Im looking for a torque converter for my th350 and im not quite sure what they mean by bolt pattern.. are they refering to where the flex plate bolts on? if so what is the difference between small and large bolt patterns? is one for a 1 piece rear main seal flex plate and one for a 2 piece? if so which is which?

 

There's only one bolt pattern on a converter.... which is the one where it bolts to the flex plate.

It does not care which type of rear main seal the block has, as long as the flex plate has the correct converter bolt pattern.

The difference between the small and large bolt patterns is {drum roll please} their size. Most 350 converters have the small bolt pattern, which is the same as the 700 converter bolt pattern. 400 converters, which also will fit in a 350, have their bolts on a larger circle than 350 converters do.