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MG MGB Technical - Temperature sender.

Are they interchangable? I have just dropped a 68 engine in a 79 car and now the temperature gauge reads almost into the red high. The engine is running at normal temperature, as confirmed with a thermometer so I'd like the gauge to read right again.

Can I put my old 79 sender in the 68 head? The threads look to be different but I can't just pull the sender out of the 68 head to see as it is seized and rounded off. Removing it will destroy it so I need to know whether or not I have to buy a new one before removing it and leaving myself B-less for a few days.

Is there any other reason the gauge could be reading high? The only different component in the system is the sender.
The Wiz

Wiz, A bad voltage stabilizer could possibly cause it to read high but that should also affect the fuel gauge reading. I don't know if the senders will interchange.

Clifton
Clifton Gordon

Wiz - The threads are the same on all the heads (3/8 BSP as I remember). There are two different senders for the electrical gauges, but I don't know what year they changed. My guess is that they changed somewhere between 68 and 79. Sounds like it is time for some big vice grips and a good dose of BF&BI. Good luck - Dave
PS. You can get new senders from NAPA.
David DuBois

Mike. Hope all is well at your house.

My notes show that the sending unit on the later cars is 5/8"-18, a standard thread. The unit seals on the tapered section at the front of the sending unit. To the best of my knowledge, all of the sending units used the same threads, a straight, non-tapered thread.

I have been told that, due to the higher temperatures involved in the later engines, the sending units were modified to read a lower temperature than the earlier cars did. Cannot say if this is true because I do not have the means to test them. I have used an IR thermometer to measure the cylinder head temperatures of RB cars which were reading about 3/4 of the way to the "H" mark. The average reading at the cylinder head was 183 deg F. On my 79, this same temperature does not move the gauge to the mid-way mark. Thus, the story may well be true.

Not sure what to recommend to you. I would, were it my car, follow Dave's suggestion and get a big pair of Vice Grips, or a small pipe wrench, and remove the current sending unit for replacement with the later model. But, if I screwed it up, I have another car to drive to work in. And, a spare cylinder head to toss on the engine until I get the pieces out of the current cylinder head. Significantly different thing than when you need the car to get to work every day.

In any event, a lot of good quality penetrating oil, sprayed on daily, would not be a bad thing to consider regardless of your final decision.

Les
Les Bengtson

I stand corrected on the sending unit threads, Les is correct and it is the same on all heads. I would second Les' suggestionto install a later sending unit to match the gauge in your car to get the correct reading. Cheers - Dave
David DuBois

Dave. You do not "stand corrected on the sendin unit threads". Nor "Les is correct". UNLESS, you have checked the threads on the earlier model units. All of my current work has been on the 18V model cylinder heads. Have not had the time to pull the cylinder head temperature sending unit on my 68 GT (18GF engine) yet. Thus, I can speak, in a realtively definitive manner, on the later model cars. I cannot, however, speak on the earlier model cars with the same sense of satisfaction. Others have claimed that the thread size was the same. However, I both know and honor your expertise. If you have taken the time to remove, measure with a set of dial calipers, or micrometer, and a thread pitch gauge, please let me know. I can then, with a great deal of assurance (because I know your abilities) state that the same thread was used on all years.

Do not mean to be a pain in the neck. You are one of the few people, in accompanyment with Peter Burgess, Chris Betson, Roger Parker, Steve S. and Clifton Gordon, who are the "real experts" and cause me to take pause and examine what I "really know". Often, it is less than I thought I knew.

Thus, since you own an "early model", please be so kind as to pull the cylinder head temperature gauge on your car and measure it. Instructions for such measurements are found on my website, www.custompistols.com/ under the "articles" section. The very same place where the articles you have sent me appear, with a very high "hit rate".

Your articles are, consistantly, one of the "top ten" articles requested. Thus, you both honor me by allowing me to post them, and add to the general body of knowledge, which honors both of us.

I am fortunate to be able to call you "my friend", and all the world benefits. Thank you for what you have done.

Les
Les Bengtson

Les - "please be so kind as to pull the cylinder head temperature gauge on your car and measure it" I have already done so. Our 66 MGB has the "old" mechanical temperature gauge in it. When I swapped the engine for a late 18V engine, I was pleasantly suprised to find out that the bulb for the gauge and its jam nut fit perfectly in the head. That is why I stated that the thread is the same on the heads, by which I meant all of the heads from the earliest 63 through the latest 80, I just didn't remember for sure what thread it was until I saw your post. In fact, that can be taken back to the 56 MGA 1500 heads. After your post that the threads are a 5/8 - 18 thread, I remembered seeing that on your web site and checking the threads on one of the jam nuts I had laying around and comparing it to the later, electrical sensors. I already knew that the later sensors would fit in any head for the B series engines, because I have made a converter so that a late MGB sensor can be used on the Magnettes with the 1500 B series heads. So yes Les, I really do stand corrected and you are right as to the thread for the temperature sensors (and to a whole lot more that you don't give yourself credit for).
Cheers - Dave
David DuBois

Thank you David and Les. The change point was 1977 according to Moss. I will remove the unit tonight and replace it with the later one. I need to do this as the needle sits slightly below or just into the red.

Engine temperature is good, I removed the filler plug in the head immediately after a hard run and dropped a meat thermometer in, it registered 190 degrees. I just need a gauge that will warn me if there ever is a cooling problem, with the needle already in the red I would have very little indication.
The Wiz

By my sources and memory, the transmitter change was:
68-71 GTR104
72-76 GTR101
77 on GTR101
Distinctly recall going nuts with these in the early 70's, until I found out that the early had black plastic, and the late red. Don't know if this holds for repro. Somewhere I have recorded resistance curves for all of them, but you KNOW I can't recall where!
To check, I went and looked at another reference (idiot!) and it says GTR104 through 75, and MTS142 after; so, a third says 104 to 75, and 101 75-on. As I am sure there was confusion in the early 70's, I think there may have been an intermediate version which is being superseded to early or late by different suppliers.
The threads are 5/8-18 because that is an SAE standard for some designation of straight thead tube fittings; you can see that the mech capillary bulb is effectively a compression fitting. It is common practice to base such things on some standard, even if it is modified a bit.
FRM
FR Millmore

Les & David,

Not only are the threads the same on MGAs and all the MGBs but on A series midgets and Rover V8s too!

I guess the common denominator is Smiths (previously Jeager) who made the instruments.
Chris at Octarine Services

Guys. Thanks for the information. Les
Les Bengtson

Remember...a cold hole is tighter than a hot hole.
Try removing your installed one when the motor is good and hot from a good hard run. Bet it can be pulled then with some vicegrips.
Wayne Hardy

This thread was discussed between 24/08/2006 and 25/08/2006

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