The Austin Healey Rear End

Have you checked your rear end lately? No jokes, please. This article is about Healeys not hemorrhoids; pinion gears not proctologists!

As I disassembled my BJ8 in preparation for replacing its well-rusted frame, I dropped the rear end out of the car and placed it in all of its rusted splendor on a pallet and tried to pretend that it would be okay just as it was when I rebuilt the car. But as the months passed and I walked by the neglected unit from time to time, I began wondering how bad the insides might be considering that the outside was caked in rust, the brake line had disintegrated to dust and the bell crank for the hand brake was frozen solid. Well, I thought someone in our Healey club must know these rear ends inside and out, after all, as a group they know every other piece on the car. But as I asked around at our tech sessions and other gatherings, I continued to get the same answer: “If it is functioning at all, do not ever open the rear end”; or, “It has to be taken to a rear end shop that knows Healeys, but there is not one around here”.

A couple of more months passed and the owner of the machine shop who is kind enough to let me fiddle with my car there, walked by the hulk of the rear end and said, “you’re not going to put that back in the car without cleaning it up inside and out are you?” I sheepishly replied that I was thinking about just sandblasting the outside and repainting the unit. Well, his disgusted look as he asked whether I was curious about the condition of the inside of the unit forced me onto the correct tract. I recognized that I had to do the job the right way, which meant complete disassembly, sandblasting, cleaning and reassembling of the unit within all of the correct factory tolerances.

I immediately had flashbacks to the Ford eight-inch rear end in which I had changed the ratios about ten years earlier. I remembered the “crush sleeve” which would not crush, the smell of 90 weight oil all over my face and clothes and the fun of trying to shim the gears lying on my back with poor light. I was younger then too.

At least this time the rear end was well drained and set up on a work table with good light!

As a result of many weeks of restoring my rear axle, working a little at a time, I have formulated some answers to the most frequently asked questions and fears about the rear axle. Here are some questions and answers.

What Type of Rear Axle Does the Healey Use?

In the early models, two types of rear axles were used. The original type was a spiral bevel, three quarter floating axle taken from the Austin Westminster models. It was replaced by a hypoid bevel (used from thereon) three quarter floating axle also taken from the Austin. This is the crown wheel and pinion gear arrangement that most automobile differentials use. The hypoid bevel rear ends have been offered in ratios of 9/33 (3.667:1) and 10/41 (4.100:1) in the 100’s and 10/41 (4.100:1), 11/43 (3.909:1) and 11/39 (3.545:1) for the 1006 and 3000. The axle is a banjo type with the differential carrier (“pumpkin”) bolted on in the center.

Why Are Owners Fearful of Opening the Rear Axle?

The inter-meshing of the crown wheel gear and the pinion gear has closely defined tolerances to which the assembler must adhere. Failure to assemble the two gears within tolerances will result in excessive wear of one or both of the gears. The result will be a very noisy or roaring rear axle. With patience and a few readily available measuring tools, the job is not overly difficult.

Does the Rear Axle Wear Out?

Some parts of the rear axle do wear and as always the amount of wear is proportional to the type of use, amount of driving and the maintenance performed. With my rear axle on the bench, I was able to wiggle my pinion flange and shaft about one-half inch and of course the seal was leaking badly. My pinion bearings were shot. If you have any wiggle in your pinion shaft beyond about 1/32, it would be wise to replace the bearings and seals. Failure to do so will result in increasing wear that will put non-normal loads on the inter-meshing gears causing wear throughout the system. Eventually, everything will have to be replaced. In order to inspect other parts for wear or to replace the pinion bearings, you will have to first disassemble the entire rear axle unit.

First, remove the drive axles (you will need a 2 3/16 inch socket for the axle nuts), unbolt the differential and remove it from the rear axle housing. You can now seal up the axle housing and have it sandblasted for new paint. The parts to inspect for wear are:

1. The pinion seal and bearings:

If the seal is leaking badly or you have play in the pinion flange (i.e. the pinion shaft can wiggle) more than about 1/32 of an inch, the pinion bearings are suspect. The pinion gear (or head) is fabricated as one piece with its shaft. The pinion shaft is the part that is connected to the pinion flange, which, in turn, mates with the rear flange of the drive shaft. You will have to remove the pinion flange and oil seal housing to expose the pinion shaft seal and outer bearing. There are two pinion bearings. One is located on the outside of the unit behind the seal at the front of the nose cone (differential housing or pumpkin), which, in turn, is just behind the pinion flange. The other pinion bearing is inside of the housing. The crown wheel carrier assembly must be removed to access the interior bearing.

Before removing the crown wheel assembly, verify that the crown wheel carrier bearing caps are indexed for reassembly. They must be reassembled the same way as they are from the factory. In addition, the spacer collars must be reassembled in the same way. The spacer collars are likely to be different thicknesses. Since they determine the position of the crown wheel and the bearing preload, they must be in the original order when reassembled.

