Bolt on Underdrive

I don't think there is any way to reverse engineer an OD to an UD. It would be easier to start from scratch. On the other hand, the low range unit of a BW1339 is already in the right general configeration to bolt into the D18 and offers a 2.57 reduction in addition to what gears you might already have-without any change in the drive train length. When I refered to using a Warn OD input gear, all you would be using is the 26/29 tooth gear and the hub. The remainder would have to be completely remachined to mate with the 1339 and there may need to be an additional support bearing mounted in the adapter plate.

 

I've thought about regearing my axles, but the drawback their is I would have to buy 2 more lockers, plus 2 gearsets. My carriers only work on 3.73 and higher. That's a lot of $$, plus I like how my CJ5 drives down the highway at 65 with the 3.73's and 33's. Guess it's tough to find a set up that will do it all. The drawback to Tera Lows and regearing the T-Case is the cost. I'd be much further ahead swapping the SM465 instead. I might try to find a 1339 low range and play with mating it up on one of my spare D18's, but I think my only solution right now is a transmission swap.

O_Negative

 

If your gonna swap transmissions, and want low gearing go with the sm420 instead of the sm465, the 1st gear in the sm420 is 7.05 versus the sm465 1st gear of 6.55. That makes final drive 65:1 instead of 60:1. You loose a little in driveshaft length with the sm420 but it is marginal.

 

The plus is I had a friend give me a SM465. A SM420 would be a great swap with better crawl, but I've been unable to find one. The other plus I believe the SM420 is lighter as well. Have to redesign my skid plate and crossmember for the SM465's size and weight.

O_Negative

 

Isn't the SM420 only synchro in the top two gears (3-4)? Unlike the rest of the truck 4-speeds (except the T-19) that have synchromesh in the 2-3-4 'driving' gears?

 

Syncro'd from 2-3, and 3-4. The SM in the name actually stands for syncro meshed.

 

Yes, that's one less blocking ring than the others. A T-18 (or SM465 I presume) also has a second gear blocking ring, providing synchromesh 1-2 (and 3-2).

Isn't this a significant driveability issue? It requires a double clutch (or speed matching) on a 1-2, 3-2, or 2-1 shift.

 

SM-420, SM-465's, and T-18's are all synchronised in 2nd gear, 3rd gear, and 4th gear. Not synchronised in first. T-19's are synchronised in all 4 forward gears.

 

I may have asked this before ...

Herm's rebuild kit for a SM420 only has 2 blocking rings, according to his site. How does that provide synchromesh for 3 gears?

 

Just because his kit doesn't come with the 3rd blocking ring doesn't mean the transmission doesn't have one

I think it's the second gear blocking ring that typically doesn't wear out and are VERY expensive to purchase. That's why he doesn't include one in the kit. Last time I checked they were still available but that's been a while.

 

Ahah.

I was assuming that there must be a reason why Herm only sells the 2 rings. Now I have an answer! :tea: