Rear brake cylinder sizes

Comparison of wheel cylinder sizes.

Lockheed rear wheel brake cylinder, twin piston comparison as found on BMC rear axles [Midget, Sprite, Austin A40] and Mini rear. Fixed cylinder, one per side.

Cylinder bore	Unipart part number	AP Caparo part number	Old Lockheed part number	Notes on original applications (not an exhaustive list; approx years)
1/2" (0.5)	GWC1126			Mini 1275GT rear - 1975 to 1976.
5/8 (0.625)	GWC1101	4241-075P	LW11075	Mini Mk1 Cooper S with servo; rear. Mini MkI (not Cooper or Cooper S) rear - 1962 to 1967
9/16" (0.563)	GWC1131			Mini MkI rear - 1959 to 1967
9/16" (0.563)	GWC1134			Mini MkIII rear - 1974 to 1978
11/16" (0.689)	GWC1129	4241-193P	LW11193	MG Midget [1500] rear - 1975 to 1979. Mini 1275GT rear - 1964 to 1975.
3/4" (0.75)	GWC1102	4241-396P	LW11396	Austin Healey Sprite & MG Midget (1098 & 1275 engines) rear, 1964 to 1975. Austin A40 Mk2, rear - 1962 to 1968. Mini MkI (not Cooper or Cooper S) rear - 1959 to 1962. Mini MkII (not Cooper or Cooper S) rear - 1967 to 1975 plus other Minis
7/8" (0.875)	GWC1115	4241-933P	LW10104	1962 MG Midget Mk1 & Austin Healey Sprite MkII rear (with front drum brakes and twin piston fixed single rear wheel brake cylinders). NB earlier cars have sliding single piston wheel cylinder of 7/8" bore in a different back-plate.

NB position of locating pin and matching hole in original rear brake back-plate may vary, so might need to drill different locating pin hole in rear brake back-plate if considering changing sizes of wheel cylinder outside original application.

Note early Austin Healey Sprite and MG Midgets (until approx late 1962) as well as Morris Minors had a different style wheel cylinder (single piston, sliding), rather than the above fixed cylinders with dual pistons..

Would MGB wheel cylinders GWC1103 (7/8" [0.875]) - MGB GT and MGB Roadster GWC1122 (13/16"? 31/64? [0.8?]) fit any of the above rear back-plates?

Rear brake backplate comparison

On the left In pictures 1 and 2 below - early Sprite and Midget (948 engine and front drum brakes) with sliding single piston single wheel cylinder (7/8 inch diameter piston bore); middle is the later 1098cc and 1275cc engined Sprite and Midget (had front disc brakes) with a single wheel cylinder with twin pistons (3/4 inch diameter) which is fixed, not sliding, and on the right is the Wolseley 8 inch.

Note do not have a picture of the 1500 engined MG Midget rear brake backplates for comparison and whether these differ from 1098/1275 backplates (may be difference on location of wheel cylinder mounting pin hole?).

First picture below shows the rear of the rear backplates and the second picture below shows the front of the rear backplates.

                                                                       Picture 1

Picture 2

 

Picture 3

Picture 3 above - comparison between early and later backplates and wheel cylinders - upper pair of backplates are early Sprite/Midget: note the sliding wheel cylinder is a Morris 1000 of 3/4 inch bore, rather than the 7/8 inch bore rear wheel cylinders for MkI & II Sprites that have front drum brakes (my 1961 AH Sprite MkII has front disc brake conversion). Lower pair of backplates are later Midget and Sprite with single twin piston wheel cylinder.