Thermocouples

[Mick Taylor]

Happily for me, the thermocouple and temperature gauge on my Classic are working perfectly , however, nothing lasts forever so it seems prudent to have a spare in stock for the day it decides to give up.

No longer available!.....so it seems doubly prudent to do the research now while I have no urgent need...I know absolutely about the Norton thermocouple, but reading a few posts on here tells me it is a 6mm threaded item that screws into the rotor housing somewhere near the right spark plug.

Having never laid eyes on one though, I'm at a disadvantage; there are loads of M6 thermocouples available on ebay and elsewhere, Googling 6mm Thermocouples and clicking on images produces hundreds, all very similar but are they likely to fit and are they correct in terms of the signal they provide to the gauge? Are better quality ones available?

Here is a typical example from Amazon...... https://www.amazon.co.uk/0-500C-Termina ... B009XPCTDO

At £3.99 it's actually double the price of most of the Chinese offerings on ebay....but in all probability exactly the same. I would happily pay for a proper (better made) item that will last and do the job...or equally happily buy one of these cheapies if they work.

Does anyone with electrical knowledge know if this sort of thing would work? I'm not worried about the block connector at the frame end, that I can sort, just the fit and function of the thermocouple itself.......

Mick.

[Richard Negus]

The challenge of this thermocouple is that it's not the conventional K-type wire. A few years ago, I did look into making a small batch but the wire, which I think was A-type, required the purchase of a 100 metre reel at something like £300.

The thermocouple works by joining two dissimilar metal wires together at the 'hot' end. This creates a small voltage displayed by the gauge, which is, effectively, a volt meter. So, if you have one Norton thermocouple with its original long lead, you can cut it in two and make another thermocouple!
The lead was originally so long so that it reached from the engine direct to the gauge on the early IP2. Later IP2 and Classic have a connector into the main loom just under the tank and a rat's nest of thermocouple wire there to clip out of the way. So long as the shortened thermocouple wire reaches that connector, then it's function is unchanged.

The 'hot' end join is made by simply silver-soldering the two wires together. The original sleeve nut that hold it in the rotor housing was made of brass for corrosion resistance. Removing it from the rotor housing often causes it to break off; stainless might be a better material.

[Mick Taylor]

That's interesting Richard......given that they are so simple, how do they usually fail? I take it from that, the thermocouple wires have specific properties that produce specific voltages, so getting the right wire is important.......which makes re-using the original wire a desirable option?

My Classic has exactly what you mention, loops of thin brown sheathed wire that has to be tidied and retained out of harm's way.

Looking at the cheapie Chinese offerings, my first thought was that the fragile-looking alloy threaded section would seize in the rotor housing .. but it seems the brass Norton version does the same thing? I suspect getting the brass holder out if it is stuck is not going to be easy?

The way forward then, if the Norton Brass threaded section cannot be re-used would be to obtain a piece of 10mm (or 8mm?) stainless hex bar to make a copy, then simply cut the 'hot' end off the existing wires to make a new silver soldered joint?........Do you happen to know the dimensions of the sleeve nut? ...... I'm no machinist, but I think I could probably turn one up.

Failing that, it can wait until the existing one fails and hopefully provides a pattern, or better still, it just keeps on working indefinitely.

Mick.

[Richard Negus]

The thermocouple itself rarely fails, corroded terminals at the connection perhaps.
The common problem is taking the thermocouple out before powder coating or replacing the housing - and the brass sleeve shears off.
As left and right housings are identical (except for the early single rotor idle ones), the fix is to use the vacant thread in the other housing. Getting the broken part of the sleeve out is very difficult as the wall is very and its a long way down the fins. An 'Easy-out' didn't work for me! You could drill it out and perhaps salvage the M6 thread, or be bold and make it M8. It might be possible to dril/tap another M6 hole just above the original.

The original wires are plastic-sleeved brown and blue, then wrapped in some sort of thin fabric sleeve. The threaded part of the sleeve is about 20 mm long with an 8 mm hexagon x about 8 mm long. Easier to drill a hole through the centre of a standard M6 hex head screw. For either 8 or 10 hex, you'll need an appropriate tube spanner to pass the wire though.

