I ordered a temp sending unit for my 1956 Mercury, the description said for 1956 Ford car but it is the wrong thread. The new sending unit is 3/8 NPT but the hole in the head is 1/4 NPT. I read an old post on here that mentioned that the 1957 is 1/4 NPT.  Can someone please confirm that. If it is, then I will just order a 1957 temp sending unit. I do not want to start using reducers/adapters. Previous owner had installed a mechanical gauge with adapters. I have verified that the gauge in the dash works, so I want to put an original type sending unit back in. I have already installed a mechanical gauge on the front of the engine in the hole for the heater return hose just before the thermostat.
Is the 57 sending unit 1/4 NPT?  If not, what sending unit do I need?
Thank you for any tips.

The 1954 thru 1956 Y cylinder heads use a 3/8” NPT thread at the temperature bung.  The 1957 thru 1964 Y cylinder heads use a ¼” NPT thread for the temperature sending unit.

The 1956 12 volt temperature sending units are unique as 1956 is a one year only item due to the electronics for the temperature gauge being different electrically from the 1957 and up units.  To use the correct 1956 temperature sending unit, you will need to change the existing ¼” NPT bung in the cylinder heads to the 3/8” NPT unit if retaining all the original 1956 gauges and wiring.

More comments always welcome.

Lorena, Texas (South of Waco)

Thank you for the info. I now have a decision to make, I don't want to do anything to risk damaging any pieces or parts. I did read another post about changing the bung. At least I know what has to be done. I have it installed now using reducers/adapters and a coupling, just as it was when I got it, but the problem is that it sits above the manifold and not in the water flow. It takes forever for it to read above cold. I have not looked at all yet but is a new bung easy to acquire? Who is the recommended source? Any tips for removing the current bung? Do they pop out fairly easily? 

stbart, the outside diameter of the bungs are the same. You can get the 3/8, 55/56 bung from Hill's (www.hillsresto.com). The part number for the 3/8 bung is 10911-AR. The price is $15, excluding shipping. Hill's probably will ship by USPS (saves money). No tax if you're outside Ohio. Your sender and gauge should be a matched set. 

I have removed/installed a number of those temperature sending unit bungs to either get to the correct NPT size for the sender or to convert a right-side cylinder head for use on the left side of the engine.  Some of those bungs were in tighter than others and I’m sure that rust and time are big players for this.  To date, I have always had the heads off of the engine in which to accomplish this.  For this, I simply used a slide hammer affair with a ¼” or 3/8” pipe nipple on its end.

While I have not changed out the bung with the head on an engine and in the vehicle, I picture it being doable.  The intake manifold being removed would definitely help in having enough room in which to accomplish this.  Depending upon the hood or firewall clearance, a slide hammer affair would get the job done.  I can also picture making a special puller that would use a large diameter bolt welded to a pipe nipple and that bolt being threaded through a 4” tall hollow cover in which to pull the bung out of the cylinder head.  Installing the new bung would be just a matter of tapping the bung in place with some sealer around its outer edge.

Always be sure that the ‘wide’ end of the NPT threads is facing out and not in when installing the bung; otherwise, the temperature sending unit will not thread into the bung.

Lorena, Texas (South of Waco)

This thread has me confused. My 6V/292 ‘55 T-Bird has replacement (post ‘56) 113 heads.  I have had to replace the sender before (NOS/6V part), and it screwed right in with no issues. Does that mean the Ford dealer mechanic who changed heads back in the 60s/70s must have used some kind of thread adapter?

6 VOLTS/POS. GRD. NW INDIANA

They either used a thread adapter or swapped the bung.



Lawrenceville, GA

Concours parts sells the bung and a tool for removing the old bung. I just used it to remove the bung from a G-head. Very easy to use, no problems. As Ted said, just put a little sealer on the new one and tap it in.

Now I'm confused. The 113 heads originally would have used the smaller 1/4 NPT 12v sender. You said that you had screwed in the 6v (3/8) sender with no problem. Unless I missed something, that means the bung was swapped from 1/4 to 3/8, then swapped back to 1/4, and now you want to swap back to 3/8. Am I missing something? Although the 55 & 56 senders are both 3/8, the 55 is 6v and the 56 12v. 

55blacktie -you are confusing two different people The original poser is not the one that mentioned the 113 heads..stbart had the original question, Daniel Tinder mentioned the 113 heads

quote]55blacktie (6/25/2023)

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54 Victoria 312;  48 Ford Conv 302, 56 Bird 312
Forever Ford
Midland Park, NJ