AND, the diameter of a bolt pattern circle is ???

[loquin]

A couple of weeks ago, I was trying to determine the diameter of a bolt mounting circle, and the only accurate measurements I could take (because it was a 5 bolt pattern) was the separation between the bolt holes.

The closest I could get for that measurement was approximately 1.76 inches.

Now, I'm sure you machinists out there already have a chart to check, or a formula written down, but after some searching, I was able to get an answer.

The formula to use is:

D = S / Sin(180/N)

where:
D is Bolt Circle Diameter
S is the Bolt Spacing
N is the Number of Bolts

If I plug 1.76 inches, and 5 bolts into the above formula, I get 2.994 inches. So, it looks like I have a 3 inch - 5 bolt pattern.

To go the other way, if you know the bolt circle diameter, and need to measure spacing for drilling holes, (when you don't have a means of indexing the holes,) the formula is:

S = D • Sin(180/N)

Here's a table to use with values for Sin(180/N):

....N.. Sin(180/N)

1. N/A(you're dividing by zero, so I guess you get a black hole...)
2. 1.0000
3. 0.8660
4. 0.7071
5. 0.5878
6. 0.5000
7. 0.4339
8. 0.3827
9. 0.3420
10. 0.3090

[loquin]

Saw an interesting machinist trick the other day, to use with your digital caliper.

To accurately measure center-to-center spacing, when the two holes are the same diameter...

1. ZERO the caliper, and measure a hole diameter
2. With the inside measurement jaws still in place inside the hole, press the ZERO button on the caliper
3. Using the same inside measurement jaws, measure to outside to outside of the two holes.

That's it. The indicated reading on the caliper is the hole center-to-center measurement.

[loquin]

If you have a center hole within the bolt-hole pattern you need to measure, then you can measure the bolt-hole circle as follows (refer to the attached diagram)

1. Measure the center hole diameter (D1)
2. Measure the separation between the center hole and any of the bolt holes (S)
3. Measure the bolt hole diameter (D2)
   .

Then, calculate the Bolt Hole circle Diameter (D) as follows:

D = D1 + (2 * S) + D2

[darwin]

On SAE bolts the diameter of a bolt is 3/16ths less then the head size. This works up to a certain size ie. 9/16ths=3/8ths bolt.