this is a crappy write up on how to swap from a 4 series ( aka 4.10 4.56 or 4.88) gear ratio 3rd member to a 3 series ( 3.73 3.56 or 3.09 ) gear ratio 3rd member on an 8" diff. It could be the same for the 7.5 but i donno, i havnt done that one before.

This write up is writen for people who have technical knowledge of these trucks. If you dont know how to pull a drive line or remove your 3rd member already then you should not be doing this.

If you do not have some mechanical knowledge i would not attempt this on your own because you can seriously damage your 3rd member, axles, or drive line. If installed wrong you could have serious driving issues that could cause you to crash.


Anyways here we go

After removing your third member, cleaning all gasget surfaces, and removing the rear flange off your drive line you are going to have basically pic #1 more or less.

Now in pics #2 and #3 you are going to see a rather large lip behind the seating surface of the third member. This is what you are going to have to grind out. As a tip I would not clean out all of the gear oil that is still in the bottom of the dif, it will make it easier cleaning out all the metal shrapnel from your grinding.

pics #4, #5 and #6 are what the finished product should look like. this will give your ring gear enough clearence to actually fit into the housing.

pics #7 and #8 are the two 3rd members, 3.09 on the left my stock 4.10 on the right. Note the size differance of the two ring gears.

Now here is the crappy part that i forgot to take pics of. The flange for your new diff probably has a different bolt pattern then your drive line. You can either have a bastard drive line made ( which costs $$ that i just dont have ) and if you were to try to swap flangs from diff to diff you are going to find that most likely you have different splines too.

To solve this I took the flange off my new diff, and the flange off my drive line, and drilled new bolt holes into the new flange useing my driveline flange as a template. You dont have to worry about centering the two flanges together because there is a lip on both of them that mates up and keeps everything alligned.

After you get the bolt holes drilled, reassemble your U-joint in your drive line, re-attach your flange MAKING SURE NOT TO OVER TORQ THE NUT OR ELSE YOU WILL BLOWUP YOUR PINION BEARING!!! and assemble the truck as you normally would with doing this repair.


hope this helps everyone and good luck

sorry for the crappy pic quality, cell phone pics at work in my cave of a shop

 

this is for pics #6 #7 #8

 

oh sorry these were both 8" diffs. the pinion setup from the 3 series is different then the 4 series my gear guy said. Something about the pinion spacing. To me it looks like the 3 series is spaced farther twords the front ( driveline ) then back.

 

A tip for people, is that lip on the driveline flange got enlarged in size between 77 and 85, so that may need a little fine tuning if you are using a new line on an older 3rd member. Those were the years of parts I swapped. Good info though, thanks.

 

Nice write up!

Mods can you move this to the Engine/drivetrain how-to section?