Reusing gears, swapping housings

[Scruffy49]

Truck shipped as a 4.3/Th400/open 2.73 rear gears with 205/75r15 tires. Getting 350 swapped, staying Th400, going with 285/70r15 rears. GPS speedometer, so no concerns in that department.

My engine donor is a 4wd with 3.42 gears. Are the splines likely the same 85 2wd vs 92 4wd? The 92 has stock 265/75 tires, so would guess 3.42 is enough gear for the 285/70s? The C10 is too low to be a tow pig, so not relevant to choosing gears, would use 3.54 if it was stock height.

Normally would not even consider this swap, but… The 3.42s are perfect, the axle they are in is scrap. The 4wd donor has a rotten frame. Trying to reuse as much as I can of what I already own, no sense in trashing viable/repurposable parts. The 92 will help put 4 other trucks (99 K3500, 49 Dodge, 69 D100, 85 C10 swb) back on the road…

Any glaring Don’t Do This jump out that I’ve missed? Thanks

 

[legopnuematic]

They should both be 28 spline axles. The change to 30 spline was 1999 I believe?

When setting up used gears, I would first check and measure backlash in the old housing and note it, I would also apply some marking compound and run a pattern to have some idea on what it looks like as is.

To note, used gears you will be focusing on the coast side of the ring gear, not the drive side for the pattern.

Installing in the new housing you want to try to match the original setup as close as you can so it continues to wear in the same pattern as it has been. Looking to match effective pinion depth, and backlash.

 

[Scruffy49]

Haven’t messed with gearing since the early 90s, mostly on 9” Fords, 8.75” Mopars or Dana 44/60/70/80. The set in the rotten 92 10 bolt look brand new, the set in the 85 10 bolt look brand new. Milling marks in the gears look factory fresh. No glitter/water in the gear oil of either.