Tru-Arc pin locks. What should I buy?

[694spd]

I missing some of the tru-arc pin retaining rings for my 340. What size do I need for the stock 340 piston?
I seems that you need to know the inside diameter of the groove in order to buy the correct ones and I not sure how one would measure it correctly....

 

[Marcohotrod]

they are based on the pin diameter = .984", and groove width. stock pistons and many others use 1 lock ring per side, some race units use double tru-arcs. the federal mogul number is LR-186, 4 per package, same number as big chevy with .990" pin