Is
your cap not sealing properly? This may be the
culprit. Hard to repair when top surface is bunged up.
If you know someone who does precision
welding/brazing; like an aircraft guy; you could get a
3-hole top plate and cut off the mounting plate and
reinstall your old 4-hole base plate; or remove and
install the tube off of the 3-hole plate.
One
thing that can help this part endure life is to be
sure the little hose that runs from the thermo housing
top plate to the rad is of VERY flexible light
material.
Even
the original hose is too stiff. The jockeying around
of the engine and a too-stiff hose contributes to the
plate fracturing or the piece on the radiator
fracturing. I'd also suggest making the little hose
longer; so it has a big gentle curve in it; this would
also tend to reduce impacts when the engine rocks and
the radiator stays still.