A few days ago I finished my first axel shortening project. Before I started I looked at a lot of axel posts and got a ton of info about models, length, ratios, the differential location, caster angles and so on, but found only a few generic tips about the nuts and bolts of the operation. It seems like a good topic for a comprehensive thread, so I am starting one. I'm sure you oldtimers will have plenty more tips and corrections for my bumbling.
The first problem was how much to shorten each side. I used the center of the spring on the differential housing as the reference point for all length measurements, since that was the one fixed point that was going to be in the same place after the swap.
On my D30 I measured from the reference point to the center of the lower eye of both yokes, and to the center of the other spring. Since I was doing a stock-width replacement, I tried to end up with the same dimensions on the D44. I also measured between the yoke eyes to use as a final reality check.
Then I got a Starrett angle/bubble protractor and measured the angles. I jacked up both ends of the Jeep, with the weight on the front wheels, and leveled it fore and aft. Then I measured the angle of the spring clamp plate, and the front of the differential housing (90 degrees from the pinion angle). Just for grins I also crudely measured the caster and found about 2 degrees.
Then I set the donor axel ('79 Cherokee) on a bench on jackstands. With a scissors jack under the pinion nose I rotated it until the spring perch on the differential housing was at the same angle as on my Jeep. Surprise, the pinion angle was within about a degree; no need to machine the perch.
Then I turned it upside down and took the same set of length measurements off of it, did the arithmetic and calculated how much to shorten each side.
Then I had a minor brain storm. I scribed a witness line on each yoke and on each side of the differential housing. I very carefully measured between these marks on each side, and recorded the numbers. Then when I was finished I repeated the measurements and knew EXACTLY how much I had shortened each side (both were about 1/8" longer than planned.) That way I knew how much to have Moser (http://www.moserengineering.com/
, $85 the set) shorten the shafts.
I put a 1-2-3 block inside the yoke, resting on the end of the axel tube, and measured down the length of the tube the amount I wanted to shorten, and scribed a line. The block was necessary because the end of the tube is about 1/8" inside the yoke, and the yoke itself doesn't have any good, machined surface to measure from.
Then I put the housing in the bandsaw and cut it at the scribed line. I then put the yokes in the bandsaw and cut off as much excess tube as I could.
Next was one of the tough parts. I clamped the yokes on a table and ground down the weld with a 4 1/2" angle grinder. Used up three wheels and made a terrible mess of grit and steel dust. I ground down as much of the tube and as little of the yoke as possible, and ground until I could see the line between tube and yoke all the way around. Then I used the gas axe to slit down the tube, and knocked the tube out with a hammer and drift.
There were a few burrs inside the yoke which I cleaned up with a die grinder, and I used a flap disk on the 4 1/2" to clean up the end of the tubes. I took as little off as I could because I figured the tighter the better.
I put the axel back on the jackstands on the bench and with the scissors jack rotated it until the diff housing spring perch was at an angle that put the yokes vertical to result in the six degrees of caster I wanted. It was easier to do this than make the spring perch level and try to measure the caster angle on the yokes.
I found a couple of pieces of bar stock that were a few thousandths under .750, that were a snug fit in the yoke eyes. (Had I not found these I would have made something on the lathe to do the job. I could see no easy way to accurately measure the yoke angle.) I tapped the yokes onto the axel and worked them back and forth until the bars were vertical. Then I set up some lights so that I could see any difference between the yokes, by sighting across the bars above the yokes, and got the bars in the yokes parallel. They were probably off by less than a degree before that, but now were VERY close. I didn't want to live with a caster pull.
Then I beat the yokes on as far as I could with a big copper hammer, checking frequently to make sure they hadn't rotated. That got them about half way. I did not put any lubricant on first because I was going to use 7018 low hy rods and didn't want any contamination.
After checking that the yokes were still parallel I put the housing in the press and started pressing them on the rest of the way. They went on a little farther, and then pressure built up to about 35 tons when my rigging exploded. I redid the rigging, heated the yoke, just a little, and pressed them on with about 20 tons. I stopped when the tube was still inside the yoke, eyeball close to the way it had been on factory assembly. That might have been where part of the final error came from. I should have measured this accurately and pressed the yoke back on precisely. Brain fart.
I set the housing back on the jackstands, cleaned it again with 1,1,1, tri to get any vestiges of oil off, and tacked the yokes in four places. I cleaned the welds and went back to lay in the money maker.
I built up a new long-side perch out of 1/4 X 2 cold rolled, TIG welded it together, and set it in place on the tube. With the scissors jack I got the diff side perch level, put the long side perch on, measured, leveled, tacked and welded.
Then I went back to the original witness marked and checked my results. As I said, the new was about 1/8" long on each side. I still don't know where that came from, but figure it's good enough for who it's for (me).
I sent the axels and shortening instructions to Moser, put a coat of Rustoleum satin black on it, and went back to take the D30 apart.
Ok guys, now's the time to add your tips, and let me know what I might have done easier or better. All suggestions welcome. Notice that I did not address any issues involved in going SOA, outboarding the springs, or going wider.
I might have been born at night, but it wasn't LAST night.