How to process back plate to guarantee Chuck's T.I.R? The semi-machined back plate has to be processed into fully-machined which is showed in the above fig before they can be mounted. The fully-machined back plate can be mounted onto 10" 3 jaw or 4 jaw self centering or independent lathe chuck (p/n 202-5435, 202-5413, 202-5475,202-5422). All back plates have 0.04" allowance, please measure the size of Chuck's D1 before process your back plates. The size of back plates's D1 should be a little bit smaller than the size of Chuck's D1. Because the gap between back plate and chuck is smaller, chuck's TIR is better. No gap is the best. back plate diameter 12.64 Spindle Nose Type Threaded compatible chuck diameter 12 mounting 2-3/8 - 6