Triumph Repair: cooling problem TR4, impeller blades, radiator cap

Question
Dear Howard,

My TR4 radiator stays cold at the bottom part as well as the waterpipe to the engine. After filling with water I first had the engine running without the radiator cap to leave air out. It seems as if there is no circulation, both waterpump and radiator are new. The engine was completely rebuild. What could be the cause?
Thanks, Ton

Answer
Hi Ton,
The TR-4 has a large bypass hose so you must use the correct thermostat with the balker ring on it to close off the bypass port when the thermostat opens. Otherwise much of the hot coolant will not go to the radiator but will just return to the engine and cause it to over heat. Another problem is the fit of the new water pump. You need to measure the clearance between the impeller and the back plate. Some aftermarket pumps were made wrong and there was an excess clearance between the impeller blades and the main case and the coolant just cavatated and did little circulating. I usually just put a lump of modeling clay on a blade and did a fit check to see what the clearance was. I would expect to see about .015" to .030" clearance. The few that I found wrong were way off like .050" or more.
Howard