Jupiter Rear Axle conversion Salisbury 3HA to 4HA

[garthwright]

Following the poor results (meshing noise and overrun whine) obtained after many expensive 're-sets' with 4.11:1 CWP set from batch made a few years back for the standard Salisbury 3HA axle, I have now successfully completed the conversion of my spare Jupiter rear axle to a Salisbury 4HA type, using high quality Jaguar gears. This seemed the right progression in line with the move of the early Jags, Aston Martins, Morgans etc. who quickly discarded the weaker 3HA, albeit the modestly powered Jupiter hardly needing it for the same reasons !

The result is tremendous so far - the Jupiter now has a whisper quiet transmission in all modes and I can start enjoying driving it again. And it should hopefully last a few years with its up to 300 bhp capability!

I will write a summary for Jowetteer in due course, but it has taken the time allowed by recent retirement and many months of research to get the right compatibilites and ensure gears are available (as long as Jaguar 4HA gears are...).
By pure good fortune I came across a local garage/classic race team run by a superb auto engineer who has Salisbury 4HA jigs for welding the modifications - as part of his contracts he maintains the only racing Sunbeam Tiger in the UK, also with a modified 4HA axle.
In summary, a rigid 4HA casing from a Reliant Scimitar 5SE was jig welded to the Jupiter tubes. The parts from a later Jaguar IRS differential (E-type/XJ6 etc) were then used with their 45:11 CWP set to give similar preferred 4:0909:1 ratio, though E-type gears are available up to 4.55:1 if a standard ratio needed.
All major donor parts were obtained via e-bay, the only significant new parts being the halfshafts to match the larger 4HA 19-spline side pinions to the Jupiter hub. These ended up so similar to the Jaguar XK120 halfshafts that I managed (but with a few months wait!) to get these put through with a batch, absorbing the slight program modification and keeping a standard price.

The design/drawings etc. will be kept with my Jupiter and the diff has been carefully labelled as a 4HA conversion - however, for the purists, if good 3HA CWP sets become available in future then the standard 3HA axle could go straight back in!

It has been a lot of work over quite a time, but so so so worth it !

[Forumadmin]

Could you offer this (or parts of the procedure) as a service to other people?
Do have a write up and pictures already, as these can go in the Gallery where you can store all the detail and not have to compress into the Jowetteer?

Great job!

[garthwright]

I am happy to share all info learned (and photos) in the process of doing this along with potential parts suppliers - however, once I had worked out what was needed, so much time was spent sourcing e-bay donor units that it would not be practical to offer a complete upgrade service - it can take a time for the right units to come up to avoid for example having to pay ~£1000 for a new Jaguar gear set on top of other work. Really you need to ensure you have all the right bits before proceeding - and there's a lot a scrap metal you don't need from these parts !

Also to beware that in the course of this project I had been offered a '4HA conversion' work by someone in the transmission trade who claimed it could be done commercially for around £700. I am very sure it can't be done properly by a third party for anything like this figure, and it was suspicion over them not telling me anything about what would be done that led me to find out for myself what is needed - any work on my one and only spare axle was going to be done with my knowledge of the detail!

I have drafted the article for Jowetteer and I will now look at a more detailed step by step version of what was done for others to judge if they want to take on something similar.

I did get inspiration to follow this route from reading of Mike Alfrey's (I think it was a Ford Falcon) diff conversion and Neil Hood in Australia kindly sent me photos of his Volvo diff conversion which was done to a much higher ratio to match fitting 15" wheels. Same general method - however the 4HA route seemed more natural considering potential CWP ratios and parts available in the UK.

[Drummond Black]

Garth,
I drove Mike Alfrey's Jupiter when I was in Australia. We used it over a distance of about six hundred miles. Initially the take off with the higher gearing was a bit strange, however, after a few miles I got used to it. ( From memory I think his rear axle ratio is 3.9 : 1 ) I found I had to use the gearbox more often than I do with my own Jupiter when out of overdrive. It did cruise very well when out on the open road.
Mike did say that if he was doing the same modification again he may consider an " Overdrive Option " rather than a higher geared axle.

