This cost effective but high quality part may well be the answer. The thermostat is thestandard 82 degrees version and the gasket (OE ref. GTG103) is of a good quality (1.5mm thick).
TRIUMPH STAG MkI & II BRAKE MASTER CYLINDER REPAIR KIT 519060
TRIUMPH STAG MkI & II REAR WHEEL BRAKE CYLINDERS & CLIPS x 2 (GWC1211)
TRIUMPH SPITFIRE MkI-III, GT6 MkI-II WINDSCREEN SEAL CHROME INSERT TRIM 613954
TRIUMPH SPITFIRE MkI, II, III & IV, GT6 MkI, II, III AC DELCO ROTOR ARM GRA2110
TRIUMPH SPITFIRE MkI,II,III & IV ( '73) GT6 MkI HANDBRAKE CABLES 121766 & 133915
Triumph Stag ignition set fits all 35D8 V8 Stag Distributors
