Hi. I've recently connected a large towel rail into the central heating. However a thread was badly cut so I couldn't screw one the tails in very far (only a couple of turns with extreme force).
I'd like to re-tap the thread, which is the normal 1/2" BSP (not sure if it's tapered or parallel though).
You can buy taps as: 'taper', 'second' or 'bottoming' ('plug'), (e.g.
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depending on the amount of tapering. However, I'm not too knowledgeable about tapping - I don't know which type is best. Can anyone advise whether I need 'taper', 'second' or 'bottoming' ('plug')?
Also, could I get away with using WD40 or engine oil as the 'cutting oil', as it's not a new thread, just a clean-up of an existing thread?
If you are just cleaning up a badly tapped thread (rather than making a new thread in a plain hole) I think you should be fine with a bottoming tap which is parallel all the way along. This type of tap would be impossible to hand start in a plain hole, which is why you'd normally start with a taper.
However, if the tap is able to go right through the hole and some way into the rail, a taper would work as you could drive it right in.
If the threads are disasterously bad, you might need a taper to start. That's the limit of my knowledge, I've never used a second tap.
Lots of oil on the tap will help it cut. Don't try to force it through in one go - back off periodically to clear the swarf.
There are people here with far more experience so hang around...
Well you need to establish this first or you can't buy the correct tap. It ought to be parallel and you should be able to tell by eye, by measuring with a vernier or by sticking a round rod of about the right diameter in and seeing if it's tighter at the far end. If it's tapered you need a BSPT tap not a BSPP one.
For cleaning up an existing thread any of those would do but a 'second' tap is the best all round choice. It'll start a new thread if necessary which a bottoming tap won't and a 'taper' or 1st tap is only really needed for starting brand new threads in tough materials. Make sure there's enough free depth of hole behind the thread for the tapered front end of the tap to go right through into or you might need a bottoming tap which will clean up a blind hole.
I doubt you'll need any cutting oil for cleaning up an existing thread. It's probably just excess paint or chrome you're jamming on. Best to keep the thread nice and clean for future sealing.
If you have an angle grinder or a dremel I can pass on an old engineer's trick. Get an old steel 1/2" bsp parallel fitting and grind three flutes lengthways into the first few threads with the very edge of the grinding wheel deep enough to go right through the threads into the core metal. In effect you're creating a rough and ready tap where the edges of the flutes you've made now act as cutting edges. You want to try and get the leading edge of each flute, the bit that'll do the cutting, vertical to the bolt centreline like a proper tap if you see what I mean. If you have the wrong side of the flute vertical, or you just create a symmetrical flute with both sides angled it won't work as well. Anyway look at any proper tap and you'll get the gist.
This should suffice to clean up and reshape an existing thread in mild steel whereas a non-fluted thread can't do any cutting and will just jam as you've found out. I use this trick routinely to clean up odd sized threads in engines when I don't have a tap of the right size but there's a decent bolt knocking about. Usually things like metric fine threads which you come across on flywheels and big end bolts etc. Worth a shot anyway before you lash out on a new tap you'll only need once. If you try uk.rec.models.engineering I'm sure someone will have a 1/2" BSP tap they can post you as a loaner.
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