Torque Coverter Bolt Pattern Question

[LT-102]

Need to buy a higher speed torque converter for my TH350 in my 71'. Some catalogs list ones for, "small bolt pattern", or, "large bolt pattern". I don't know which I have. What size are each, and how are they measured?

[mikep]

168 tooth flexplates use the larger pattern . 153 use the smaller pattern. Sometimes they are called a TH350 pattern and sometimes TH400 but its not always the case and with these trucks it often isnt as most come with the larger pattern whether they have a TH400 or a TH350 or not.

Its a good idea to replace the flexplate if you have any worn teeth and most do while you have the transmission out. A new flexplate runs about $20. Mine did. New flexplates usually have dual bolt patterns so it render the whole discussion worthless.