O-rings I have bought new - there are some "bastard size", nothing is impossible.
It`s quite easy to make a new mould for O-rings.
I have also compared new to old...
Reason why this O-ring is not available from Yamaha is that they get this complete part from Mikuni pre assembled.
Maybe its possible to get original parts for the pump directly from mikuni as the pump design is also used on a lot of other more modern bikes like Aprilia RS125, scooters etc.
http://www.ebay.at/itm/ROTAX122-123-APR ... SwAPlXgg~q" onclick="window.open(this.href);return false;
@Bengt Weil: The pressure topic for O-rings is more valid for axial sealing (ring is between 2 flanges).
With outside pressure you stretch the ring over the smaller groove diameter, with inside pressure you compress the ring with the outer groove diameter.
You just need to imagine what happens when the pressure goes inside the groove.
If there is inside pressure and the ring is stretched over the inside groove diameter it has to grow until it`s pressed to the outside diameter to seal => not good as there is a lot of movement in the O-ring then and it will fail over time.
But I guess in 99% of the cases it will not fail even if the design of the groove is wrong...
For radial sealing you stretch it over the inner dimension when the groove is in the inner part and you compress it by the outer diameter when the groove is in the housing.
Simple reason for that is the assembly proess, you have to avoid that the ring starts to twist in the groove and will not be squeezed between the 2 parts while pushing together both parts.
Always lubricate the parts during assembly with the fluid it should seal.
NBR O-rings: engine oil
EPDM O-rings (cooling system): DON`T use engine oil! As far as I know silicone oil/grease is what you sould use.
For Brake parts (might be SBR material) I`m using this one:
http://www.pinzgauer-techshop.ch/daten% ... spaste.JPG" onclick="window.open(this.href);return false;
As you can see - to buy an "o-ring kit" is more or less useless because there are so much different materials.
I like to use Yamaha parts because I know they are the correct material.
Interesting info to this topic, but it`s german...
http://www.o-ring-prueflabor.de/de/fach ... -versagen/" onclick="window.open(this.href);return false;