This gasket was originally developed by Howley Racing way back in the mid eighties for what was then part of the factory Rover Group racing effort - the Group A Metro Turbo. The engines developed by Howley Racing were giving 220+bhp and 185lb ft running 16.4psi boost - quite a chunk of power in a car little bigger than a Mini! Despite 10" wide sticky slicks, the engine chirped the tyres in each upward gear change. Even more impressive when you consider this was still on it's single 1.75" HIF SU carb!
All that was basically done was to take a standard production head gasket (the then all over silver TAM1521 - a great gasket it it's own right) and add a reinforced fire ring by over-lapping the existing one with an extra fold of metal. As a consequence the block needs to be machined to accept this gasket - rendering it useless for any other gasket fitment and any further over-bores (even to 73.5mm) unless a top-hat type liner was used. These two points alone should give cause for much soul-searching before going ahead with their use.
Despite reasonable availability in the UK, they never caught on in competition because of their limited usage and cost! In the UK, racing regulations and poor racing fuel availability makes it pointless trying to run super-high static compression ratios. The highest probably being 13.7-1. But even at these levels, head gasket failures are minimal. They have, however, seen a rise in popularity in the USA where static compression ratios up to 15-1 have been used - more regularly 14.5-1 in GT5 I believe - due to allowance and availability of high octane racing fuels.
As a guide line I would say that these gaskets are probably only worth considering where block bores are not to exceed 0.040" (maximum diameter the reinforced fire ring can deal with) and where static compression ratios exceed 14.0-1 and standard head gaskets simply are not cutting it. But make sure this isn't due to some other reason (see 'Cylinder Head - Torque settings and gasket hassles' in Calver's Corner) before taking the plunge.
The machining data for correct fitment is :
Centred on each bore centre -
Fire ring recess OD in block top - 3.060"
Fire ring recess depth in block top - 0.027-0.029"
Note: recess corner between base of recess and wall of recess is to have a small 45-degree chamfer in it - NOT a sharp, dead square corner.