What size is this manifold plug?

I’m starting to scout parts for a mega squirt conversion and this looks like a good spot to put the two wire coolant sensor. Does anyone know the thread size on this plug? I assume I’ll need an adapter from this to the sensor. It’s a 2 x HD8 manifold from I believe a 420. Thanks

If you are talking about the plug on the side of the manifold on the triangular pad surrounded by three holes, it is not normally tapped. Originally, that was the location of the Otter switch that controlled the cold start enrichment device. If it is tapped, it will be whatever the person who tapped the hole made it.

I don’t see the 3 Otter switch tapped mounting holes, although they may be buried under the sealant/pipe dope that surrounds the plug.

To fit without bottoming out, it is almost certainly a tapered pipe plug. It looks like 3/8 or 1/2 pipe–you could measure the diameter and match it with the size. Not sure where you live…if the US it’s probably an NPT (US National Pipe Tapered) thread whereas if in the ROW it’s probably BSPT (British Standard Pipe Tapered). The hex recess drive is more often associated with NPT plugs. Note that NPT and BSP are very similar–slightly different diameters, and thread pitches differing only by one thread per inch for a given size. IMHO.

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You are right, looking closer, I don’t see them either. Do we know the manifold part number? Thought I had a 420 manifold, but the closest I can come is C30375, which I believe is probably E-type. I tried both 3/8 BSPP and BSPT and both were no-go. Looks like a SAE 5/8-18tpi straight thread. Two different twin carb manifolds below, both the same thread.

Evidently that’s the standard temperature sensor thread, although there is some dispute (see link below). Mine (Mark X manifold) is poorly fit with sealer by the PO, so I’ve never removed it. Seems like it should have a washer if the thread is straight, but who knows? Jaguar did use UNF threads for similar purposes–the plugs in the oil galleries are 1/2 -20, not BSP.

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I would concur with that. 3/8 BSPT and BSPP just don’t fit. 3/8 NPT is also 18 tpi, someone could confuse a 5/8-18 fine thread for 3/8 NPT. It would take a tap to thread it in, though. 3/8 NPT is slightly larger than 5/8 fine thread.

Thanks for all the info. There are def not three holes there, just the main plug, so I guess it’s a non-original situation. I can’t seem to find my calipers to measure it so I guess I’ll just wait till it’s time to install the sensor and pull the plug to match it. The part number I found on the side is C43679 if that means anything to anyone!

I’ve never seen one with that sort of hole in it.
Think it’s something non standard. You need thread gauges and a “Zeus” table. If you can determine the TPI and diameter then you should be able to “ google” it.