I need to drill and tap a 3/4-24 hole in an Indian transmission cover for the filler hole used on the '52&53 Chiefs. I am thinking that without some sort of fixture or guide, the tap will just ream out the hole rather than cutting threads. It is a plug tap, which is all I could find. I plan on starting from a hole reamed with a .71" dia reamer. Right now, I have the cover bolted to a steel plate and held in a mill vise on my Bridgeport. I have so far drilled the boss to 1/2" and will go to 45/64 then use the reamer. I am thinking that I can use my drill press to tap the hole, by turning the spindle by hand and feeding the tap with the down feed crank. Comments?
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Tapping 3/4-24 Hole
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There is a relatively cheap device available to help with this operation. It consists of a .500 shank with a spring loaded point to put pressure on the tap. The point has a 1/2" of travel or so and you can adjust the pressure a bitfor small hole/lightwork. Keeps near perfect alignment. If the tap doesn't have a centering hole you will need a driver/tap holder that does.
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Before I would use a plug tap and try to get a true 3/8-24 thread, I would do a test-tap in some scrap metal. Seems like I tried something similar once and the results were not satisfactory. You can check with V-Twin (Tedd Cycle), perhaps they have the correct tap available.
Lonnie
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