This is a problem I ran into during my engine build. I have a factory cast iron intake manifold with the large thermostat opening. I bought the Spectre 4736 Water Neck due to the fact that it had 3/8" threads tapped into it. I wanted to be able to use my new temperature gauge along with my idiot light. The problem that I ran into was the fact that the thermostat that correctly fits the new water neck would fall into the intake manifold!
Here, you can see the difference between the big hole factory water neck and my new chrome water neck.
I measured the factory water neck at 2.57". The thermostat that goes with the factory water neck measured at 2.49"
I measured the aftermarket water neck at 2.15". The thermostat that goes with this water neck measured 2.12" outside diameter.
Here is the opening on the intake manifold, measuring at 2.21". You can see where there would be a issue! It is larger than the thermostat opening on the chrome water neck, allowing the thermostat to fall right into the manifold.
My fix for this was to make an adapter plate. I tried to purchase one online, but I was e-mailed and told they were discontinued!!
I used 22GA steel sheeting to make my plate.
I began by cutting a 1 3/4" hole in the plate with a hole saw.
After cutting my first hole, I used a gasket to line up where the hole for the bolts was supposed to be. I drilled them out using a 3/8" drill bit.
I also used the gasket for the outline of where to cut my plate out at.
For cutting the plate, I placed it in a vise and used a 4.5" angle grinder with a cut off wheel. Cut through it very easily!!
Here is a picture of the plate after I cut it out.
Here is the order I installed everything.
-Water Neck -> Thermostat -> Gasket -> Adapter Plate -> Gasket -> Intake Manifold
Here you can see how the plate holds the thermostat in place and does not restrict the flow of coolant at all.
Before the final installation, I made sure to put a good coat of sealant on everything so I wouldn't have any coolant leaks.
Here it the final installed product, with the temperature sender in place.