Temp hole thread size?

[Vancraft]

We are installing a later 230 into a ‘36 Plymouth and I see that the later head has a very small hole in the head, I assume this was an electrical gauge. I need to run the factory mechanical gauge. What size thread is the early heads? I would like to drill and tap the head to use the ‘36 mechanical gauge. 

[Sniper]

Where is this hole located? Because if it's above the number 6 cylinder that's supposed to be sealed with a pipe plug when not in use and its purpose is to mechanically set the ignition timing. Generally speaking coolant temperature probe goes in the side of the head not the top.

Edited November 18 by Sniper

[clarkede]

Sniper - Just including a picture I happen to have that shows the plug you're referring to on the top and the temperature probe fitting on the side.

[oldodge41]

I don't know the thread size for the temperature probe, but I remember it was also different on my '48 230 head than my original '41 218 head. I solved it by using the '41 head on the '48 block. If memory serves me right, mine looks like the one in the picture clarkede pictured. I think, memory not so great, that the '48 head had a smaller diameter hole in the side of the head. Both were Dodge engines.

Edited November 18 by oldodge41

[desoto1939]

they used a Gland Nut to thread into the side of the head. It is a brass nut and then the tube from the end of the mechanical temp gage threads into the gland nut.

 

Also notice the welsh plug directly above the gland nut. If you have to remove the bulb from the gland nut they ususualy get stuck in the glad nut. By twisting the tube you might break the bulb and then it is a pain to replace the entire temp gage and also costly.

 

SO, Drain the engine then pull out the welsh or otherwise known as freeze plug. You will then see the end of the tube inthe opening. Spay lot of PB Blaster in the area and let is soak over night.

 

Then take a soft mallet and then try to tap out the bulb from the inside of the hole take you time, might have to respray more PB Blaster to loosen up the tube or you might get lucky and it will pop out.  Clean around the opening put in some sealing material and then put in a Brass Welsh plug.

 

Open the attachment with pictures and this will make it easier to understand the process.

 

The little nut next to number 6 sparkplug is used to time the engine.

 

Rich Hartung

 

How to safely remove the temp gage end from the side of the engine head.docx

[Young Ed]

The original posters problem is that in the mid 50s somewhere they went to an electric gauge with a sending unit. Those heads have a significantly smaller hole than the mechanical gauge ones. 

[Sniper]

I want to say that holds a 3/8 npt but I'm not 100% sure I don't have a flat head that old

[clarkede]

6 hours ago, desoto1939 said:

It is a brass nut and then the tube from the end of the mechanical temp gage threads into the gland nut.

I learned the hard way about the bulb getting stuck and man-handled it a little too much and broke the capillary tube. I forget why I have two broken capillary bulbs.  Perhaps one is from when my father worked on it years earlier.  I was able to find a relatively inexpensive replacement online in excellent condition for about $90. I kept the old nuts and gauge to remind me how it all fits together if/when I need to take it out again. 

[Jerry Roberts]

Pipe thread size that goes into the head ;  1/2 inch pipe . The tube thread size on the nut that goes into the gland nut ;  O.D. 5/8 inch x 18 threads per inch . The gland nut has a beveled seat for the ether bulb . 

[OldPly]

My 48 p15 has a large temp bulb, motor has been changed (I think mid 50's crate motor) with small hole in head 

Member Keith has a video about drilling out the hole (but his cars are 37's) it can be done as far as I know it's pipe thried 

Photo What someone in the past did ? (Must have been a pipe fitter )

[Dave72dt]

If that's a pic with the small temp gauge hole, looks like 3/8 pipe to me

[Vancraft]

Thanks for the replies. It is indeed 1/2 pipe. I drilled it and tapped it to the correct size to accept the capillary tube which I did indeed break trying to remove. I even removed the Welch plug on top but could not get it to budge.