oldtin 921 17 #1 Posted September 23, 2014 The trans in my 1979 C-121 jumps out of hi range all the time, so what I want to do is install another trans and tear this one apart and rebuild it. It has been a great machine all summer and winters since new. What trans can I swap in while I tear the original apart? Thanks guys! Share this post Link to post Share on other sites
doc724 925 #2 Posted September 23, 2014 Any 8 speed from 1978 C series and 300/400/500 through end of production will be a drop in replacement. However, not all of the 8 speeds were 8 pinion differential like you have in your C121. Some of the low hp 300 series were 4 pinion differential (meaning not as robust). I think all the 4 pinion models had 1 inch axles while the 8 pinion models had 1 1/8 inch axles. If you put in a 4 pinion trans, just remember to go easy and get your original back into the machine. 1 Share this post Link to post Share on other sites
jeremi3210 735 #3 Posted September 23, 2014 If you get a 4 pinion all the internals exempt the axles and differential are the same. So you could use parts off the 4 pinion to fix the 8. Share this post Link to post Share on other sites
oldtin 921 17 #4 Posted September 24, 2014 Thanks guys,that is what I wanted to know. Share this post Link to post Share on other sites