I tried initially to use 'K' wire but the ratio of millivolts/degree was wrong and the gauge needle hardly moved.
Just searched 'thermocouple types' and found 8 including K. None looked familiar so I've no recollection what the correct one is. I'll see if I can find out.
R.

[Mick Taylor]

I would never have known there was a redundant second tapped hole in the left rotor housing.....and it probably wouldn't have occurred to me to use it anyway unless I was absolutely certain it was suitable, so that is well worth knowing, thank you.

This is hypothetical (for me) at the moment as mine is working perfectly, but having the information to make a replacement is priceless.

Just finding an M6 (8mm head) stainless bolt to adapt would be worthwhile....unless there is clearance for a 10mm head? as they are commonplace.

Many thanks, Richard.


Mick.

[Richard Negus]

Sorry to say this - but if your thermocouple is now in the right housing, it suggests that there's already a broken one in the left .

[Mick Taylor]

To be honest Richard, I haven't even looked, I just read the location on a post and took that as gospel.....I might even have got that wrong

I don't think it will have been touched; insofar as I can be certain of anything, the only time it has ever been meaningfully interfered with to my knowledge was when you worked your magic on it and made it run properly

Without looking I would be confident there will be a nice virgin spare threaded hole in whichever side should be vacant.

I did manage to find some M6 screws with an 8mm head though.....and in stainless too.... I'll order a handful of these just in case the need ever arises.

https://www.ebay.co.uk/itm/M6x20mm-Thre ... SwnTdaTMPm

The flange could be turned down if needed.......they supply them in various lengths......is 20mm suitable? Is there anything special about the silver solder joint apart from being large enough to prevent the wires migrating up the sleeve nut?

Mick.

[Richard Negus]

My photos of your engine from 2012 (!) clearly show that your thermocouple is in the standard, left, position.

I like the look of the bolts you found - just wondering where I could use them.

Just a blob of silver solder, without setting fire to the plastic insulation, and then pull the softened insulation back down to the blob. There might even be a crimp-able metal sleeve that would do a better job. Something like the small brass nipple of a throttle/choke cable.

[Mick Taylor]

I take it the solder 'snot' should not touch the sleeve bolt?.......or is that irrelevant as it is simply reacting to temperature, not connections?

How about something like this for a crimp fitting?

https://www.ebay.co.uk/itm/100Pcs-bag-s ... 0005.m1851

They could be crimped with the exposed wire inside, then cut to length with end cutting nippers which would seal it and reinforce the crimp at the same time....or is stainless the wrong material for this?

I spent ages finding those M6 (8mm hex head) bolts, I knew they should exist because Honda used this bolt type for engine covers which means someone will have reproduced them in stainless..but no mention of a reduced hex on the listing, even though they have one

[Mick Taylor]

Also available in brass and aluminium....

https://www.ebay.co.uk/itm/Single-Brass ... XwiB4EqKgA

[Richard Negus]

[quote="Mick Taylor"]I take it the solder 'snot' should not touch the sleeve bolt?......

That poses interesting questions. Yes, the 'snot' is clamped to the housing by the sleeve bolt, but surely if either of the bare wires touch and is grounded, it wouldn't work?
Does the sleeve bolt really need to be so long, given that tightening torque can only be minimal? Shorten it by 5 mm and use a 6 mm long crimp sleeve that will accept wire and insulation, and crimp it onto bare wire only at the open end?

[Mick Taylor]

Hmm.......having never actually seen one of these items I'm in the dark, but it must be quite awkward to clamp the hot end against the rotor housing without some part of the wires touching the sleeve nut......and it follows that, if the 'hot end' is clamped against the rotor housing then surely the entire thing is earthed.....or am I missing something?.....

Here is a thermocouple on ebay that happens to have decent photos of the business end.....I see no insulation between the hot end and the sleeve nut.

https://www.ebay.co.uk/itm/RS-Pro-J-Typ ... Sw-wJaGW9P


Mick.

[Mick Taylor]

Looking for something else on the Venhill site, noticed this item, which looks ideal as a basis for a thermocouple.....pre drilled with an 8mm head and a cavity at the hex end to fill with silicone to protect the wires.

https://www.venhill.co.uk/cables-amp-co ... 00-20.html