The MG TC Rear Axle

A rear axle rebuild has been one of many tasks undertaken by myself during the ongoing ground up restoration of TC0894. The following article forms a record for myself and in doing so I hope it may prove useful to other restorers.

The first task was a total strip down to a bare axle casing. This revealed that the bearing journals were badly worn as the wheel bearing carriers could be removed by hand without any need for a hub puller or slide hammer. Aside from this, other faults included a stripped thread in the casing for one of the differential housing mounting bolts, various graunch marks on the differential housing indicating something had been flying around inside the casing at some point and many casing rivets weeping oil.

The purchase of TC0894 also included a spare chassis from TC4429 and this had both front and rear axles attached. An inspection of the spare rear axle casing and differential revealed that it was in better condition than TC0894’s although the bearing journals were just as worn so I decided to use this one for the restoration.

The axle casing was treated to a thorough internal/ external clean by soaking in a drum of solvent at a local sandblasters, followed by an external sandblasting to return the casing to bare metal. At this stage a quick coating of etch primer was applied to stop any surface rusting occurring.

The rear axle on a TC is a three quarter floating design and as such the wheel bearings need to be a good fit on the axle journals in order to stop the bearing inner races spinning on the journals. Having inspected the wheel bearings and associated axle journals it is very apparent that this particular feature is a very poor design. The width of the bearing is about 4mm greater than the axle journal width resulting in approximately 22% of the bearing sitting unsupported over the axle threads.

Axle journal wear is a well documented problem and this reduction in contact area between axle journal and bearing bore further highlights the importance of a good fit between bearing and axle journal.

However, the $64,000 question – “what is a good fit?”

The British engineering industry has standardised the tolerances of fit within two British Standards, BS 1916 (Inch) and BS 4500 (ISO Metric). Tolerances have been classified into the three main ‘Limits and Fit’ groups of clearance, interference and transition.

As a Design Engineer by profession I am involved with ‘Limits and Fits’ on a daily basis but for those who are not familiar I’ll add a brief description.

A ‘clearance fit’ describes a tolerance condition where a bore is always larger than the mating shaft.

An ‘interference fit’ describes a tolerance condition where a bore is always smaller than the mating shaft.

A ‘transition fit’ describes a tolerance condition where the resultant fit on assembly of mating shaft to bore can be either clearance or interference.

The class of tolerances are defined by a letter and number and bores are always given an upper case letter, i.e. H7 and shafts a lower case letter i.e. s6. These alpha numeric notations are listed in the previously mentioned British Standards and the actual tolerances to be applied for a particular class of fit will vary depending on the nominal diameter of a component.

For example, take the H7 tolerance applied to nominal bore diameters of 55mm and 110mm. For the diameter 55mm the H7 tolerance is +0 / +0,030mm = 55 / 55,030mm diameter. For the diameter 110mm the H7 tolerance is +0 / +0,035mm = 110 / 110,035mm diameter.

In the context of the TC rear axle bearings a fit needs to be selected that will result in an interference fit between journal and bearing on assembly. The internet is a great source of information and I studied many bearing companies’ technical data sheets to ascertain the particular fit required for this specific application. The nominal diameter of the axle journals is 40mm and I finally chose a shaft tolerance of m5 (+0,009 +0,020mm) = diameter 40,009 / 40,020mm (1.5752 / 1.5756in)

After a lot of searching I found Avanti Engineering, a company not too far from where I live, who could metal spray and finish machine the bearing journals to my requirements.

A couple of rivets on the casing had displayed very slight oil weeping. Some people have suggested putting fuel tank sealer inside the casing or brazing the rivets to cure this common fault but after further internet research it was recommended by the Technical Department at POR15 to apply two coats of their excellent paint over each rivet head and the surrounding base material, in line with their specified application procedure, to act as a seal. This was an easy fix but only time will tell if this actually provides a long term solution? The whole casing was then treated to brush coats of primer and semi-gloss black top coat.

I do not propose to describe the differential rebuild /modification as this is covered in great detail by the renowned expert Roger Furneaux in his excellent publication TA/TB/TC Differential Modification & Setting-up, which is a ‘must have’ and without which, I would have had little idea of exactly how to tackle this task.

