Which Radiator Cap

[FloydP]

Can anyone tell me if I need a 7lb or a 13lb radiator cap for a 1967 mk1 GT6
Cheers,
Floyd.

[rotoflex]

The Mk1 & Mk2/+ were originally fitted with a 7 lb. cap.
The Mk3 was originally fitted with a 13 lb. cap.

The Mk3's cooling system is a 'no-loss' system that squirts coolant into the overflow bottle as it warms, then sucks it back into the radiator as it cools.

As far as I can tell visually, there's no difference between the radiators on all of them (except for the emission control Mk3 models that had a vacuum switch doodad stuck onto the radiator).  It is likely that all the magic is in the Mk3 radiator cap design.  I'd think that if you'd like the no-loss system, you could just make sure that the overflow tube goes to the bottom of the overflow bottle & use the Mk3 radiator cap.  Keep an eye on the temp gauge for a while, & if it goes hottish undo the rad cap to the first click, turn on the heater (ENJOYABLE IN A GT6, DON'T LET ANYONE TELL YOU DIFFERENT), if that doesn't do the trick stop to let it cool down, & limp home to get the 7 lb. cap.

[FloydP]

Thanks rotoflex, it had the 7lb cap on it when I got it and I've not had any problems, I just wondered if it was the correct cap as all the parts suppliers just seem to list the 2 types with no info on which one goes with which. So thanks again for clearing up the confusion, I think I'll replace it with another 7lb one.
Floyd.

[heraldcoupe]

The change to 13lb caps was made on most Triumphs during 1969. There should be more specific information in the parts lists and workshop manuals for the specific models.
Most caps are made with different lengths to the locating tags to identify between 7lb and 13lb caps. Those on the 7lb caps are generally 7/16" long, the 13lb caps have 11/16" tags. The radiator will have corresponding lugs, so fitting a 13lb cap can not be done without noticing.
Cheers,
Bill.

[rotoflex]

Thanks Bill, that's very interesting.  So you could get a 7 lb. cap on a radiator that originally had a 13 lb. cap, but not vice versa.

I wonder if there was any difference in the position of the overflow pipe to accommodate the 13 lb. no-loss cap design, or if the cap & its corresponding lugs on the radiator neck are the only difference.