I need some technical help and I know this is the place to get it! The new heads I bought from Summit have smaller holes for the temperture switches then the stock heads had. I am not a pipe thread expert but i think the stock holes are 1/2" pipe threads and the new heads only have a 3/8" pipe thread hole. Summit thinks I can use a sending switch from a 89-91 Camaro or Firebird. Will the calibration be the same and how can I check it now that the engine is out? Also why does the 74 vette Auto trans have a sending switch in both heads? The drivers side is a single post switch which I see from the wiring diagram to be the gauge switch. If I am reading the diagram correctly the passenger head switch is called a dual temperature override switch. Should I just plug this hole since I am not using any emissions? Not sure i understand the complete operation of this 2nd switch. Both switches use a green wire, but the connector on the passenger side with the 2 terminals has only one lead but is wired from the one terminal to the other then into the wiring harness.Hope all this makes sense. I dont want to have to use an aftermarket temp gauge.

The temp sensor in the pass side is for the vacuum advance/TCS(transmission controlled spark) system. It would either allow/block voltage to the vac. solenoid, depending on engine temp. If you have removed the vac advance solenoid on the engine, then the sensor is not needed.

I've heard many folks having the same issue with the thread size in these aftermarket heads. You can either go with the smaller temp sensor, which may or may not be accurate, or you can install the sensor somewhere else, such as in the thermostat housing. The reading in that location will not be a "true" engine temp, but it beats nothing.
hth

Joel and forum, when u say the solenoid on the engine, are you talking about the one on the front of the intake passenger side, that has vacuum lines and an electrical connector about 3 inches long and 1 1/2 round? If this is the one, am I going to need it with the modifications that I have done? Also do you know of any other model cars that may use the smaller thread pattern that I could make work on this head? I may have no choice but to eliminate it and will it matter? Thanks for your knowledge and help!

The TCS system controls the vacuum advance depending on what gear the trans is in. For the '74, it would only allow vacuum advance with the trans in 4th gear(3rd & 4th on earlier models), or when the engine is cold, or overheated. There was also a timer/delay relay in some of the systems, to delay the vacuum for 20-30 seconds after the trans went into high gear. It's a weird system, but it was necessary to pass emissions. Since the '74 is no longer required to pass emissions, you can remove all of the parts of the system if you want. If you are going to aftermarket heads, intake, carb, etc., I'd say get rid of the TCS set-up...it prolly wouldn't work correctly anyway.

I don't know of any other year that would use the smaller temp sensors, but it is possible the later cars did('81-'82 maybe?). Perhaps someone here with a later car could verify. I know there are several folks here that have gone thru the same problem with the temp sensor when installing aftermarket aluminuminum heads, so someone here should have the skinny on how to do it...and have a correct temp reading.

I moved my sensor to the intake manifold between the thermostat and the head. I found that the temp did not vary by much and the thread size is the same as the stock sending unit.

In your first post, you say it is an automatic car, so you should not have any of that TCS stuff.  It is not unusual to find  odd modifications on a car that old, so I am sure Joel is right in saying you should just take it all off.  I also agree that your aftermarket intake should have the correct holes in the passage across the front of the intake.  That should give a very accurate reading.  I also suspect that you could use the later Camaro sender in the head.  The only way to be sure is to install it and see, or put it in a pan of water, hook a guage to it, and heat the water.  It is not like GM to change a calibration for no good reason.

Larry

the 89-91 sending switch is what i am going with. this alllows me to keep the same wiring connector without cutting any wires. I purchased it at NAPA part # TS6641. My cost was $27. Just thought i would let anyone else thinking about it have the parts number. I should be firing up the car this weekend if all goes right and then i will let you know how it worked.

Switch worked fine. reads good and not leaking.