Speed rating?

[jagnut66]

Hi,
What is the speed rating for the standard 948 axle diff? It's not that I want to go charging about but I would like to know if it will sit at 70 mph comfortably. Failing that I will need to continue searching for a later Morris 1000 axle casing (I have a 4.22 diff).
Many thanks,
Mike.

[liammonty]

70 mph 'comfortably' is a subjective one - to put it into perspective, either a 4.55 (948) or 4.22 (1098) rear axle will allow this with a standard gearbox, but the revs will be high with either. According to the online calculator I used, engine revs at 70 mph would equate to 4650 rpm with a 4.55:1 final drive, or 4300 rpm with the 4.22:1 final drive. In my experience, the 3.9:1 final drive from 1275 Midgets, for example, helps a lot (3975 rpm at 70 mph), but unless you have at least a 1098 engine in good shape, it will be too much for the engine. My 1098 Traveller, with a strong engine, cruised noticeably better with this final drive than the standard 4.22:1. the calculations above are all assuming 145 80 R14 tyres - if you have 155 80 R14s fitted, the revs will be marginally lower at the given speeds.

You don't need to change the axle if you are planning to do so due to the potential lack of filler if you swap the diff - you can do as I and others have done, and drill the casing at the appropriate level, then weld a nut to the outside of the casing. You then have threads for a filler - in my case, I used a short bolt. Others just fill the axle with the measured quantity of gear oil through the breather, though of course you have no way of checking the level in the case of leaks.

[jagnut66]

Hi,
How high up would I need to drill it? I could guess from pictures but if it's a set height and someone knows I'd be grateful.
Best wishes,
Mike.