Rear Axle Overhaul by Fred Gauci
For those who can remember way back in April 1999 I wrote about rebuilding my 1928 Austin 7 rear axle, now I had the opportunity to rebuild a Ruby ( D type ) rear axle. Rebuilding the rear axle of an Austin 7 requires removal from the car and a complete strip down and examination of all the internal parts for wear and or damage. This particular axle I had to rebuild had suffered a broken tooth on both the crown wheel and pinion that have to be replaced as a matched pair. There are numbers engraved on both the crown wheel and the pinion that will easily identify them.
First of all drain all the oil out of the axle by undoing the drain plug at the bottom then remove the wheels, brake drums, shoes, cables, hubs, back plates, woodruff keys etc. I do this while the axle is still on the car as it makes stripping down a lot easier to do with it held in place. Mark the rear prop shaft flange to aid reassembly then remove the four bolts from the flange. Undo the rear shock absorbers. Place a jack under the axle to take the weight. Now undo the nuts on the cotter pins of the spring pins and with a drift knock the spring pins out, then remove the axle from the car.
So now you have the thing out you will need to clean off some of the oil, grease and muck from outside the axle case. I use paraffin brushed on to loosen it all up then lots of old rag to wipe dry. (You can also use Gunk or Jizer). This will take a considerable amount of time but it needs to be done before you separate the axle case or all the loose muck will find it’s way inside.
Now to start stripping it down. I use small plastic boxes with lids to keep the nuts and bolts that you have just removed and stop them from getting lost. It may also help to label the boxes with the contents so that they all go back to where they came from on the axle.
The job is best done on a bench with the O/S axle tube held in a vice with the torque tube facing upward. Carefully undo the six bolts from the torque tube flange and lift it off taking care to note exactly the number of shims under the flange. Keep these very safe as you will need to put them back when you build it up again. Once the torque tube is removed put it in a safe place for further stripping later. Knock back the lock washers and undo and remove the six bolts holding the two halves of the case together but be ready to catch any oil that was left in the case before it ends up on the floor or on your shoes. The N/S axle tube should now just pull off and you will see the crown wheel on the diff carrier, the diff carrier bearing and half shaft sticking out. (You won’t be able to remove the half shaft until the diff carrier is removed and the two halves separated.) Now you can pull the diff carrier with crown wheel and the two half shafts out of the case.
What you are left with in the vice is an almost empty axle case, I say nearly empty as there are felt oil seals and retainers in both the axle tubes which have to be removed. To do this you need to undo the crown wheel adjuster collar covers which are located on the outside of the case secured by two ¼ BSF bolts. Now, looking inside the case where the half shafts enter the axle tubes the adjuster rings can be unscrewed and removed. These have oil return screws machined in the bores and are handed left and right . If they are reversed when put back in they can quickly drain the oil from the axle so check that they are both marked. Once removed you will see a spring ring which holds the felt housing, felt and cover plate. These are removed (from both sides ) and placed in a box for safe keeping. Now both halves are empty, move them out of the way to continue to strip the differential.
Before you start undoing the nuts and bolts take some time to check to see if there are any mating marks on the two halves of the diff carrier ( this is most important when you reassemble the two halves together again ). If you find no markings, mark them with a small punch on the two halves so they match on reassembly. Once that’s done knock back the tab washers and undo the bolts. I always undo any bolts in a diagonal sequence so as to avoid any distortion, This also applies to tightening them as well. Place all the nuts and bolts in a container but discard all the tab lock washers. Don’t be tempted to re use any locking washers as when you straighten them they will crack and break off with possibly disastrous consequences.
Carefully prise the two halves apart taking care not to damage them and remove the six planet gears from their posts and put them to one side and slide the two half shafts out from the carrier. At this point you can remove the two carrier bearings from the outside of the two halves. This is done with a thin pin punch through the two holes provided but if you are going to re use the bearings make sure that they go back on the same way they came off. They are A/C thrust races and the side that‘s marked with “THRUST“ must face the adjuster rings. Once removed you can give them a wash in paraffin and check for wear. I always renew old bearings as a matter of course as they are relatively cheap.
It is now time to clean and check all the components for wear and damage. I place all the small bits first into my Parts cleaner, this has an electric pump which washes the parts with paraffin and by using an old stiff brush will remove even the most hardened dirt and old grease and will be nice and clean for reassembly. Once you are satisfied the parts are clean, dry them off with old rag or paper toweling and put them away.
