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Old 11-24-2003, 06:45 AM
243 243 is offline
Bleedin' Gasoline
Join Date: Apr 08, 2001
Location: Houston, TX
Posts: 2,477

I want to use the early 80's Blazer/Truck centercaps on my aluminum slot wheels. The caps are bolted on from the front with 7/16" bolt head cap screws. They may be 3/8" bolt head but I can't remember exactly. I understand the tap is a different size than the bolt head but I can't remember what size it is for the bolts at the moment.

What is the correct procedure for tapping aluminum?

I have tapped lotsa holes in steel but the single attempt in aluminum resulted in a broken tap.
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Old 11-24-2003, 09:35 AM
andy d andy d is offline
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Join Date: May 06, 2000
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1/4 20 screws generally have a 7/16s head. i just re-tapped some 3/8 holes and the tap said right on it to use a 5/16 hole. so i guess i would try a 3/16 hole for a 1/4 thread,if the tap starts to bind, try a 7/32 drill. you are using oil, right?
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Old 11-24-2003, 09:51 AM
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tgreese tgreese is offline
Join Date: May 29, 2003
Location: Medford MA USA
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Aluminum tends to clog the flutes of the tap, so use a lot of oil and run the tap out and clean the flutes every few turns. Here's a table of drill sizes for various taps:

Here's another one:

[ November 24, 2003, 04:54 PM: Message edited by: tgreese ]
Tim Reese
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Old 11-24-2003, 10:33 AM
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Millerluck Millerluck is offline
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Join Date: Sep 08, 2000
Location: Blissfield Michigan
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13/64 is the tap drill size for a 1/4-20 tap.

Don't use the taps they sell at the hardware store they are junk in my book.

Get a good high speed steel tap fom an industrial supply store. A two flute spiral point tap is what you need to get. Spiral point taps keep the chips forced ahead of the tap so they are less likely to clog it and break it.

One and a half times the diamater of the bolt thread depth is plenty of threads most times.

Drill one and a half times the diamater of the tap deeper then you want of thread depth. This gives room for the point of the tap and a place for the chips to go.

Easy math so you never need a thread chart....

1/4-20 tap drill size?

1 dev 20(thread pitch) = .05

.250 (Tap dia) - .05 = .200

Closest drill in a standerd drill set = 13/64


3/8-16 tap drill size?

1 dev 16 =.0625

.375-.0625 =.3125 (5/16 drill)

Good luck

Larry Miller

[ November 24, 2003, 05:43 PM: Message edited by: Millerluck ]
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Old 11-24-2003, 10:33 AM
Bozz Bozz is offline
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You might try a machinist tap. It has only two cutting rows and the rest is open to get rid of the metal fine's.
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Old 11-24-2003, 02:11 PM
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TexasJ10 TexasJ10 is offline
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There are guys on the Home Shop Machinist website that swear olive oil is the best tap lubricant you can get. I have tried it and it seems to work really well at keeping the tap from galling with aluminum threads and it seems to keep the new threads from breaking off. On aluminum you also have to make sure you get a straight start. Other wise the tap seems to be going in OK but it is just tearing up the threads you just made.
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Old 11-24-2003, 10:02 PM
243 243 is offline
Bleedin' Gasoline
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Thanks for the tips!
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Old 11-25-2003, 02:03 AM
joe joe is offline
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Location: PNWet, USA
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Even hdw store taps are ok for alum but as Tim mention above don't try to tap the hole in one continuous shot. Back it out some every couple turns to unload it and go slow.
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Old 11-25-2003, 03:53 AM
Quartermaster Quartermaster is offline
Join Date: Feb 13, 2003
Location: Enfield, Ct.
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In addidtion to all the tips mentioned above it may be noteworthy to know that the standard taps have a lead of about 3-5 threads. Meaning the thread is not going to be as deep as the hole and you must drill deeper (as mentioned above) to get the depth.

More math:
1/4-20 tap. 1/20=.05
.05 X a lead of 5 threads = .25 short on thread depth. A bottoming tap can be used also which has a lead of about 1-3 threads.
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Old 11-25-2003, 04:25 AM
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FSJ Thing FSJ Thing is offline
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And then a finishing tap which has NO lead! Funny thing is that I can't find anywhere that sells bottoming and finishing taps...If I hadn't used them so much on my ship I'd doubt that they existed!
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