What size hole for 7/8-18 Tap?

CluelessNewB

CluelessNewB

Active Resistor
H-M Supporter Gold Member
What size hole would I need for a 7/8-18 UNF Tap?

Yes I know it's an odd size. None of the reference material I have lists it and I couldn't find it on the web either. This strange thread was used for spark plugs, tie rod ends and speedometer and tachometer cables. I need to make a cap to go over a tachometer cable fitting on an engine. The total length of the fitting is about 3/4" but I can make the cap longer. Since this is for a boat used in salt water I may make this out of HDPE. I was considering aluminum but I would rather avoid dissimilar metals.
 
There's usually a large latitude of possible diameters. I'd start with the requirements for 7/8-20 and 7/8-14 and split the difference. Here's a chart that I refer to a lot.


http://www.shender4.com/thread_chart.htm


Ray


CluelessNewB said:
What size hole would I need for a 7/8-18 UNF Tap?

Yes I know it's an odd size. None of the reference material I have lists it and I couldn't find it on the web either. This strange thread was used for spark plugs, tie rod ends and speedometer and tachometer cables. I need to make a cap to go over a tachometer cable fitting on an engine. The total length of the fitting is about 3/4" but I can make the cap longer. Since this is for a boat used in salt water I may make this out of HDPE. I was considering aluminum but I would rather avoid dissimilar metals.
Click to expand...
 
For any standard (60 degree) thread in aluminum, I've always found the following rule of thumb handy:

Base diameter - 1/pitch = tap drill diameter, then go up to the nearest standard drill size.
For steel, you can use the "aluminum" value for small sizes, or go up to the next available size for large threads.

For 7/8" - 18 in aluminum, since 1/18=.0556, this gives .875-.0556 = .7294, or about 46.7/64
Next standard drill size would be 47/64, or .7344

PS - checking rule of thumb against the table that Ray sent out:

. . . . . . . Table . . . . . . . . . . . . . . . .Rule of thumb
. . . . . . . Alum . Steel . . . . . . . . . . Aluminum . . . . . . . . . . . . . . . . Steel
7/8 - 14 .8125 .8281 - - - .8036 = 51.4/64 => 13/16 = .8125 . . 53/64 = .8281
7/8 - 20 .8281 .8438 - - - .825 = 52.8/64 => 53/64 = .8281 . . . .27/32 = .8438

Good agreement for this size thread. But note that the table goes to next size for all threads in steel.

- hman
(John Herrmann)
 
As hman wrote, if you subtract the pitch from the diameter, it gives you the correct hole size.

When I found out about this, I shared it with my supervisor. He wasn't happy he spent all that time memorizing the tap drill charts :rofl:
 
I think you've got a typo...

0.875 - 0.0556 is 0.8194. = 52.44/64 or rounding up, 53/64 = 0.8281.



hman said:
For any standard (60 degree) thread in aluminum, I've always found the following rule of thumb handy:

Base diameter - 1/pitch = tap drill diameter, then go up to the nearest standard drill size.
For steel, you can use the "aluminum" value for small sizes, or go up to the next available size for large threads.

For 7/8" - 18 in aluminum, since 1/18=.0556, this gives .875-.0556 = .7294, or about 46.7/64
Next standard drill size would be 47/64, or .7344

PS - checking rule of thumb against the table that Ray sent out:

. . . . . . . Table . . . . . . . . . . . . . . . .Rule of thumb
. . . . . . . Alum . Steel . . . . . . . . . . Aluminum . . . . . . . . . . . . . . . . Steel
7/8 - 14 .8125 .8281 - - - .8036 = 51.4/64 => 13/16 = .8125 . . 53/64 = .8281
7/8 - 20 .8281 .8438 - - - .825 = 52.8/64 => 53/64 = .8281 . . . .27/32 = .8438

Good agreement for this size thread. But note that the table goes to next size for all threads in steel.

- hman
(John Herrmann)
Click to expand...
 
According to Machinery Hanbdbook 7/8-18 UNS class 2A thread shows minor diameter .815 min - .828 max.
 
Look up the minor diameter for 9/16" - 18. Subtract that from 9/16", which give you the deepth of an 18 tpi thread. Subtract that dimension from 7/8". You then have the minor diameter of a 7/8" - 18 thread.

Bill
 
Ray C said:
I think you've got a typo...

0.875 - 0.0556 is 0.8194. = 52.44/64 or rounding up, 53/64 = 0.8281.
Click to expand...


My error. You are absolutely right. Apologies!:angry:

- hman
(John Herrmann)
 
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