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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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if you have everything lined up in the mill, finish it right up there, either by hand, like you were going to do on the drill press or in back gear under power -
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.Comment
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I got it done successfully. Put the tap in a collet (15mm, which I got from MSC) and with the Bridgeport in neutral, hand fed the tap and turned the spindle by hand. Once I got three or so threads cut, I put a vise-grip on the tap to turn it. Came out just fine.Comment
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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.
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