Summary: Have a 92' 240sx auto and a 93' FD auto. They both use a Jatco 4R01a transmission. What makes a transmission share the same model number? Internals, casing, or is it just an random assigned number to the years they were built?

The auto trans in my 240 is a weak link, so I wanted to beef it up and possibly use the extra auto trans from my FD, but where the similiarities begin and end is all beyond me. Any input is great appreciated.

Thanks!