The crown wheel is then removed from the carrier by knocking it off. I used a big rubber mallet and a block of wood. This is a tight interference fit on the carrier and may be difficult to remove. A matched crown wheel and pinion was sourced, the pinion was removed from the torque tube and checked for correct pre-load with feeler gauges and as the pinion was correctly set up in it’s torque tube this was inspected and cleaned. The bearings were serviceable and reassembled.
At this point only temporarily fit the prop shaft flange ( not the torque tube anchor socket ) as you will need to turn the crown wheel when making the meshing adjustments. The crown wheel was then placed on the carrier and partly pressed on using the same method of removal until I could draw it on using the original bolts. One bit of advice, before you put it on make sure both mating surfaces are spotlessly clean and you can use the bolts to line up the holes and that concludes the strip down.
Next is to clean the axle case inside and out and in preparation for the rebuild. At this stage you can assess what parts are needed and ordered. What you will probably need are the two diff. carrier bearings, two hub bearings and a rebuild kit which will include all the gaskets, seals, locking tab washers and woodruff keys. Once the axle case is cleaned and dry I drill a 3/16th hole on the underside of each axle tube at the end just under where the spring pin goes through, this is to let any oil out should it get past the oil seals and will help keep oil from contaminating the brake shoes. I then run a 5/16th BSF tap in all the threaded holes to clean out the threads, you don’t want to break a bolt half way through and have to strip it down again to remove it. Then give the axle case, brake back plates, brake drums and torque tube a good coat of etch primer and later a couple of coats of black cellulose paint. Once that’s done and the paint is drying you can start assembling the diff carrier.
Start by pressing on the two diff carrier bearings making sure that the THRUST sides are facing out towards the adjusting rings then you can start building up the carrier by inserting the six planet gears (three in each half of the carrier) on their shafts making sure that you lubricate them first, and then you can insert both the half shafts again with a bit of oil in the oil return bushes. The two halves of the diff carrier can now be joined together making sure that the mating marks line up. Before you finally bolt it up for good you must check the run out of the crown wheel with a DTI (dial test indicator ) . There should be no more then three thou (three thousandths of an inch ) run out. This is done by supporting the diff carrier on the bearings, and with the DTI against the crown wheel, zero the gauge and slowly turn the carrier noting the read out. Unfortunately when the teeth on the crown wheel and pinion broke it also damaged the carrier to the extent that the run out was over 20 thou which meant sadly that the carrier was scrap so another carrier was sourced. The original one was stripped down and the new one was re built. This time it was within the three thou limit. The bolts were given a final tightening with new locking tab washers and the run out re checked. Once satisfied all was well the tab washers were knocked over.
Before you go any further the outer hubs need to be lapped on the half shafts to make sure that they are a good fit. Using some fine valve grinding paste clamp the half shaft in the vice using some pads to protect them and smear a little grinding paste on the taper and rotate the hub forwards and backwards until there is an even matt grey finish to both surfaces, you will need to mark or keep the hub with the same half shaft when re fitting. The diff carrier was then put to one side for final assembly.
Back to the axle case, the inner oil seal cups were checked and the new rubber lip seals were pressed in. I did this in a vice sandwiching the cup and seal between two bits of aluminium and slowly winding them in making sure that they were the right way round and went all the way in and straight. They then were inserted into the axle tubes with the washers on top and the spring clips to hold them in place. This can be quite difficult as the spring clips must go into a groove and has a tendency to spring out and sail through the air and hide in the corner of the garage. Once they have been secured smear a little grease on the lip of the seal to lubricate them and screw in the adjuster rings making sure that they fit in the correct side. They are marked L and R. As I have said before this is very important as they incorporate a thread that winds the oil back and if reversed they can empty your axle very quickly.
The next stage is to fit the torque tube to the axle case with the six bolts you removed and the shims that should be in one of the boxes you put aside for safe keeping. The shims must go back in the same position and thickness as when they were removed, ( you should have made a note of this on removal ). As the torque tube I am using is not the one that was removed I will also have to adjust the position by adding or removing some of the original shims I took out but I will do this later on in the setting up but for now I will just put them in. The shims must go in between the distance piece and the case flange and not the torque tube and distance piece, now tighten them down evenly and as always do it diagonally.
