Fitting failure of water temp sensor adaptor - what went wrong?

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Pete Bags
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Fitting failure of water temp sensor adaptor - what went wrong?

Postby Pete Bags » Tue Sep 08, 2020 12:33 pm

As per my earlier thread about fitting an electric water temp sensor, I went ahead at the weekend, but it ended in failure. I broke the sensor adapter fitting. I'd like to know what went wrong, so I don't repeat this when I get a replacement part.

It all started well - I drained enough coolant out (using a siphon, faster and less messy than disconnecting lower hose!) and removed the blanking bolt from the cylinder head. I was surprised at how much force was required to remove it, but there was no evidence of any gasket. I then carefully cleaned the thread.
before.jpg
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Of the three adapter fittings that came in the kit, one was far too small, the other two looked the same, but with slightly different threads. No gaskets came with this TIM kit, but as the original blanking bolt did not have a gasket, I didn't use one or even any ptfe tape. The adapter went it very easily, and I tightened it up until the flange stopped at the cylinder head. I had thought that it might be a tapered thread, but it is not. I then inserted the actual sensor, again without ptfe tape, and this was a tapered thread.
during.jpg
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All looked good! I refilled with coolant, and ran the engine up to temperature. I then noticed that there was a slight water leak between the adapter and the cylinder head. Feeling somewhat deflated, I waited for the water to cool down, drained it and removed the combined sensor and adapter. I then put some ptfe tape on the adapter thread, and re-inserted the sensor/adapter unit. As the adapter seemed to tighted up a bit too easily all the way again, I was concerned that I had not put enough ptfe tape on to stop any leak, so I removed it again, and decided to put a 'decent' amount of ptfe tape on. I started to screw in the unit, and with the additional ptfe tape it felt quite a bit stiffer (great I thought, it might not leak now!) when suddenly the head of the adapter sheared off. After a moment's stunned silence, I might have uttered a rude word, but I managed to remove the body of the adapter and replace the original blanking bolt. I didn't use any ptfe tape, and like it's removal, was just as tough to do up tightly again - but at least no leaks.

I didn't think I had used excess force, but of course the adapter is hollow and is a softer material. Rather than make the same mistake again, any tips or thoughts as to creating a water tight seal, but still allowing an electrical circuit for the electric sensor to work? The final photo shows the sheared adapter, with some of the ptfe tape that had filled the void between the end of the thread and the sheared of head - I removed other bits of the ptfe tape from the cylinder head thread.

Many thanks!
after.jpg
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philthehill
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Re: Fitting failure of water temp sensor adaptor - what went wrong?

Postby philthehill » Tue Sep 08, 2020 12:53 pm

Normally the taper face on the bottom of the sensor makes contact with the concave taper in the head and that is what makes the seal.
The transmitter sensor usually have a parallel thread so are you sure that the transmitter sensor thread is taper.
The brass adapter is only there to make up the difference in size between the hole in the head and the size of the transmitter.
The outer thread of the adapter do not look to be correct. The thread in the head is 5/8" x 18TPI UNF. That adapter thread looks too coarse.
If you do need to seal the adapter and transmitter use Loctite 234 Loc 'N' Seal.

As an aside that cork thermostat housing is getting squashed out and could fail. It is better to fit the later paper gasket lightly greased.
These are the gaskets I use and they are good:-

https://www.ebay.co.uk/itm/CLASSIC-MINI ... %3A2334524


Steve Phillips
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Re: Fitting failure of water temp sensor adaptor - what went wrong?

Postby Steve Phillips » Tue Sep 08, 2020 1:35 pm

often you will find that adaptors are machined with an undercut just under the head, often this will be down to the core size of the thread and looking at your picture the diameter where the PTFE tape is does look reduced.

If this is the case then this is a weak point on the adaptor as you have unfortunately found out, in my experiance if a fitting will screw in until the head of the fitting touches the cylinder head then it either needs a copper washer between the two or as Phil says good thread sealant.

if you go the copper washer route you will need to clean all the paint and rust off the cylinder head first using medium to fine emery cloth on a fat block or piece of wood, using either a washer or sealant means you shouldnt have to tighten the adaptor to the point of breaking.

Steve
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ampwhu
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Re: Fitting failure of water temp sensor adaptor - what went wrong?

Postby ampwhu » Tue Sep 08, 2020 7:42 pm

Why didn't you fit a capillary type gauge? Always thought the Tim products weren't any good. Just buy a Smith's gauge and associate fittings.

les
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Re: Fitting failure of water temp sensor adaptor - what went wrong?

Postby les » Tue Sep 08, 2020 8:41 pm

In your initial thread you said the threads looked different, knowing that, I’m surprised you fitted that particular adapter. Doing so would at least have caused that leak and probably had something to do with the breakage.


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Re: Fitting failure of water temp sensor adaptor - what went wrong?

Postby Trickydicky » Wed Sep 09, 2020 8:03 am

You have used too much ptfe tape, that and the quality of the brass fitting caused it to shear when you tightened it up.
I once fitted a central heating radiator with extension tails on the end which constantly weeped when filled. I made exactly the same mistake of using too much ptfe and in the end used a smear of thread sealant which cured it.
After trying to tighten up the tail with too much ptfe I ended up splitting it in half as a crack appeared down it’s length. If you don’t want to use ptfe or thread sealant you could try a copper or fibre washer on the back of it.
Richard

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Declan_Burns
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Re: Fitting failure of water temp sensor adaptor - what went wrong?

