snapped radiator bleed screw what size thread?

16 May 2010
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United Kingdom
Had a less than stellar day yesterday, bleeding an old rad in which I'd recently fitted new TRV and lockshield and managed to shear then bleed screw end off.

I've since discovered a variety of online retailers who sell a variety radiator bleed screws (I've ordered a multipack with an option to return unused ones).

However my conundrum is as follows:

I need to get the remains of the old screw out, or put a new one in the other end. The design is less than optimal, in that there's no removable boss, and the screw is shrouded by the radiator top. Its approx 5mm in dia, which I can probably drill out but I could do with knowing the thread form so I can run a tap through it to clear it - or chicken option - drill and tap a fresh hole in the other end.

So any suggestions as to the thread size and tap to use? I'm guessing its a taper (most plumbing stuff seems to be) as it would then be self sealing.

The website selling the screws suggests a range of sizes around 4.6 to 4.8mm for this application, and 30TPI - casual searching doesn't bring up any taps of that size. Is there something else I should be looking for?
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1. The thread is likely to be BSW, could be BSF or BSP, unlikely to be metric.
2. Measure across the tops of the threads. Needs to be accurate so something like a vernier caliper or micrometer.
3. Look up the OD you find on the internet. Look at BSW, BSF and BSP and see if you can work out what the thread is.
4. Difficult, seeing how few threads are left, but if you could measure the thread pitch you could get it dead on.
5. Be much easier to PART drill out the existing and use an Easyout.
6. Don't forget they screw into a conical seating, which is a 90 degree one, not the same as an ordinary drill bit. It is not a full diameter through hole.
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Or be really radical and fit a new rad , that thing must be at least 30 years old
Maybe double that - also the brass screw goes straight into steel = bi metal corrosion.
Or be really radical and fit a new rad , that thing must be at least 30 years old

New rad isn't really an option, as we have plans to move it in a year or two, and I'm not sure it's size would be suitable for it's new location.
tried to remove old screw remains, failed miserably as brass was too soft and my screw extractor tools just extracted bits of it. Ended up drilling it out, tapping to m6 and epoxying a regular machine screw into the hole, and then fitted an easibleed as |Chris_w recommended. Now have a fully bled and toasty radiator.

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