Old school differentials - need expert opinion

@AModelCat

 

What trans do you have? If it’s old enough to have sq100s I’m betting it only has an RTO trans in it which has OD in the back box only, compared to an RTLO which has it In the front, you could essentially gain half a gear by gong to an rtlo 13/18 speed, 3.90s are basically the perfect ratio with an rtlo and 11r24.5s

 

I am 99% sure it can, my uncle had an 87 Pete that went from a 10 speed to an rtlo167-18 and the Speedo still worked, a dedicated tranny guy should be able to confirm/deny

rto is 1:1 and .87, RTLO is .87 and ,73 in the #8 position, so you are basically adding half a gear, should be able to go air cooled instead of water as well

If you are planning on keeping the truck, which I assume most of us with the old junk are, I would start there before the diffs, if you still aren’t happy then I would look for a modern cut off with the gears you want

 

Swap the two high gears in the main box and that will give you exactly what you want.
if I'm not mistaken 14613 was the last transmission you could do that with. That will give you a low, medium and high. It works very well with 3.90 gears. Relatively inexpensive to do, and quik.

 

never had a chance to run one of them, but from what I’ve heard they whine and run hot

 

I heard that years ago but we've had three done over the years and not one ran hot or whined. Our mechanic that did them said if they were slap wore out ( not to be confused with someone on TTR) they might run hot.

 

The sqp’s well fit in the carrier. They are just 100’s with a oil pump and filter for higher weight capacity. Takes a standard old school chevy v8 oil filter. My ‘67 pete has them in 4:11’s.