What is the size of the double roller chain used on the engine/ transmission coupler???  

I checked the chain on my 555 and it's a one inch pitch (80) but the rollers are 1 inch wide whereas typically 80 roller chain rollers are 5/8 wide. This is somewhat of a special chain so I don't know who sells it.

What tractor is it for? 

 

101 senior! 

I gotcha. When I did the clutch job on my 55, I had to have a new chain as the old one wouldn't free up. I lucked out as DanP had a NOS one still in the original box (I even saved the box just because I thought it was cool.) You might ask him or Terry Kessel. When I get home I'll measure the chain on my 55.