Before you remove the pinion head and shaft, you must measure the height of the installed pinion. It is not likely that this will have to be changed but it should be checked. The factory manual notes the use of a dial indicator on a magnetic base with a factory setting block tool number 18G.191.B. From my measurements following the factory procedure, I believe that the setting block is just a steel block with a thickness of .750 inches. I was within .001 inch on the measurements when I used a steel block of this size. Follow the procedure in the factory manual or one of the after market manuals to obtain the mean height of the two bearing bores. This method will give you the height (the number on your dial indicator) which must be added to the number etched on the head of the pinion gear (the unbracketed number). It is then added to the bracketed number etched on the head of the pinion gear (which is the tolerance for the machined height of the pinion head). The final number should be plus or minus .001 inch from zero as set by the factory. If your final number is not zero, any variation from zero is adjusted by varying the thickness of the pinion head washer.

As an alternative, you can assume that the factory (or PO) setting is spot on and place a machinist’s flat steel bar across the flanges of the differential housing and measure the perpendicular distance from the steel bar to the pinion head. This will give you a relative measurement against which you can check when the pinion gear is reassembled with the new bearings.

The two pinion bearings are Timpkin ™ (tapered) bearings and are set up and shimmed in the same manner as the front wheel bearings on later model Healeys, one on the outside of the casing and one on the inside of the casing with a spacer and shims in between. The two bearings are tightened against one another with the large castle nut behind the pinion flange. The factory spec for the shimming is a torque rating of 12 -15 inch pounds needed to rotate the installed pinion shaft. NOTE: That is inch pounds not foot-pounds. How do you measure inch pounds? An inch pound torque wrench, which you will have to buy, borrow or rent. For reference, 12-15 inch pounds is a slight but noticeable (not moderate or heavy) amount of resistance when one to rotates the installed flange using a full palm grip (How’s that for technical terms!). The spec for the bearings with the seal in place is 18 inch pounds, so, an oiled seal by itself has a resistance of about 3 inch pounds. This spec is different than the one for the front wheel bearings, which just calls for no endplay.

Once you have the shimming job complete for the bearings, you should check the pinion head height to verify that the height position is the same or within .001 as it was before you removed the pinion shaft. If it varies by more than .001, you should consider changing out the spacer washer under the pinion head to the correct thickness. Just use the difference in your two measurements and adjust the spacer washer plus or minus, as appropriate by that difference.

NOTE: You will see references in the manuals to the numbers on the pinion head. These are absolute numbers relating to the factory setting block. These numbers are used if you follow the factory setting procedure as described above. They are not necessary if you are using relative numbers based on the alternative method also described above.

Okay, so now you have the pinion bearings and seal in place perfectly and the pinion head height set correctly but the crown wheel assembly is still sitting on the bench. That brings us to step two.

2. The crown wheel/pinion gear and crown wheel bearings:

The crown wheel bearings are not as subject to wear as the pinion bearings. These bearings are ball bearings. If they need replacing, the new bearings should be the same width as the old ones. Ball bearings are one piece and thus are manufactured to close tolerances relating to width (unlike Timpkin ™ bearings which are two pieces). If you need to replace the bearings, measure the old ones (they should be a metric dimension: did you know that almost all bearings are manufactured to metric dimensions, including the Healey ones?) and replace with the same size. If you cannot obtain bearings with the same exact dimensions, you will have to adjust the shimming so that a) the position of the crown wheel is the same as before replacing the bearings and b) the overall thickness of the two bearings plus all shims remains the same as it was with the old bearings.

The differential bearings must be preloaded by .002 on each bearing. If the overall dimension of the differential collar shims plus the bearing thicknesses remains the same, the preload will be correct.

Now onto checking the backlash

3. The backlash setting:

If the pinion height was set correctly and the crown wheel reassembled in the same position, the backlash should be correct. To check the backlash use the trusty dial indicator again. Set up the differential on its pinion flange so that the crown wheel and pinion are on top. Immobilize the housing so that the entire unit will not rotate (perhaps a helper?). Set the magnetic base of the dial indicator and adjust the indicator arm to rest on one of the teeth of the crown wheel. Now, move the crown wheel back and forth against the pinion head (without moving the pinion head) to see how much play or “backlash” there is in the intermeshing of the gears. The correct figure for the amount of backlash for any pair of crown wheel and pinion is etched on the back of the crown wheel. The range of tolerances for backlash which I turned up in my research are BN1 .005-.008; BN2 .008-.012; BN4 to BJ8 .005-.007.

If you are not sure, you can always check the meshing of the gears by using a suitable amount of engineer’s blue on the crown wheel, then rotating the gears with a load (helper again?). The pattern should show a smooth contact patch in the center of the crown wheel’s teeth. Refer to Figure 1 for adjustments.