If I hook a Megajolt and standard water temp gauge to the water temp sender will they interfere with each other?

I didn't bother with the temperature sensor when I used a megajolt - never missed it

The megajolt compatible temp sensor isn't compatible with the mini gauge temp sensor - the resistance is very different. The simplest solution would be to put your megajolt's temp sensor in the water drain plug hole in the back of the block.

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did I tell you that I won a trophy?

I'll have to try that I was going to use it as a thermofan switch

I can't see why you need a specific temp sender for a MegaJolt. Maybe for a MegaSquirt? You do need a separate sender, but not a specific sender. Separate, as you'll be able to calibrate it for the MegaJolt if it's a shared output from the sender, but the gauge would also require recalibration, and that's just not worth the effort. They would interfere with each other. A constant, workable interference, but not one worth the effort.

I've got the standard sender (well, one in the standard spot in the head) set up for my temp gauge (currently not working), and I've got one in the thermostat housing (a VDO sender - just happened to be what was already in there) which is hooked up to the MegaJolt. I then borrowed an infrared thermometer, worked out the scale of resistances, and put that into the MegaJolt. So, my ignition is further advanced below 60º, and starts to retard as it gets closer to 100º.

That said, Simon's right - you don't need it hooked up in order to run. Mine's hooked up to adjust the ignition as mentioned, and it also controls a pc fan as a thermofan mounted on a heater matrix used as an auxiliary radiator, connected to one of the auxiliary outputs. That said, if you're running more than a pc fan, you'd probably still want a relay or something in the circuit, as the outputs aren't meant to have more than 500mA of draw on them.

What is the thread of the drain plug in the back of the block

I pulled a Cooper S engine out of a car this morning. I have just checked the block drain plug and it seems to be 1/4 BSP. Also checked a Morris 1100 S engine and it has the same plug. Hope this helps.

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1970 MK II Cooper 'S' Supercharged 1310 with Megajolt.
1970 MK II Cooper 'S' Original resto project (SOLD)
1962 850 Original. (Under restoration)
1973 Leyland S (My first car. Now rebuilt, so my kids can learn how to drive a real car).
VMCI # 215.

I have been to a few auto parts stores now and none of them seem to stock a temp sended that will fit into the hole, I am also unable to find an adapter that will allow me to fit a different threaded sender. For those that have taken this path, how did you get a temp sender in there without changing the thread?

I have been looking into using my Megajolt to run my Thermofan and think the way Tadhg has done it is the easiest way to go. Just get a threaded thermostat housing and temp sender to suit. Use this for the Megajolt and the sender in the head for your gauge.

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1970 MK II Cooper 'S' Supercharged 1310 with Megajolt.
1970 MK II Cooper 'S' Original resto project (SOLD)
1962 850 Original. (Under restoration)
1973 Leyland S (My first car. Now rebuilt, so my kids can learn how to drive a real car).
VMCI # 215.

Or you can buy one if the spacers to go under the thermostat housing with the thread in them, too. Sure, it's yet ANOTHER thermostat gasket, and might mean you need longer bolts, but it might be easier to source or cheaper (though I suspect not).

Something I didn't point out before - the sender in the head is best for the gauge, as it's more accurate during warm up. Because it isn't reading water blocked from the heat source by the thermostat. Whereas, if you're only running a fan, then having inaccuracies below thermostat temp is of no consequence to you.

Now, to further complicate things... I did mention I've modified the advance curve on my engine dependent on coolant temp. If you're running a standard setup with the MegaJolt getting its readings 'behind' the wall of the thermostat, this could see you running extra advance for too long (as the coolant below the thermostat could be at 80*, but the coolant above the thermostat might still be at 55* - it's plausible). I don't have that problem, as my auxiliary radiator comes off the heater and is unrestricted (something I might well change for this winter), and so I've always got some water flow through the thermostat housing.