Harold and Susan Vordos wrote:
"john" wrote in message
...
snip---

I have never tried it but I would try a tap standard 1/4 20 and grind
off all but one row of threads for clearance. Leave four full teeth on
the existing row and grind the rest off. Run the feed at 5 tpi. You
may need more relief because of the greater lead so you may have to
grind off the back side of the four teeth left to get the added
clearance needed.


John


That won't work-----not even close. The helix angle is wrong, so you won't
achieve the lead you require,

The lead and the helix angle are two separate items. The lead is .25 on
a four start 1/4 20 screw. The lead is determined by what you set your
lathe to cut at.

and even if you could, the teeth wouldn't have
proper clearance.

If you have any skill in tool grinding you can increase the relief
without touching the front profile of the four teeth to accomodate the
increased helix angle caused by the larger lead. The front profile of
the four teeth doesnt change for a four start thread.



John