AFAIK the 3 basic styles GM-H 'big nut' master cylinders go like this.
The early one with the rounded ends starts at HK & finishes at 7/71 (end of HG).
The m/cyl with the 'squarer' ends (top pic in post #2) starts at 7/71 (HQ). This means that the last 6 months of LC (7/71 to 1/72) also got the older style rounded unit. This then fits all Holdens & Toranas prior to 1/77.
The larger/wider casting appeared for 1/1/77 (as S pack has said). This was for LX Toranas with a CLX chassis prefix & HX Holdens with a BHX chassis prefix.
As far as interchangeability is concerned. The later larger m/cyl will bolt straight on with the pipes swapped, but the early rounded one needs a different length push rod. Aside from that it will bolt on & plumb up as per standard.
All of the above excludes the baby Toranas (HB/LC/LJ/TA 4-cylinders)
Hi Dr Terry,
A very timely thread for me.
Just to clarify, my January 72 XU1 should have this type of big nut? I did suspect as much but had trouble getting confirmation from other people, just bought the wrong one too but no time to fit it so I'll wrap it up and try to swap it at some stage.
LC round top master cylinder.jpg 73.79K
I got this pic from another thread.