The swap to later axles is raising the diff ratios from 4.71: to 3.54:1. It would be quite possible to fit the 4.71 crown wheels and pinions from the SIII axles to the new units, but since the Tdi should cope with the 35% gearing increase fairly easily, I have chosen to benefit from the increased speed, decrease in engine and transmission noise and increase fuel economy on the road – the standard gearing is a little too low, even with the Roverdrive engaged. The speedo will need recalibrating, but there are plenty of companies to do that.
Ideally, I’d use 4.1:1 diffs (KAM Diffs sell the crown and pinions for the Rover axle, and Dana crown and pinion gears are freely available in the US for the Salisbury diff, if you can get them shipped), but that would cost over £1,000 in parts alone. I’ll see how I get on with the existing 3.54s, and if they’re too tall, I’ll get the 4.1s later.
That leaves one problem – changing the diffs raises the high ratio gearing, which is fine on road, but also raises the low ratio gearing, which is a big problem off road for torque, low speed crawling and engine braking on hill descents. The fix is to use SII Suffix B transfer box low ratio gears, with their 2.81:1 ratio. Suffix C and all later Series transfer boxes (except V8 and 1-ton models) use 2.35:1 gears. These Suffix B gears are a direct swap for the later transfer box main shaft low gear wheel and intermediate gear cluster. The Suffix A gears use the same ratio, but the intermediate cluster’s internal diameter is much smaller, so don’t get that one by mistake.
The gearing reduction will offset most of the increase of the 3.54 diffs, though not quite all of it. With 4.1 diffs, the low range would actually end up lower than on a standard SIII, so the final gearing would give the best of both worlds – about 17-18% higher in “high” (ignoring the overdrive, which gives a further 28%), and lower in “low”.
At the moment, I’m just getting the parts; I don’t need to fit them until the axles are installed, but I don’t want to leave ordering them until later for fear of them becoming unobtainable – the limited number of Suffix B SII boxes built means that supply of these gears is probably rather limited too. I got the intermediate cluster (521330) from PA Blanchard for £50 and the main shaft gear (235438) from DLR for £30, both Genuine Parts. These were the cheapest genuine prices I could find.
Hi, have ordered these parts for the same reason you have, when I split transfer box the new gears are same as those fitted. Any chance of confirming part numbers or at least number of teeth so I can work out if I ordered correct parts or not?
Thanks, Phil.
Hi Phil,
Are you sure they’re the same? They look almost identical, and the only way to tell for sure is to compare the gears to each other directly, just like in my photos – the difference in diameter is just too small to see unless comparing side by side. I’m afraid I don’t have the part numbers any more as I threw their packaging away (one of them came in a Britpart box for a different component anyway – it had been stored in its sealed plastic bag loose and the box had been used for potage). If the output shaft gear is black, as my new one was, then that is a good indication. Of course, it’s not impossible that you already have a SII SuffB box anyway – there’s no way to tell unless you know the serial numbers or count the teeth on those gears as they are otherwise identical to the last of the SIII 4cyl transfer boxes, or someone may have made the same mod (knowingly or accidentally) on a previous rebuild of the unit…
Try removing the intermediate gear cluster to compare the spur gear diameter to the new one – it’s not a difficult job to get that out and refit it if the new parts are incorrect. Dunsfold Land Rover supplied one of my gears and Craddocks the other, so they can both provide part numbers if you still need them.
The B Affix intermediate shaft has 22 straight cut teeth on low range, and 39 straight cut teeth on the high gear wheel.
Standard later stuff has 36 and 25. The price of the intermediate gears is now between 160 and 220 sterling depending where you source them and genuine gear wheels up to 90 sterling (oem from eviolution are 30 sterling) (either early or late spec) written in FEB 2020.
Thanks – that may be of use to others.