In short, my rebuilt differential comprises new bearings, a Roger Furneaux Crown Wheel and Pinion set (4.625:1 ratio) to allow TC0894 to run easily in modern traffic when finished and one of his modified pinion seal housings incorporating a modern oil lip seal.

New bearings were fitted into the bearing carriers by a friend who had access to an oven to warm the carriers to ease assembly and a suitable press. Loctite 243 was also used to aid the interference fit of bearing to carrier.

The use of Loctite in an interference fit assembly is suitably described in the Loctite Design Manual which I have and basically, no metal surface is perfectly smooth when viewed under magnification and the actual metal to metal contact between the interfacing components is only 25 – 30% of the total surface area even with the most extreme of interference fits. Loctite will fill the gaps between two interfacing components, formed by surface imperfections, thereby increasing the effectiveness of the interference fit.

I also purchased a pair of Roger’s axle nuts that incorporate modern oil lip seals to prevent the differential oil contaminating brake shoes, which is another common problem. However, a major factor in this well documented problem (i.e. Sherrell) concerns the fit of the axle hub to the bearing carrier. The flange on the rear of the axle hub must make contact with the outer race of the wheel bearing in order to ensure the bearing is ‘clamped up’ on assembly preventing axial movement in the carrier and in order to be certain of ‘clamp up’ there must be, on final assembly, a clearance between axle hub flange and bearing carrier flange. (See the drawing below).

When a component is designed, tolerances are applied to all dimensions shown on the engineering drawing to make production practical and cost effective.

General tolerances on length and width etc will be ±.010in (0,25mm) and if there are important features mating with another component on assembly tighter tolerances will be specified i.e. the rear axle bearing journal diameter. As a consequence of these tolerances every component manufactured in accordance with the same engineering drawing will vary dimensionally to a small degree.

In the case of the rear axle bearing carriers, each one can assemble on either the LH or RH axle journal. The resulting clearance on assembly, if one exists, between axle hub flange and bearing carrier flange can vary depending on the tolerances on bearing width, bearing carrier depth and axle hub rear flange length.

This clearance can be checked after the bearing carriers and half shafts are fitted to the rear axle but I used the following method prior to final assembly in order to take advantage of ‘selective assembly’.

Selective assembly means swopping components around until the optimum assembly combination is achieved, resulting from the dimensional variations of similar components as described above.

Therefore ‘on the bench’ assemble one bearing carrier (complete with bearing) onto a half shaft omitting the paper sealing gasket that fits between the two flanges and then assemble a brake drum to the axle hub and bolt up to achieve the final assembled condition. However, the important part in this method is to assemble the brake drum ‘back to front’ with the inner diameter facing away from the half shaft. This would of course be no good for braking or wheel fitment but does importantly allow full access to the joint between axle hub flange and bearing carrier flange.

Once the assembly is bolted up, feeler gauges can be used to see if a clearance exists between axle hub flange and bearing carrier flange. If a clearance exists it may well be less than the thickness of the paper sealing gasket that is supposed to be placed between the two flanges on assembly.

However, if no clearance exists, trial assemble the same half shaft with the other bearing carrier (complete with bearing) and repeat the clearance check.

Hopefully, by selective assembly, you can identify which bearing carrier should be assembled with which half shaft to achieve a clearance.

As has been well documented elsewhere (i.e. Sherrell) the case of zero clearance can be rectified by making up metal shims to fit between flange on the rear of the axle hub and the outer race of the wheel bearing to re-instate bearing ‘clamp up’. (See drawing at the end of the article).

The trial assemblies I carried out resulted in a similar clearance on each assembly, which is less than the thickness of the paper sealing gaskets I’d purchased. I will therefore use a Loctite sealing compound in lieu of the gaskets to ensure clamp up is maintained on final assembly. In theory, using axle nuts with the integral oil seals negates the need for any seal between the flanges but I feel it is still advisable to include secondary sealing to cover the possibility of nut seal failure.