We are now getting close to what is the most important and tedious part of the rebuild and that is the meshing of the crown wheel and pinion. This is done using engineers blue or a thinned paint, (red oxide with a little thin oil is ideal). I always use engineers blue as I happen to have a tin. When meshing the gears you must use the banjo gasket you are finally going to use as any difference in gasket thickness will make a difference. With the torque tube bolted in place I hold the case in the vice with the torque tube in a horizontal position and lower the diff carrier with the half shafts into it taking care not to damage the inner oil seal. Then screw the adjuster ring so that it just contacts the carrier bearing. Place the dry gasket on the case and place the other axle tube on. You may have to gently tap it to seat the bearing. Once this is done bolt the two halves together using the original bolts and bring the adjusting ring into contact with the bearing.
Now you have the differential in place and roughly adjusted, turn the pinion by the flange a few times to make sure that there are no tight spots or any signs of trouble. Any problems at this stage will have to be investigated and fixed. The next step now is final adjustments. This can take quite a considerable time as it can mean the crown wheel and carrier may need to be removed many times and re marked with engineers blue before you get the optimum setting you are looking for. What you will be aiming for are the markings on both crown wheel and pinion teeth to be in the correct place for smooth and noiseless running with maximum longevity with regards to wear. Also the correct backlash, which is the small amount of play ( between 6 and 10 thou ), is needed for correct running.
With no backlash the teeth will run hot and will wear out quick and eventually break, and with too much backlash the gears will make a clonking noise which is detrimental to the teeth and will eventually break. For this I made up a measuring bar from a length of 1 inch flat mild steel. I drilled two holes at one end of the bar to line up with two of the bolt holes in the prop shaft flange. I then marked between the centre of the holes and measured exactly 7 inches from the mark which I sawed off. The movement of this bar will tell me when the backlash is correct by clamping a pointer on the axle case at the 7 inch point and then measuring the distance the bar moves. This turned out to be way too much and I had to move the adjuster rings on the case till I got a 1/16th of an inch movement. This represents about 6 thou clearance on the pinion, as the movement of the bar is amplified by the length of the bar. When you adjust the adjuster rings the one that’s on the crown wheel side if turned clockwise will move it inward and into mesh and turning it anti clockwise will do the opposite. Don’t forget to adjust the one on the other side at the same time and only do it one notch at a time.
Once the correct backlash has been achieved it is time to make the final adjustments to properly mesh the crown wheel and pinion together. The markings on the crown wheel should be towards the heel of the teeth i.e. towards the inner part of the tooth and the pinion towards the front of the teeth, so that when they are under load the tooth contact would be towards the total length of the teeth. So now the diff carrier is removed again and a thin layer of blue is put on to about six teeth. I use a small paint brush for this as I find it easier to paint the blue on rather trying to use matches or the like to spread it on. I will also need to adjust the pinion on this axle as it’s not the original torque tube I am using. If the markings are too high or too low on the tooth I will need to either add or subtract the number of shims.
I have positioned the axle case in the vice with the O/S tube in the horizontal position and with the torque tube pointing outwards so that I can rotate the pinion from the prop shaft flange. Once the teeth have been marked slide the carrier into the casing taking care not to damage the oil seals and push it all the way in. You may have to give it a light tap with a hide or rubber mallet to seat the bearing. Then place the large gasket and N/S tube on and again seat the bearing. Now tighten up the six bolts ( diagonally) and with a little resistance on the crown wheel through the drain plug hole turn the torque tube flange in both directions to transfer the engineers blue to the teeth on the pinion. Once you have done this undo the bolts and remove the N/S tube and pull out the diff carrier and check where the markings are on the teeth. The first attempt showed that it was way off and also the pinion needed adjusting as the markings showed that it was sitting too high on the teeth. I loosened off the torque tube and removed one shim from the top and one from the bottom and retightened the torque tube. This time when I tried it I achieved the correct setting. When you do this you may find that you have to do this several times to get it right.
Next is the meshing part, this again is done by removing the carrier and re-marking the teeth. The job is best done with clean teeth for every time you adjust. I used rag with some paint thinners ( but make sure that the area is well ventilated ). I find this removes all traces of the blue. Once again paint six or so teeth on the crown wheel and bolt it together again and rotate the flange back and forth. Undo it and check the markings again. If the markings have moved to the outer part of the teeth you need to move the crown wheel out of mesh by turning the adjuster ring anti clockwise, again only one tooth at a time. The opposite adjuster ring should also be turned (this ring should only be finger tight ). Now keep doing this until you achieve the correct markings. This can take quite a while to achieve but you must keep trying until you get the best results and as close as possible to the correct adjustment. Once you have done this and you are satisfied with it then lock the adjuster rings with the adjuster collar covers using a new gasket and some gasket compound. I only use Wellseal jointing compound. I find it’s the best one available and won’t set hard like the Red Hermetite and won’t dry up like Blue Hylomar. It’s been around for years and I know many restorers like me that swear by it.
