Measuring Bores and Threads

An outside diameter is easy to reach. A bore is not. You cannot see inside it, your tool has to find the true diameter by feel, and the hole may be tapered, oval, or barrel-shaped. Threads are harder still, because the size that matters is one you cannot touch directly. These are the skills that separate someone who can read a micrometer from someone who can measure.

What You Will Be Able to Do

• Choose the right tool for a bore's size and tolerance.
• Measure a bore with a telescoping gauge and a micrometer.
• Set and read a dial bore gauge.
• Check a bore for taper and out-of-round.
• Identify a thread's pitch, and calculate pitch diameter from three-wire readings.

Choosing a Bore Tool

A transfer tool has no scale. It takes the size from the hole, and you read it with a micrometer. So you have two chances to add error, once in the hole and once at the micrometer.

Using a Telescoping Gauge

A telescoping gauge looks like a T. Its spring-loaded plungers push out to touch the bore walls, and the handle locks them.

1. Choose a gauge that fits the bore's size. Clean the bore and the gauge.
2. Squeeze the plungers in and lightly lock the handle.
3. Put the gauge into the bore at a slight angle, tilted so the handle leans toward you.
4. Release the lock so the plungers spring out and touch the walls.
5. Snug the lock just enough to hold, but not tight.
6. Rock the handle through the center in one smooth motion, pulling it toward you. The plungers are pushed in to the true diameter as they pass the widest line across the bore. Do not rock back.
7. Lift the gauge out without bumping it.
8. Measure across the plungers with an outside micrometer. Use a very light touch. The ratchet alone can push the gauge in, so feel for a light drag while you rock the gauge between the faces.
9. Do it three times. If the readings disagree by more than about 0.0005 inch, slow down and repeat.

Getting a repeat takes practice. Try it first in a bore of known size, such as a ring gauge.

A small-hole gauge works in a similar way. Turn its knob to expand a split ball until you feel light drag, pull it out straight, and measure across the ball.

Using a Dial Bore Gauge

A dial bore gauge has a sliding contact and an indicator on a long handle. It does not read the size directly. It shows how far the bore differs from the size you set it to.

1. Pick the anvil or extension for the bore size.
2. Set the gauge to zero in a master ring, or in a micrometer locked at the target size.
3. Put the gauge into the bore, guide end first, and let the centralizer settle it.
4. Rock the handle gently front to back. The needle swings and reverses.
5. Read at the point where the needle reverses. That turnaround is the smallest reading and it marks the true diameter, because any tilt makes the distance across longer.
6. Add or subtract that value from the master size.

Worked example: the gauge is set to zero on a 2.0000-inch ring. In the bore, the needle turns around at +0.0006. The bore is 2.0006 inches. Watch your plus and minus. On many bore gauges, a larger hole lets the plunger move out, and the needle moves in the direction marked plus when it is set up correctly. Always confirm on your gauge.

Finding Taper and Out-of-Round

One reading is not enough for a close bore, because a bore is rarely a perfect cylinder.

1. Measure at the top, middle, and bottom of the bore.
2. At each depth, measure twice, 90 degrees apart.
3. Write the six readings in a grid.

• Taper is the change from top to bottom. In this example, the 0-degree line changes from 1.5004 to 1.5000, a taper of 0.0004 inch.
• Out-of-round is the difference at one depth. At the top, 1.5008 - 1.5004 = 0.0004 inch.

If the bore must accept a mating pin, take your reading in the same direction the pin will enter and at the depth it will reach. The size where the parts meet is the one that counts. Engine cylinders, hydraulic bores, and bearing housings are all checked this way.

Thread Basics for Measuring

Each thread has three diameters. The major diameter is the largest. The minor diameter is the smallest. The pitch diameter is in between, and it is where the width of the thread equals the width of the groove. Pitch diameter decides how well the threads fit, and it can't be read with a caliper.

Pitch is the distance from one thread crest to the next. An inch thread is named by threads per inch, and pitch = 1 divided by threads per inch. For 1/4-20, the pitch is 1/20 = 0.050 inch. A metric thread gives pitch directly: M10 x 1.5 has a pitch of 1.5 mm.

A thread pitch gauge is a fan of toothed blades. Find the blade that sits in the thread with no light showing, and read its number. This identifies the thread. It says nothing about the size of fit.

The Three-Wire Method

Three small hardened wires of a known size let you measure pitch diameter with an ordinary micrometer. Two wires sit in the threads on one side, and one sits on the other. You measure over the wires, then calculate.

For a 60-degree thread:

• Best wire size: W = 0.57735 x P
• Pitch diameter: E = M - (3 x W) + (0.86603 x P)

Here, M is the measurement over the wires, and P is the pitch.

Worked example: a 1/4-20 external thread has P = 0.050. The best wire is 0.57735 x 0.050 = 0.0289 inch. You measure M = 0.2585 inch over the wires.

E = 0.2585 - (3 x 0.0289) + (0.86603 x 0.050)

E = 0.2585 - 0.0867 + 0.0433 = 0.2151 inch

Compare that with the pitch diameter limits in the thread table for that class of fit.

Tips for good readings:

1. Hold the wires in place with a little grease, a wire holder, or a rubber band.
2. Keep the micrometer's faces flat across the wires, and use a light, steady touch. Heavy force can dig the wires into the threads.
3. Take readings at more than one spot along the thread.

For shop-floor pass or fail work, a thread ring or plug gauge is faster. Three-wire work is for setting tools, checking ring gauges, and answering a dispute.

Common Mistakes

Check Your Understanding

1. A bore gauge is zeroed on a 1.2500 ring. In a bore, the needle turns around at -0.0003. What is the bore size?

1.2497 inches, assuming your gauge reads minus for a smaller bore. Confirm its direction on the master.

2. Why rock a dial bore gauge instead of holding it still?

Any tilt gives a longer distance across the bore. The smallest reading at the turnaround is the true diameter.

3. Your telescoping gauge readings in one hole are 0.7502, 0.7495, and 0.7509. What is your next step?

The spread is too wide to trust. Check your lock pressure and your rocking motion, and repeat until the readings agree.

4. What is the best wire size for a thread with a pitch of 0.0625 inch?

0.57735 x 0.0625 = 0.0361 inch.

Key Takeaways

• A transfer tool gives you two chances to add error, so go slowly.
• Use one smooth rock on a telescoping gauge, and read a bore gauge at the turnaround point.
• Check a close bore at three depths and two directions.
• Measure in the direction the mating part will enter.
• Pitch diameter decides thread fit. A pitch gauge only names the thread.
• The three-wire method turns a micrometer reading into a pitch diameter.