LSD - Bad trip!

[erictharg]

No - not acid! Limited slip diff. Bought one last week from Peter May and proving a hassle to install. Easy in theory. Pull existing diff. 20 mins. Easy. Measure backlash of crownwheel (to replicate same mesh with new centre section). No problem (.006 to 007"). Remove centre section - 4 bolts. Remove crown wheel. 6 bolts. Remove bearings from centre section. Aaah. Tricky. Can't get a puller under them &they fall apart when you try (took ages to find that last ball on the floor under the bench!). Ended up cutting up the old centre section to get the inner race off. Messy. However, now have the bearings reassembled ready for shimming to get the same preload. But first need to work out a way to get the bearings off without damaging them each time I need to swap the shims during set up. Jan - any tips? It does look as if there is more room behind the bearings with the LSD than the OE centre section so hopefully can rig up some support plates to allow use of the press. Will give it another go today - needs to be sorted for Snetterton on Sunday!

[Westfield 129]

I have done this myself a few times, but now leave it to a local pro who charges me about $200 US to do the job so long as I don't install the bearings on the new carrier, and don't take apart the old one... And he complains that removing the bearings will damage his puller... He has done 5 for me so far, and a few for my friends. He does it in about 2 hours (labor charge including shims). Actually, I think that his actual labor was closer to 45 mins...

The trick is to take the measurements from your original diff as to the precise locations of the bearings in relation to the crown wheel and the carrier, (measure the mounting flange for thickness, and then you can use the flange for your measurements) then measure your shims and make any corrections there before you put the new one together. In theory, anyway. This works if you are using the original crown and pinion. Distance from the flange to the bearings is the critical measurement. Use a quality dial caliper.

I find that this method works best when the new LSD is precisely the same dimensions as the original carrier. Seriously, I was able to simply swap bearings and shims for a perfect fit with the original crown and pinion more than once. And then TranX changed their machine contractor, and everything was off by just "that much". But using the measurements, I was able to get the proper lash on the first try.

I wonder if the 3J diff is more accurate in comparison to the original carrier (or current TranX). My experience is that the carriers are quite accurate from manufacturer to manufacturer, most of the time.

If you measure the carrier carefully, you can determine just how much it is different from the original, and then correct it with the shims prior to installation. The diff is usually less than a few thou... The carriers are usually quite accurate in relation to the original part. so if you get the measurements right, about 60% (or more) of the time you can get the lash right on the first try if you are using the original crown and pinion. Most of the time, the shims that are installed on your old diff will be about what you need.

Note that if you replace the seal and crush sleeve, that can also change the setting.

I usually install new diff bearings, so I am shooting for a single installation once I figured out the shims (or if I lost a ball from the bearings).

I always remove the bearings with the carrier in an oil catch pan, with a towel lining the bottom, the better to catch the loose balls...

I have pullers with fingers, and a puller that uses a couple of flat plates that bolt together beneath the bearing and usually (but not always) grab the center and pull the bearing off without drama.

If you have a BMC factory bearing puller, it is probably easier to do the job.

If it looks like the bearings are not seating all the way, find a larger deep socket to press them home. Sometimes, the socket that is used to press the bearings onto the new carrier are too short, or a bit too small in diameter.

I seal the diff casting using Hondabond, or any RTV that is specifically for differentials. I usually don't bother with the paper gasket.

And one more thing. If you have the rear jacked up, and you have put in your "friction modifier" or "LSD Additive" (you will need it to prevent chatter, even if you are using an LSD compatible lube), you might slightly overfull the diff, and you will get a leak. You may try to replace the seal a couple of times, but the problem will persist. The problem may not be the seal, but an overfill situation. With the car level, on the wheels, remove the drain plug and let the excess run out. The leak will likely stop.

You did get some race axles, right? If you didn't, you will find yourself stranded when the stock ones break, usually when making a turn at an intersection, or driving out of a drive way far from home...

My advice to most builders is to have a pro do this job if they can afford it. Often, the price to do this is silly, so shop around for a tech that has done this a lot, and charges for the actual labor time. I watched my guy do this, and it took a lot less time than the two hours he charged me for. And... If you want to do it yourself, take proper measurements first, then give it a go. Make sure that you do this in a bin so that the balls are easily recoverable.

[erictharg]

I measured the new (3J Driveline) centre and it was as close as I could tell (with a digital caliper) the same as stock so I started with the same shims as the original set up. However it gave me too much pre load on the bearings so I reduced the shim thickness by half on the bearing furthest from the crownwheel. That gave me .006" backlash vs the original .007-.008". Close enough I think (lucky?). Odds are if I tried to get closer I'd end up with too much slack. Pre load on the bearings seems OK. Filled with Castrol 373 on the level (!) and test run in the air it sounded fine. Sunday will tell - Snetterton. Axle shafts are stock but "Growlerised" and shot peened. We'll see how they go! I always carry spares for racing. I'll continue to drive with my sympathetic right foot.

[Westfield 129]

I think that you nailed it. Excellent job! It was likely that your original shim setup was OK if there was any slack at all, and the bearings were pressed home. Generally, to get any change, BOTH shims have to be changed to move the crown wheel. Sometimes, all you need to do is swap the shims left to right.

In any case, your gear lash is right on, and the diff is quiet. Good Job!

However, all the treatment in the world wont keep the drive plates or the axle splines attached to the stock axles. The axle will twist in half at the splines, or the drive plate will fail (unless you have the double bearings, and that wont help the splines... The thing will just twist in half! (Pictures in one of my albums on the other sites).

Note that the failure of the axle will feel like the clutch failed. The car will drive forward, but there will be lots of slip. Whatever you do, don't remove the engine to change the clutch. Jack up the rear and spin the wheels first. One will spin easily. Hopefully, you will be able to get the stub out without having to remove the diff from the case AGAIN...

The problem is not axle hardness. The actual metal is not up to the stress. Too soft. You will see this at the conical failure when the axle twists in half where the splines start. The drive plates are too thin, the welds not strong enough... I have broken 4 at the splines, two at the drive plates. Nice mix of left and rights...

No amount of treatment will work (tried that, didn't work any better than a stock axle). All broke during mundane daily driven (always making turns after a stop, or pulling out of a driveway), and none were abused, other than to take all the HP during cornering loads (drives through the canyons, where wheel spin was the norm), as there is no wheel spin. Even the stock engine has enough torque to break the axles.

Tires used to break axles were 165/13, 185/70/13 and 5.00X15.

Seriously, these rear axles are well qualified on axle failures. Get a set of Peter May axles, or any race type axle that is chromoly with a bolted on machined drive plate. You can get away fine with the stock rear wheel bearings, but double bearings do offer better stability to the rear axle ends. You really don't want to have problems taking apart the rear axle when you snap one and it jams in the diff... 'Been there, done that!

You have less than 8 hours before the axle fails, unless all you are doing is freeway driving. A trip to the track will usually ensure a broken axle on the way home, pulling into your driveway.

[erictharg]

I'll see how it goes at Snetterton, but will plan on a pair of Peter May axles before Spa! Just more money...sigh.

[Westfield 129]

The sad reality is that if you build a proper LSD rear axle with double bearing hubs and a clutch type LSD, the cost is around $2,000 USD. Maybe a little less expensive if you do your own welding and install the LSD yoursel, and get a good deal in the LSD.

However... You will be amazed at the way the car rockets off the corners. You may even have to give a click or two of compression damping at the rear to balance the extra grip if it develops some understeer. It's like having another 20 HP, and you will be able to accelerate through the corner without waiting for the wheel to drop back to the pavement. Have fun passing all those cars.