Now remove the N/S axle tube again for the last time and give the big round flange gasket a coat of Wellseal and using new locking tab washers bolt up the two halves together and knock over the tabs. Using new fibre washers replace the drain and filler plugs and the breather on the torque tube.
You can now bolt on the brake back plates again using new tab lock washers or spring washers. Next stage is to assemble the brake shoe adjusters, shoe return springs and levers. It’s a good idea at this stage to smear the adjusters with some copper anti seize grease. Now find the two inner hubs, which should have been cleaned with all the other parts when stripped off, and if still fitted remove the old felt seal and retaining plate. You can ether fit new felts or a modern lip seal but if using a felt seal smear the hole in the middle with a little grease and if using lip seals you can press them in or carefully tap them in. I find a large socket if you have one works well. Next fit the new bearings making sure they go in straight all the way in using the same method. I find pressing them in with the vice and some thick aluminium strips to prevent damaging anything works best. Also check the three wheel studs for any sign of working loose. If there are any loose ones they can often be tightened up using a ball pein hammer on the rivets.
Once you are satisfied that all is well the inner hub can be placed on the axle. It is worth greasing the bearing at this point by pressing the grease well into the track and spinning it to circulate the grease. Using a new lock washer and large nut secure the hub to the axle. You will notice that in the axle tube there is a hole, this is for the pointed bit on the lock washer to fit into and stops it from rotating. Now tighten up the lock nut and check that the hub rotates freely then bend over the lock washer on one of the flats on the nut. Of course this has to be done on both N/S and O/S. Next fit the woodruff keys on the half shafts making sure that they are a nice fit. You may have to file them to get a nice fit in the shaft and also the outer hub. Now find the hub gaskets and give them a coat of Wellseal on both sides and place them over the wheel studs then push the outer hub onto the inner hub lining it up with the axle key. You may have to tap it on with a hide mallet as the bearing should be a tight fit. Once on, screw on the hub retaining nut but don’t try to tighten it fully yet as this needs to be done when the axle is back on the car. You can now fit the grease plugs (grub screws) in the hubs and brake drums with the countersunk screws making sure everything is turning freely.
If you haven’t done so already refit the torque tube to chassis socket with the large brass ball adjusting nut and the two securing bolts and the prop shaft flange using a new split pin through the castellated nut. Once this is done the axle is ready to go back on to the car. As written in Haynes manuals the procedure is the reverse of removal i.e. spring pins, brake cables, prop shaft bolts, torque tube anchor, shock absorber arms and links. You can also fit the road wheels on but will need to remove them one at a time when finally tightening up the hub retaining nuts but they will help to stop the diff from turning as the nuts have to be done up really tight on the half shaft taper to stop them from breaking the axle key.
Now with a bottle of SAE 140 gear oil fill the rear axle. I personally never fill the diff to the plug level as I think this is over filling it. I jack up the front of the car and put it on axle stands then I remove the filler plug and fill the axle until the oil is just starting to flow out of the plug hole. I then leave it for a while till it stops. By raising the front of the car it lowers the filling level so that when you lower the front the filler hole rises and the oil level is lower in the axle case and not over filled. One word of warning as some of the bearings in the diff and torque tube have phosphor bronze cages never use EP 140 oil, this oil is for Hypoid axles and has sulphur and phosphorous additives which will attack the yellow metal in the bearings and will result in a major failure. Oh, don’t forget to tighten up the filler plug.
Now jack up the rear of the car and place it on axle stands and adjust the brakes. I tighten up the adjusters until the drum locks then just back off the adjuster until you can just rotate the wheel freely without any binding. Now lower the car keeping one side only raised and remove the wheel. With a suitable size socket and bar and a length of pipe apply the hand brake and tighten the hub nut as tight as possible until the castle nut slot lines up with the hole in the half shaft and secure it with a new split pin. Do the same for the other side, replace the wheel and the job’s done. All that’s left to do is a test run to make sure all is well.
Now go and pour a pint of your favourite brew and give yourself a pat on the back for just successfully completing a major overhaul on your rear end and go enjoy driving your Seven….