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Odd sized tap and drill needed - expensive

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I need a 12.8mm drill bit, and an M14*1.25 tap (Metric Fine). This is to drill and tap one piece of steel plate to accept a spark plug. Unfortunately, it is too big to fit on the lathe, so I can't cut it that way. I would buy one, but it is looking like £17.70 for the drill and £15 for the tap, plus p+p. That is somewhat steep for one damn hole. Does anybody know where I could borrow one from, buy one cheaper than that, or alternatively, get hold of an M14*1.25 nut. I could turn a nut thin enough to be able to screw onto the spark plug on the other side of the plate.

Any suggestions gratefully accepted.


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A 1/2" drill is 12.7mm - more than close enough for the job. You should be able to get a good HSS one pretty much anywhere for under a tenner.

As to the tap, well I'm afraid £15 is about right.

Could you could drill a clearance hole and weld a nut to the outside of the plate?

Regards, Glen.

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12.8 is the preferred size, however, ½ inch is 12.7mm and this will be more than adequate when using a standard fluted tap. In fact a lot of reference material shows 12.5mm as the tapping drill size

Last night I tapped 14mm x 1.5 thread for an air temp sensor collar. The material was hard as it used to be a rocker shaft. Drilling at 12.5mm and then tapping using Trefolex cutting compound produced an easy, first class thread.



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In fact, if you're working in a soft material like aluminium for a disposable/consumable part, you could use the spark plug to cut the thread. Bung the scrap man £5 for as many old plugs as you can carry in a bag and put a hacksaw cut at an angle on each side of the thread (a "farmer's tap"), then screw it in.

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Thanks for all of the replies. I'm making a jet engine... I have a new lathe/milling machine and really want to learn how to turn things properly on there, so I decided to get a project going, and i'm producing all of the fittings, plus most of the other parts on there - unfortunately, this part is a little too large for it. I've learnt quite a lot of 'what not to do' already. The first part is a combuster - basically you have a lined tube, with holes in the inside tube. Blow in lots of air and propane, and huge amounts of heat should come out of the end. This links up to an old turbocharger, which in turn creates loads of air. Route this back to the beginning, and it should be self sustaining.

I can't believe how long fabrication takes.... Turbocharger's father has found me an old turbocharger that spins. I have made flanges for all of the connections to it, and am in the process of making pipes that link the burner to the turbocharger - 120mm at one end, 60mm at the other - with a nice smooth transition. It takes bloomin ages to get all of the cuts right - heat up the pipe, bash it into shape, weld it up, grind it back, weld the end flanges on etc etc etc. All I'm left with 9 hours later is a piece of pointy pipe, which looks as though it wouldn't take 5 minutes to make. Still, I'm enjoying every minute of it.

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Why do you think I'm called Turbocharger? There's a pile of ten of the things at home, and they interest me because there's as much engineering in one as the rest of the engine...

Hi ,

If you're still after a 14 x 1.25 tap...I've just found a couple in the garage/workshop whilst having a clear out. I've checked it against a spark pl;ufg from my MG ZS 180 and it looks correct (thread form that is).

I'm dropping sandbags Disco into Wooton-under edge on Thursday for a service . I'll leave the tap with my mate in Parts if you can collect it.

Just post it back when you've done.

Let me know if this is OK!!



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Thank you very very much for the offer, but I found the correct sized nut in a machine shop in Gloucester. I turned it down so that it was about 1/2 as thick as it had been, and have used that.

You are very kind to offer, though.

Will post some pics up when I get it running. At the moment, i seem to be buying loads of stuff from e-bay, like oil pumps, pressure gauges etc. Hopefully, it will all be worth it.

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I'm impressed Keir!

It's one of those must do projects I'll probably never have time to do!

If you get it going, I saw a nice one on the web made out of a ring of 6 truck turbos feeding the same exhaust and generating a very respectable amount of thrust!

If you need any help or anything - just ask & I'll do my best!

If you need a test pilot for the jet tea trolley - there was a chap I met once named Bickerton (I think?) who had the ideal build and emboldened with half a can of shandy seemed to be up for that kind of thing. ;)


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