> So can I use a TR4 radiator cap on a TR3??
Early TR4 used the same 1" reach neck as the TR3. However at some point,
the radiator changed and the later radiator used a 3/4" reach neck (similar
to many other cars). The later cap will screw on to the early radiator and
may even appear to seal, but will not hold pressure properly.
So if you want to use a 7 psi cap on your TR3 radiator, an early TR4 cap
will do. Note that, the last time I looked at a Stant catalog, they listed
only the later TR4 cap, without any notation that it would not fit earlier
TR4.
Another option is to have the filler neck replaced with a 3/4" reach neck.
My radiator shop charged less than $20 for this (as part of other work),
which gives me a much larger choice of radiator caps (including the Stant
"Lev-R-Vent" series, which I rather like). But if you go this route, be
careful to remember that the neck has been modified. The original 1" reach
cap will still go on, and result in the relief pressure being way too high.
One of our local club members suffers from CRS and had a terrible time
figuring out why his upper radiator hoses kept blowing up like balloons ...
-- Randall
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