In 2012/3 Garth Wright modified the rear axle to take a 4HA crown wheel and pinion of ratio 4.1:1 made for a Jaguar XJ6.
New custom-made half-shafts were manufactured and fitted, different housing etc obtained for the crown wheel and pinion obtained from a Reliant Scimitar.
See full details
here on page 15
After quite a few years trying to achieve a quiet rear axle for my Jupiter, I eventually had to re-assess my options, having decided that I couldn’t live with the irritating noise any longer. I re-directed my quest, having to hand at this time one spare axle with worn standard 4.56:1 ratio CWP gear set, and the axle in the car with a new 4.11:1 ratio CWP gearset from a batch made a few years ago.
This latter axle had proved impossible to set without a high pitched whine on the overrun and general meshing noise, even with numerous time consuming and expensive adjustments of pinion pre-load tension and gear setting.
Salisbury 3HA gearsets are not in supply, the other early cars which used them (Morgan, early Jaguar XK120, and Aston Martin DB etc.) having long since been up-rated to the stronger Salisbury 4HA units, which continued to be used by Jaguar and other sports cars for many years continuing through the E-type and other IRS models.
I had read of the Mike Alfrey’s Ford Falcon conversion, and Neil Hood in Australia kindly answered my enquiries with useful photos of his Volvo axle conversion, although this was for a much higher ratio to suit fitting 15 inch wheels.
However it did give me the inspiration to proceed along the same lines of grafting a differential with available gears onto the Jupiter axle tubes.
I decided to follow the Jaguar 4HA route. The 4HA gears and differential parts are larger, so you cannot just re-machine the 3HA casing. 4HA gearsets of many different ratios are available, though rarer unless very high Jaguar prices are paid – but they are out there! Specifically, high quality Jaguar sets were supplied up to at least 4.55:1 for some E-types.
I wanted if possible to stay with the 4.11 ratio that had at least been the one good point of my noisy 3HA set.
To recount the whole project researches with ups and downs would fill a book and not all be good reading, so herewith a brief summary of what was actually done.
Ingredients: Donor items noting parts used from each:
· One spare Jupiter rear axle. Parts used: Axle tubes with suspension mountings, handbrake/Panhard rod bracket, hub fittings and via e-bay:
· One rigid Salisbury 4HA back axle ex-Reliant Scimitar. Parts used: Differential casing and the internal differential spacer/thrust block only
· One Jaguar XJ6 IRS differential unit with 4.1:1 ratio later series ‘thick crownwheel’ CWP gearset. Parts used: All internal gears and differential parts, pressed steel back cover with integral breather.
New custom parts:
· Two new halfshafts were made in EN24T with induction-hardened splines, to my custom design to match the Jupiter hub ends to the larger diameter 19 splines of the 4HA differential side pinions. The resulting shaft is very similar to the Jaguar XK120 shafts and although I had to wait months for a batch of these to be made as my two were put through as modified members of the same batch to hold the price down!
These are Garth Wright's photographs (this page and overleaf) of his recent Jupiter rear axle work - perhaps of interest because of the scarcity of any crownwheel and pinion sets for the standard 3HA Jupiter axle and ratios.
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- gw2.jpg (12.9 KiB) Viewed 226 times
- gw3.png (87.42 KiB) Viewed 226 times
- gw4.png (97.21 KiB) Viewed 226 times
I also took the opportunity to redesign the outer seals to a Nitrile metric size (36 x 58 x 12) to negate the need for the difficult-to-source Payan leather ones.
The ex-Scimitar axle tubes were cut each side of the differential casing, and jig welded to the Jupiter tubes which had been cut off to the exact length to maintain the axle width, and the handbrake/Panhard rod bracket ground off and re-welded – a tight fit to the bigger housing.
The differential was rebuilt using the Jaguar parts but with the central thrust block from the Scimitar (note: this part is not used in the Jaguar IRS units which have stub axles). Luckily the Jaguar CWP gearset in my e-bay acquisition was in excellent condition so this avoided paying Jaguar prices for a new set. I had however located three similar sets at re-builders if needed. These gears are 45:11 (4.0909:1) so with the higher teeth count are arguably better than the 37:9 (4.11:1) teeth of my noisy 3HA set.
The pinion splines of the 4HA are the same as the 3HA, as is the pinion seal (which can be bought as for E-types in Nitrile with integral gasket and dust seal as better than the Payan type). Hence the standard 1140 series flange for the propshaft was re-used, with the 4HA pinion shaft pre-loaded with a crush tube rather than shimming.
That’s it in a nutshell – easier said than done! There are many points of detail that have been fully recorded and filed with all the drawings and details of suppliers including who holds the programme for the halfshafts – these notes will stay with the car. The casing is also fully tagged as a 4HA conversion with its ratio. But for the purists, if some good 3HA gears appear, then the original axle could go straight back in. That said, the 4HA axles can take up to 300 bhp so I’m hoping it will be there for a while!
The result is a transmission that is as whisper quiet as I dared hope for - no sound in any mode, just that lovely Jupiter exhaust burble. Once more, a pleasure to drive – all we need is some good cruising weather!
I am indebted to a great engineer in Tony Absolom of Automotive Solutions who runs a local service garage and classic race car support team. He undertook much of the work with skill and enthusiasm, not to mention his 4HA jig, superb welding, and database of Jaguar differential ratios.
I’m just glad for my part that I appear to have got all the dimensions right!
And thanks for the patience of many JCC members who have supplied eartime, parts, and contacts along the way!