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Help with making a custom thread tap


Sep 17, 2004
Ottawa, Ontario, Canada
I have an old aluminum cylinder head with a spark plug plug hole that was repaired with a bronze pinned threaded insert. This insert has come loose allowing gas to leak. The pins have been ground out and the insert threaded out easily as it was loose. No standard replacement insert can be found because of space restrictions a custom insert has to be made. For this I need to make a m25 1.25 tap. There is not space for a more coarse thread. The spark plug is m18 1.5.
Pictured is a tap made from water hardening steel. It will not bite and pull in aluminum, even pushed with the tailstock. I have tried 2 taper angles, 5 and 3 degrees, no difference. The scraping angle in the flute is 90 degrees or a little under. I have made smaller taps in the past with good results.
Any advice or help will be much appreciated.
I thought most plugs were M14...but I guess some are bigger.

But...why use M25? It sounds like you are making your own repair inserts? Use a 1" tap...you can buy them from McMaster with all sorts of fine threads.

If it was me, from what I can tell.....use a 1" tap, make an insert with M18, and use LocTite or some other means to glue it in.
from what I can tell.....use a 1" tap, make an insert with M18, .

isn't 1.25 mm more or less .050"? So a 1" - 20 tap is about $30.

and use LocTite or some other means to glue it in
I've been told i'm superannuated so may have missed it - do they now make loctite that is good for cylinder head temperatures?

I imagine you are probably avoiding welding it up in order avoid having to bore and put new valve seats in, as well as guides, and face the head?
If that is not a factor, have it welded and then start over with the correct size sparkplug tap.

Per your actual question, i have several tap grinding (pointing) accessories, and they put radial relief on the cutting lip. It maybe counter intuitive, but i think your lead taper might be too shallow, if you get the right radial relief. Look at the tops of the ground threads - they have more reaming surface than "pulling" surface - there are no nearly sharp threads to dig in and start pulling before the hole is already reamed out.

Your tap has no back-off so basically, you are trying to screw a bolt into a hole with no existing threads. That normally doesn't work.

You could possibly get it to cut for a few parts by hand-grinding radial relief starting right behind the front edge of the flutes. Maybe.
Thanks for the replies. Several of you have nailed the issue I believe. There is no radial relief. I’ll figure out a way to create this.
Upon further thought the lead taper angle is probably a compromise between chip size and cutting edge length.
Any guide lines as to how wide the thread portion relative to the flute width should be? This ratio is about 1:3 for the 1/2" taps I have.
End view.
1 st spark plug insert tap end.JPG
For taps with no relief,the best method is to reduce the cutting edge to a minimum.........you can also freehand grind a relief on the coned lead..........Id also mention ,if you ground the flutes and edges ,you may have drawn the hardness of carbon steel ......especially on the fine points of the cutting edge.