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SJ410 and JB43 Differential Compatibility
https://www.auszookers.com/forum/viewtopic.php?f=5&t=57781		 Page 1 of 1
Author:  Lando2048 [ Fri Nov 01, 2024 1:08 pm ]
Post subject:  SJ410 and JB43 Differential Compatibility
Hi, everyone! I'm new here, but I'd like to get some information (and advice) from you guys.

I've got an 84' SJ410 (as indicated on the chassis) Suzuki Samurai (at least the shell is 100% a Samurai?). My differential ratios are 4.1 and I have the model where the transfer case output acts as a drum for the handbrake, as well as the flange for the rear driveshaft (handbrake is on the transfer case, not the diff).

My diff has recently began to give me a horrible backlash, very strong jerk between drive and reverse. I know it's the diff as when I have the handbrake up and switch between revere-drive I don't get the jerk or body twist.

My question to you guys is: Are the JB43 guts (the assembly that goes into the differential housing) compatible with the SJ410 housing? The JB43 diffs are easily accessible (and affordable) where I'm at, so I'd like any information with regards to the swap; like if I'd also need to swap the axle shafts etc. (that'll suck because I literally just changed the rear bearings and seals.)... Any information or advice is greatly appreciated.
Author:  sideways [ Sat Nov 02, 2024 8:14 am ]
Post subject:  Re: SJ410 and JB43 Differential Compatibility
They're not really compatible. An SJ410 has 6.5" low pinion diffs front and rear, a JB43 has a 7" diff in the rear and a 6.5" high pinion in the front.

So the rear is 100% not compatible, it is just a different family of diff.

The JB43 front is a relative of the old suzuki 6.5" diff but it's high pinion 22 spline. I'm sure with enough stuffing around (perhaps a custom tailshaft) you could fit one to the front. A high pinion wouldn't be good in the rear as it would be running on the weak side of the gears, you would also have to source 26 spline side gears and it may well have clearance issues with the underbody.

Your diagnosis of the problem doesn't make sense to me. Chock the wheels, put the gearbox in neutral, the transfer in 2wd, hand brake off. Climb under the car and wiggle (don't be gentle) things around to find the play. If you can move the pinion flange up and down at all then your diff needs rebuilding/replacement. Your problem could also be a uni joint, worn tailshaft spline, worn splines/bearings inside the transfer case, worn spider gears, worn axles splines ect ect.

In any case, a 6.5" suzuki diff is easy enough to DIY rebuild or a workshop could do it.
Author:  Lando2048 [ Sat Nov 02, 2024 11:35 am ]
Post subject:  Re: SJ410 and JB43 Differential Compatibility
sideways wrote:
Your diagnosis of the problem doesn't make sense to me.


Fair enough, here's how I determined it was the diff (all tests done in 2WD):
1. (Background) I've engine swapped it and I made the connection between the tranny and the transfer case solid (took the flanges that go on the new tranny, and transfer case to a machine shop and had them friction weld it together)... So there is 100% no "wiggle room" between the tranny and transfer case.
2. I replaced the both U-joints on the rear driveshaft thinking that was the issue and problem persisted.
3. Decided "It must the transfer case or tranny shifting (twisting) when I go between gears" so I tested that by pulling the handbrake (stopping the rear drum on the transfer case from moving) and shifted between gears... There was no jerk or twist.
4. I then decided to unbolt the driveshaft from the transfer case alone and remove that half-shaft and then rotate the other half-shaft (on the diff)... And guess what? The darn thing has almost 90° of free rotation (I can rotate quarter turn before the gears hit each other).
Hope that was a good enough test for you :wink: (I am also on my way to attempt your version as I didn't check for "rock", only rotation.

What I am yet to determine is why when I put it in drive and the diff jerks, the whole body feels like it twists forward and to the driver side, then settles when I put it in neutral or drive off.
Author:  Lando2048 [ Sun Nov 03, 2024 12:41 am ]
Post subject:  Re: SJ410 and JB43 Differential Compatibility
sideways wrote:
They're not really compatible. An SJ410 has 6.5" low pinion diffs front and rear, a JB43 has a 7" diff in the rear and a 6.5" high pinion in the front.


Yeah, what I really want to know is if I remove the guts and axels, can I swap JB43 guts and axles into the housing?
Author:  Gwagensteve [ Sun Nov 03, 2024 8:12 am ]
Post subject:  Re: SJ410 and JB43 Differential Compatibility
The answer is no.

As Sideways pointed out the rear differential and axles are completely different. Whilst the axle spline is the same nothing else is shared. The JB43 assembly is larger in every dimension and no parts are interchangeable.

You have however provided some more information that's a little bit alarming. Whilst it's not related to your current problem it 100% WILL cause issues in time. Directly coupling the output of the transmission to the transfer input will result in heavy side loadings of the transmission output shaft and transfer input. This normally shows as heavy bearing noise from the transfer. The gearbox rotates around the crank under load and thrusts fore/aft. The transfer rotates around the RHS transfer mounts, which, being below the crank means the input to the transfer moves left/right and up/down in an arc. The two motions are opposing and that's why Suzuki fitted a jackshaft.

Many (~20) years ago there were a few cars built with a kind of cradle that linked the gearbox and transfer together so they moved as a unit but the accuracy of this alignment is very difficult to get right. These days with 3D scanning and CAD modelling it would be easier but I'd argue it's still not the preferred solution.

Significant "twisting" of the car/suspension when shifting from drive to reverse is completely normal. In low range, my car rises and falls through easily 4" when I shift from R to D - it's just the effect of the drag in the torque converter. I have to "settle" the car by shifting D to R and letting it engage the gear, which loads the suspension up and levels the car before I shift to P, otherwise it sits all twisted up when parked. Whilst my car is very softly sprung, it's just an example that there is a lot of torque being transmitted to the wheels even when the car is at idle.
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