what is an sm420, duno????

i belive it is a four speed trans for trucks in the 50's

Here ya go...........http://www.off-road.com/jeep/tech/convtrans/gm/gmsm420id.html

They are great for cruisers and would be a fairly inexpesive swap for that 73 FJ40 of yours. If you like a 67 to 1 crawl in 1st gear!!

First of all the sm420 has been hashed a lot in the past couple of weeks. If you do a search for it you will find everything you want to know (almost)
Use the search, it is your friend :flipoff2:

:D

Easiest answer is that it is a 4 speed tranny that was used in chevy trucks (and other heavy duty vehicles) from the 50's untill 67 (apparently it was built untill 72ish but was not widely used after 67) It has a 7.05 1st gear ratio that makes it very valuable... Gotta love the crawl that a SM gives you...

A better question is how do people feel about the SM420's that they are running. :D