Rear wheel cylinders, MGB and Midget
The MGB and Midget rear wheel cylinders are part of a range identical in design apart from the bore size, which ranges from 5/8 to 7/8 (I think I have seen 15/16 too) in 1/16 steps. The sizes as fitted originally are:
- MGB roadster 13/16”
- BGT 13/16” early cars, 7/8” later cars (change point about 1968)
- Midget ¾”
The Mini and Metro used smaller sizes of the same series.
If the cylinders used on the same axle are different from side to side a braking imbalance will result, leading to instability under heavy braking. In order to reduce the likelihood of mixing up the cylinders on the same axle, there is a roll pin which protrudes from the rear and engages with the backplate, and this roll pin is in a different position for each size of wheel cylinder.
If the roll pin doesn’t match the hole, you have the wrong size cylinder for your car. This is particularly important to note if you are replacing one cylinder only.
If the pin in the cylinder you took off was missing or the backplate has been fiddled with to make it match, it will pay to check the wheel cylinder on the other side to make sure it is the same size.
Sometimes it is desirable to change the cylinder size to improve the front/rear braking ratio – for example, race cars usually need to effect such a change due to the differing characteristic of race grade linings or how the car has been lightened. Changing to another size cylinder in the range is an easy way to do this.
When you change cylinder size, you will therefore have a mismatch between roll pin and backplate which will have to be addressed. While the simple way is to remove the roll pin, you also remove the protection that you/someone may in the future inadvertently be supplied the wrong cylinder and end up with different sizes on each side. For that reason it is preferable to either modify the backplate (plug & redrill) or fit backplates off the original application.