Definately a good modification for a Jupiter.

Drummond

[Forumadmin]

Pictures here in the Gallery of Mike Allfrey's conversion.

This one is probably the most informative.

[garthwright]

Hi Drummond

I did at least with the noisy 4.11 (37:9)set in the original axle have the chance to test this ratio over quite a few hundred miles as I tried unsuccessfully to bed them in. It was good in that although there may be the the need to use first for start off a bit more often when hill starting, it was otherwise very usable and great for cruising.
Hence the use of a very similar Jag ratio and I think the 4HA diff (has much thicker crownwheel) with the 45:11 tooth count gives the opportunity for a smoother result anyway.
Guess the overdrive option would be more flexible but can't quite contemplate the complexity - i.e. have had enough bits to maintain without one of those as well!

Here I will confess that while out cruising with my new quiet rear end the other day, I relaxed and missed a gear, taking a tooth off the first gear train....
Fortunately have sourced a replacement recn'd g'box and someone the rebuild this one as a spare, so it's back to the garage for a few days.

As they say, pride comes before a fall...

regards Garth

[Forumadmin]

Ah first gear. Best avoided. I very seldom engage it, and do it VERY carefully and slowly and with little right foot. Except once doing a standard quarter at the MIRA circuit breaking my rule religiously followed for 20 years, got the fastest time but with a stripped first gear. Never again!

[vicboddy]

Hello Garth, this is very interesting! Could you post the contact details for the garage mentioned? I have been thinking about back axle changes for some time, especially as I have a spare Healey 3000 axle (4HA) and both 4:1 and 3:9 ratios available. I believe some older London taxi's used a 4:3 ratio, but dont know if this is correct (or relevant). And if not a good idea to post the contact details, my email address is vic.boddy@hotmail.fr. Many thanks Vic

[garthwright]

Hello Vic. Let me speak with Tony to see what work he is happy to take on - I say this because the work he did for me was done in his own time between running his busy garage, working to the detailed drawings and dimensions which I specified from measurements on my axles and with me muttering over his shoulder to take responsibility for them!

The essentials, apart from good welding skill of course, are a jig to bolt onto the back of the 4HA casing to accurately set the Jupiter axle tubes and mountings in line, and then obviously the diff set-up experience. So a good engineering shop with these capabilities could do the work. Otherwise the critical bit dimensionally is to measure the resultant axle width (targetting the same as the original Jupiter - mine ended up within 1mm), which with the longer 4HA thrust spacer in the centre will define the halfshaft length allowing sufficient shimming of the hub end bearings (I allowed max 1.00mm per side shimming to be safe).
The program for the halfshafts is with Orson Equipment in Dudley West Midlands under part No 1H1076.IH - it would I am sure be easy to slightly respecify the length to suit if need be.

As a note there were a couple of precautionary things I did in addition to the axle work. I could not be absolutely sure that there was quite enough take up on the prop shaft splines as the 4HA is 10mm longer on the snout and my original shaft was already close to it's shortest! Rather than just shorten the existing shaft I bought a new prop shaft from Propshaft Services at Heathrow which is 10mm shorter - these shafts can be supplied new and accurately balanced within 24 hours at £130.00 each. At only 10mm difference this shaft can be used with the standard axle - there's plenty of longer spline movement.
Also the casing at the pinion bearing is slightly bigger on the 4HA. Again the Panhard Rod clearance looked OK but because of the geometry it's difficult to be 100% sure on fully defelcted axle so the old corroded rod was replaced - I made a new one in high tensile tube with a short reduced section one end in HT steel at 15mm dia over the axle snout section. I note this as there are probably dimension differences between Jupiters so just to be sure. Garth