After quite a few hours of research and measuring I have this figured out.
I measured the O-ring that is supplied by Yamaha in the 5LB-14390-00-00 kit, bought some O-rings at my local hardware store and also ordered some online. I compared them all.
The Yamaha O-ring for this application is 1.5mm (thick) x 7.0mm (I.D).
I do not have a durometer to measure hardness, but by "feel" I would say the Yamaha O-ring is 50 or 70 duro. Duro 50 O-rings are not commonly used so I went with Duro 70 / 75 for the O-rings I ordered.
For material, the choices are Buna-N* or Viton. Buna N is the most commonly used material for O-rings in general. Viton has a higher resistance to temperatures and chemicals than Buna-N – and is more expensive and less common. I don't know what the stock Yamaha O-ring is made of. From what I could find online, I believe that Buna-N should work as well as Viton in our TW carb application (if anyone else could confirm it would be great).
(*Buna-N is the same as Nitrile.)
elime, thanks for your work in getting me started in the right direction on this. You are correct that the 2-010 O-ring is the closest non-metric size to the Yamaha O-ring. I could not find any local stores that carried metric O-rings, so for anyone that wants to by local (in the US) I would say that the 2-010 (also referred to by its nominal dimensions of 1/4" O.D. x 3/8" I.D. x 1/16" thickness) is your best shot. However, it is definitely a different size than the stock Yamaha O-ring that is metric and it would probably be a tight fit inserting the valve seat into the carb with a 2-010 O-ring.
For a comparison, I put the various O-rings on the valve seat and then measured the outside diameter of the ring after it was in place on the seat. Here is what I found:
- The stock Yamaha O-ring from the 5LB-14390-00-00 kit: 0.410 inches
- The 1.5 mm x 7.0 mm Buna-N O-ring purchased online: 0.409 inches (in other words, the same size as the stock Yamaha O-ring)
- 1/4" O.D. x 3/8" I.D. x 1/16" O-ring from hardware store: 0.431 inches (this is the Parker 2-010 O-ring)
- A 1.6 mm x 7.1 mm Buna-N O-ring also purchased online: 0.416 inches (it was listed as a standard metric O-ring so I bought it just as a trial)
Measurements of the O-rings in their un-stretched state (not on the valve seat) also confirm that the 1.5 mm x 7 mm matches the Yamaha O-ring.
I bought a Viton O-ring in 1.5 mm x 7.0 mm from theoringstore.com and I am going to use it.
I also bought 10 Buna-N O-rings in the 1.5 mm x 7.0 mm size. If anyone has a need, I will be glad to send you one free-of charge in the US. Just PM me your name and address.
elime, maybe you could add this O-ring to your excellent O-ring sticky in the Technical Write-Ups section?
STUFF I LEARNED
Standard O-rings are traditionally specified by:
Inside diameter (I.D.)
Cross sectional diameter (thickness)
Material (i.e., Buna N, Viton, Silicone, etc).Non-metric O-rings are sometimes listed by their nominal sizes (in 1/64, 1/32, 1/16, 1/8, 1/4, 1/2" increments) which are not quite the same as their actual measurements.
Metric O-rings are listed by their actual sizes.