The only difference between the boxes is the ratios. Strength etc is exactly the same. If you have to have close ratio set you have 2 x options:

1. Convert to M21 ratios: 2.54, 1.83, 1.38, 1 :1.

2. Convert to XU-1 ratios: 2.54, 1.83, 1.25, 1 :1.

(Standard M20 is 3.05, 2.19, 1.51, 1 :1).

Option 1: Buy a new M21 input shaft, M21 cluster and M21 3rd gear.

Option 2: Buy a new M21 input shaft and XU-1 cluster.

Both options buy a rebuild kit and put it all back together. Only thing to check is that your current M20 has a neoprene front seal (later than about 1973). If its an oil slinger type youll need to buy another V8 input shaft cover with the right seal.
Also you might need other parts too if they are worn out, like 1st, 2nd, reverse, main shaft, synchros etc. these are the same for all boxes. All parts can be bought from Kingswood Country - speak to Reece. Ive got some of the parts here but you might as well buy the lot off Reece.

Warning - dont change to M21 or XU-1 ratios if you have a 3.08 or taller diff. You and your clutch will hate it!



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