S, M and A calipers

[hmd]

I know the S are alumium calipers.
What is the different between the M and A calipers and how do you identify them.

TIA.

[Bill Verburg]

A & M are both multipiece iron calipers

A - 3.5" mounts 2x48mm pistons

There are several versions of the M
all have 3" mounts and are multipiece iron calipers
front M's have 2x48mm pistons, rear M's have 2x38mm pistons

and there are wide A & M from '84-89 Carrera

These are A & M

[djpateman]

Some of the M calipers also have the 42mm pistons. These were used on the 914 front (no spacer for non-vented rotors), and the '84-89 Carrera rear in wide form (14mm spacer for 24mm rotors).
The 914 calipers have the inlet pipe in a different location and were used up to 1972. Later 914 fronts were similar, but slightly wider for thicker pads and were different castings. AFAIK all M calipers use the same pads with the noted exception of the late 914 with the thicker (but same face size) pads.

[Bill Verburg]

Some of the M calipers also have the 42mm pistons. These were used on the 914 front (no spacer for non-vented rotors

As I know next to nothing about 914s I'll take your word on that.

There were M front, again w/ w/ 2x48mm pistons used on the earliest 911s w/ 18mm solid rotors rather than the vented 20mm used on S and all other 911 '72-'83

The part #s for the 914 front calipers are
914/6 (I always thought that these were the same M used on 911 w/ 18mm solid rotors)
911.351.907.00 /L
911.351.908.00 /R

914/4 -'72( I always thought that these were L's but am not at all sure)
311.615.107B /L
311.615.108B /R

914/4 '72 -(as above)
914.351.425.00 /L
914.351.426.00 /R

and there is a difference between the 914/4 and 914/6 rotor but again I haven't a clue as to what the difference is. There were several different versions of the M fronts used between '64 and '72 but all 911/912 used 2x48

[djpateman]

914/6 front calipers are the basic M 2x48mm pistons, spaced for the 20mm vented 911 rotor. Vented front rotors, unique solid rear rotors. The 914/6 GT had even more unique vented rear rotors.

914/4 front rotors are a VW 411 part.
The early 914/4 front calipers look identical to the 911 rear calipers in both 2x38 & 2x42 versions except for the lack of spacers to widen them for the vented rotors, and the fact that the line inlet goes in the back behind the piston.