Series 3 LT76 Trans - Rear Drive Castle Nut won't tighten

Gearbox is out of car, on the bench. Trans is in gear, and I can tighten down on the rear drive pinion castle nut but it never tightens. It goes on until I can see the hole for the split pin, but then something "lets go" and the whole spline spins. No bang or pop or anything scary sounding, it just starts spinning.

The gearbox is out because I had a problem last year where the car would not move when in gear. I pulled the trans to find the bellhousing full of oil, so assumed the problem was leakage and ruined clutch, so that's what I dealt with. I replaced the primary pinion oil seat and refurbed the parking brake and all linkage and seals. I did not separate any of the cases. Putting it all back together and after bolting on the parking brake and pushing on the rear drive flange and nut as described above, that's where I'm at.

Any thoughts from those that are more familiar with the inner workings of the gearbox?

Thanks,

Don

As the box is out of the vehicle you need to lock the shaft to stop it turning. Tighten the park brake adjuster so that the brake drum cannot turn & if that is not enough , engage 4x4 and lock the front output shaft from turning with a lever someway, as the front & rear will then be locked together. Picture attached showing cross section through main and transfer box

Attachment 13844

There wants 2 B a heavy, machined washer under your castellated nut. B sure it is there!! If not that is probably the problem

Quote:

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Originally Posted by roverp480

As the box is out of the vehicle you need to lock the shaft to stop it turning. Tighten the park brake adjuster so that the brake drum cannot turn & if that is not enough , engage 4x4 and lock the front output shaft from turning with a lever someway, as the front & rear will then be locked together. Picture attached showing cross section through main and transfer box

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Thank you @Roverp480. Excellent visual there. I see that locking the front output shaft is key. On a side note, I had not yet torqued the nut on the front output shaft, so will address both of them.

Thanks @Charles1943. Yes, indeed, the heavy washer is in place.

Just wanted to say thanks for all the replies and input on this question. In order to tighten the castellated nut for the front and rear drive pinion, it is necessary to lock the drive so it doesn't rotate when you're trying to tighten that nut. As pointed out, if the gearbox is on the bench, you don't have the benefit of the brakes and tires on the ground to prevent spinning of all the gears when you're tightening one of the nuts. I made up a quick tool with 1/8" x 1" steel bar to bolt to the front output flange that bears on the aluminum bellhousing when tightening the rear pinion drive bolt. When tightening the front drive pinion, I used a longer piece of the steel bar to bear on the bolts at the rear to prevent rotation. Hopefully this picture will clarify what I'm trying to describe.

Attachment 13869