Postby Declan_Burns » Wed Sep 09, 2020 8:54 am

Or a "Dowty" washer.
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Re: Fitting failure of water temp sensor adaptor - what went wrong?

Postby jaekl » Wed Sep 09, 2020 12:41 pm

As a plumbing point. Teflon tape is not a sealant. It's a lubricant to allow the natural sealing properties of TAPERED threads to be tightened farther to ensure a tight seal and perhaps ease disassembly.

Pete Bags
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Re: Fitting failure of water temp sensor adaptor - what went wrong?

Postby Pete Bags » Wed Sep 09, 2020 3:32 pm

Many thanks for all your replies - very much appreciated.

@philthehill: I used the adapter that seemed to fit the thread the best - the photo below shows the alternate adapter that did not want to be screwed in - the threads are very slightly different. Thanks for spotting the thermostat housing gasket - I have ordered the suggested replacement. Is standard bearing/trunnion grease acceptable to use?
threads.jpg
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@ Steve Phillips, Trickydicky, Declan_Burns and jaekl: I do think I used too much PTFE tape - I didn't know that PTFE is a lubricant rather than a sealant, every day is a school day! I think that the idea of using a washer makes sense - the Dowty type look very interesting.

@les: My error in not making myself clearer - the adapter threads looked different to each other, but one was a good match for the removed blanking bolt. I bought the kit from Minispares, so assumed one of the adapters would be correct.

@ampwhu: I did think about a capillary type gauge, but as the gauge is fitted in the driver glove box (along with my rev counter) rather than 'on show', a gauge which can simply alert me to any obvious water temp issue was my primary goal. I drive with the glove box lid open so I can see the rev counter, and shut it when parked, as it also contains all sorts of other bits and pieces.

I was surprised that the sender unit/adapter kit does not come with a suitable washer - I cannot believe I am the only person to have had this problem before. I look forward to reporting back with what I hope is a successful completion of the task soon.

Thanks again all.

Pete

les
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Re: Fitting failure of water temp sensor adaptor - what went wrong?

Postby les » Wed Sep 09, 2020 3:38 pm

I hear what you’re saying Pete, thanks.


geoberni
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Re: Fitting failure of water temp sensor adaptor - what went wrong?

Postby geoberni » Wed Sep 09, 2020 5:21 pm

I'm glad I didn't go the same route as you.

I just brought a sender, such as
https://www.morrisminorspares.com/elect ... cc-p829965
and then connected it to an old Smiths gauge I brought at an auto-jumble for £4. :wink:
The gauge had adjustment holes on the back so I could tweak it to be mid Green Range at normal Thermostat operating temperature, thus no compatibility issues between gauge and sender.
No having to put up with Centigrade markings on my 1950s car. :)
Basil the 1955 series II

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Pete Bags
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Re: Fitting failure of water temp sensor adaptor - what went wrong?

Postby Pete Bags » Wed Sep 09, 2020 5:39 pm

geoberni wrote:
Wed Sep 09, 2020 5:21 pm
I'm glad I didn't go the same route as you.

I just brought a sender, such as
https://www.morrisminorspares.com/elect ... cc-p829965
and then connected it to an old Smiths gauge I brought at an auto-jumble for £4. :wink:
The gauge had adjustment holes on the back so I could tweak it to be mid Green Range at normal Thermostat operating temperature, thus no compatibility issues between gauge and sender.
No having to put up with Centigrade markings on my 1950s car. :)
If only I had known.......! Out of interest, with this sender, did you use a thread sealer?

geoberni
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Re: Fitting failure of water temp sensor adaptor - what went wrong?

Postby geoberni » Thu Sep 10, 2020 9:34 am

Pete Bags wrote:
Wed Sep 09, 2020 5:39 pm
geoberni wrote:
Wed Sep 09, 2020 5:21 pm
I'm glad I didn't go the same route as you.

I just brought a sender, such as
https://www.morrisminorspares.com/elect ... cc-p829965
and then connected it to an old Smiths gauge I brought at an auto-jumble for £4. :wink:
The gauge had adjustment holes on the back so I could tweak it to be mid Green Range at normal Thermostat operating temperature, thus no compatibility issues between gauge and sender.
No having to put up with Centigrade markings on my 1950s car. :)
If only I had known.......! Out of interest, with this sender, did you use a thread sealer?
I did put a small twist of PTFE tape right at the top of the threat (imagine it looking like a thin length of string), just to give a cushioning effect. Whether it adds anything, I've no idea. I just like the idea of some sort of gasket under the head of it (in the hope it might make removal easier if I ever need to).
Last edited by geoberni on Thu Sep 10, 2020 10:07 am, edited 1 time in total.
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oliver90owner
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Re: Fitting failure of water temp sensor adaptor - what went wrong?

Postby oliver90owner » Thu Sep 10, 2020 9:52 am

IIRC, most of these electrical temperature sensors are thermistors which change from somewhere between 2k and 200 Ohms (some may be only 600 Ohms cold?). So likely only needing a shunt or series resistor to make them compliant with the usual meters used for indication.

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Re: Fitting failure of water temp sensor adaptor - what went wrong?

Postby Declan_Burns » Fri Sep 11, 2020 7:33 am

Oliver,
The standard temperature sender for Smiths instruments is GTR 101 or Intermotor part no. 5271. It has a 5/8" UNF thread. The resistance is 800ohms @20°c and 50ohms @90°C.

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Declan


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