| | |
When you realise how many different ratios there were in the Minor's hey day, it is easy to see how some cars end up with completely the wrong one. The axle ratio is often quoted as being 4.55 to 1. This simply means that for one revolution of the wheel, the engine has to do 4.55 turns or revolutions. From this you will gather a very rough check of the ratio can be had by just jacking up the back of the car and seeing how many turns of the prop shaft are required to turn the wheels once. But it is only rough, as some ratios are very close (i.e. 4.55:1 and 4.22:1). It's no good trying to change a ratio, unless you are increasing either the engine's size or power. The ratio is worked out to suit the engine brake horse power output, and the torque curve of that particular engine size. Generally the van ratios were lower to enable the low compression engine to pull the weights imposed on it.
  There are two types of axle/diff. unit around. The early type fitted to 803cc cars had the filler in the diff and the drain in the axle casing, but the later type has both filler and drain in the axle casing. You can end up with some odd combinations of either two drains and no filler, or vice-versa. Although it is possible to interchange these complete units as a diff or 'banjo' casing, the component parts are not interchangeable. I make a point of this because the only alternative ratios available would be from a second hand vehicle and you must be careful which type you buy (unless of course you are buying the complete axle/diff assembly).
Stronger half shafts from the 1275 'Spridgets' are a direct replacement part. Remember your speedo will need recalibrating when using an alternative diff-ratio. The following chart gives an idea of what you may find if you search round the breakers yards.
| Ratio | No. of Teeth | Diff Assy Part No. | From Model | | | | Morris Minor | | 4.55 | 9/41 | (Nuffield Axle) | Series MM (std) | | 5.285 | 7/37 | (Nuffield Axle) | Series MM (optional) | | 4.22 | 9/38 | ATA7326 | 1098cc car | | 4.55 | 9/41 | ATA7093 | 948cc car | | 4.55 | 9/41 | ATA7093 | 1098cc van | | 5.373 | 8/43 | ATA7073 | Series II 803cc and GPO van | | | | Other Models | | 3.727 | 11/41 | BTA551 | early Riley 1.5 | | 3.9 | 10/39 | BTA1222 | later Riley 1.5 | | 4.22 | 9/38 | ATA7326 | early Sprite/Midget | | 4.875 | 8/39 | 2A7230 | Austin A55 van | | 4.55 | 9/41 | ATA7093 | early A40 Mk1 | | 4.22 | 9/38 | ATA7326 | later A40 Mk2 | | 3.727 | 11/41 | BTA551 | Wolseley 1500 | | 3.9 | 10/39 | BTA1222 | later Wolseley 1500 | | 3.9 | 10/39 | BTA1222 | later Sprite/Midget | | 4.22 | 9/38 | ATA7326 | *see below | | *fitted to Sprite/Midget from HAN724731/GAN2 16183 to HAN9 66226/GAN4 77591 |
The teeth numbers are etched on the side of the crown wheel and can only be seen with the diff out. Divide the pinion teeth into the crown wheel teeth and that gives you the ratio ... 9 into 41 equals 4.55 ... 4.55:1 means about 4.5 turns of the engine to one turn of the wheel, which is where we came in.
| |
