I would think it's a 10 3/4",but you never know until you remove it.The parts store should be able to match one up to your original disc.Hear are some part numbers from napa.

T-5 clutch disc part numbers

9" disc is 4190

9 7/8" disc is 4201

11" disc is 4212

Tim

1951 3100 Chevrolet
1951 Chevrolet Suburban Carryall
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