The axle threads and bearing carrier studs were a little damaged but careful work with a thread file prior to final assembly enabled the new replacement axle and hub nuts and to run smoothly. Replacement oil scrolls in each end of the axle casing were fitted.

To accomplish this, I used two lengths of 12mm threaded bar joined together with a threaded connector, some 12mm nuts (all available at most DIY outlets) along with some suitably sized washers to pull out the old scrolls and fit the new ones. I acquired thick stainless steel washers from a fastener dealer at an autojumble. One had to be ground down to match the outer diameter of the oil scrolls and became the extractor/fitting washer. The other was slightly larger than the end diameter of the axle.

To remove an oil scroll, lock the extractor washer centrally on one end of the threaded bar with two nuts locked together on either side of the washer, insert the bar through the axle casing with the extractor washer up against the oil scroll and fit the other washer at the opposite end, up against the axle, and slowly wind a nut down the bar with the aid of a large spanner to pull the oil scroll into the axle casing and onto the threaded bar.

To fit a new oil scroll, place the oil scroll on the threaded bar against the locked washer and insert the bar through the axle casing with the scroll against the axle bore it will assemble into and fit the other washer at the opposite end, up against the axle, and slowly wind a nut down the bar with the aid of a large spanner to pull the new oil scroll into the axle casing until correctly seated. This action will be a little jerky during fitment as the scroll overcomes assembly friction in spurts but providing the process is not rushed a perfectly positioned oil scroll will result with ease. The oil scrolls on both of my axle casings were set in .30in (7,6mm) from the axle ends and I replicated this on fitment of the new scrolls.

However, as a footnote and result of a recent closely associated internet query on the TABC Yahoo website, it has been suggested that ‘the oil scrolls are not necessary, indeed undesirable because the seals within the new hub nuts need to be lubricated to prevent them running hot’. Therefore, despite having a reputation of being fairly useless at retaining the axle oil, having overlooked this important fact, I plan to remove them, which will be easy, to ensure good seal lubrication.

Final assembly of bearing carriers to axle journals necessitated initial careful use of hammer and drift (large socket) to ensure no contact with the bearing races occurred and when sufficient axle thread was visible the slotted lock washer was fitted along with an axle nut which was tightened to drive the bearing fully home against the step between axle journal and oil seal diameter. The new replacement hexagonal nuts make tightening and final torqueing to 130lbft very easy.

More facets of TC0894’s restoration will be penned as and when the spanners have been active again.

Steve Cameron

Contacts:
Metal Spraying ~ Avanti Engineering, Tipton, West Midlands (UK) Tel. 0121 557 1153


2 thoughts on “The MG TC Rear Axle

  1. Denis Dunstan says:

    I read this well written and researched article with interest as I am working on the axle of TC3715 at the moment. Could it be that the bush near the axle tube end and the associated land on the half shaft have two functions, one to aid in restricting oil flow along to the hub and the other to give added stiffness to the half shaft. The single row ball bearing in the hub will allow the flange to deflect whilst cornering quickly or when subjected to a heavy vertical load. In this situation the half shaft becomes a lever. If there is no support along the axle tube then that load must be supported by the side gears in the differential, thus resulting in accelerating wear in that component.
    I would add that when I started my career in road vehicle engineering semi floating and three quarter floating axles were commonly used in small lorries. These observations relate to those halcyon days. This bending action is commonly referred to in text books of that era.

    Denis Dunstan

  2. Simon Johnston says:

    I found this article when researching the topic of fitting new rear hub bearings on my J2 and many of the problems that Steve describes – worn bearing surfaces, dodgy threads, etc. – surfaced on the J2. (It’s the same setup, after all, and the car is fifteen years older!)I too am resorting to good ol’ Loctite but I would point out that the one that Steve used when fitting the bearing to the carrier – Loctite 243 – is actually a thread lock adhesive and Loctite make a range of ‘Retaining Compounds for Cylindrical Assemblies’, i.e. for securing bearings into housings or onto shafts, which might be more suitable. I ended up using Loctite 638 (which can accommodate wear of up to 0.25mm/0.010″)on the bearing surface of the axle casing – time will tell how effective it is.

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