[TR] Radiator Caps

[Randall]

> The TR2 through
> TR3A engines use a 4lb cap and the TR4 through TR4A engines use a 7 lb
> cap.

> What is the deciding factor that makes the difference?

Personally, I suspect there was no real reason except they originally
thought 4 psi was enough, and later decided that 7 psi was better.  

But others think it was due to the change in heater design.  The TR2-3B used
a circular heater element that internally had wide, flat tubes that weren't
very strong.  With only a little bit of internal corrosion, they can fail
from too much pressure and dump scalding coolant onto the occupants.  The
TR4 & 4A had a more modern, round tube heater core.

AFAIK, early TR4s used the same radiator and thermostat as later TR3s.  They
did eventually switch to a pellet type thermostat, but I think that wasn't
until later, maybe 1965.

Certainly there is nothing in the engine itself, as the original engine in
TS13571L is running quite happily with a 10 psi cap (and no heater).

-- Randall