That photo of the restrictor reminds me of what they used to do 30-40 years ago with racing engines. Bet they don't do that any more. I've raced A series engines for more than 20 years (started in 1988) & not once has an engine builder, machinist or any competitior suggested doing this. I've always run a 62 degrees celsius thermostat as against the standard 72 degree one, mainly to get the thermostat open earlier. The opening in the thermostat is smaller than the outlet anyway, so the larger housing shouldn't be an advantage.

"neck down" wont hurt anything, water goes through it just fine. The A15 had smaller ones than A12.

The correct one depends on the car you put it in, the cylinder head you are using and the radiator outlet positions.

This is the right one for stock Datsun 1200:


More pics and details

thoughts anyone??

I just wonder if it really matters which inlet, the stock A15 one necks down pretty severely.....

I'm putting the A15 back together & thought I'd use this other H2O inlet port due to it being a larger diameter (thought was more flow, less restriction)but looking at it closer the hose end is quite a bit larger then the stock A15 hose so that creates a minor problem but I begin to wonder if the smaller A15 stock one is like that intentionally to restrict flow to help get to temp faster?? I put in a 180 degree F thermostat.

This is also an H72 head, not the stock A15 head. I then out of curiosity took off the thermo cover on the A12 semi built motor I have (I didn't build it)to see if there was much difference between the 2 covers & they 'd actually restricted the flow even further by eliminating the thermostat completely, plated it off & drilled a much smaller hole - would a racing engine want this type of set up & why?

the motor



the head



Stock A15 intake on left, not sure what the right one is off, anyone know - its the larger port one...






A12 w/small flow hole...



thoughts anyone????