Dial Bore Gauge
For Pump & Case Bores

The number one comeback in a home rebuild is not a burnt clutch. It is a pump that will not hold pressure, or a converter that walks in the hub bore, or a servo that leaks past the seal on the third apply. All three come from a bore that was out of round or worn oversize, and all three were invisible to whoever put the unit together because they measured with a caliper, or did not measure at all.

Bore wear in a transmission lives in the range of one to five thousandths, and it is almost never uniform. It is oval, and it is tapered. A caliper reads one chord across an unknown angle at an unknown depth and gives you a number that is confidently wrong by two or three thousandths. A dial bore gauge reads true diameter at a known depth on a known axis, and a 0.0001 inch gauge lets you see the taper develop across the bore length. That difference is the entire job.

Why 0.0001 Inch and Not 0.001

Most parts-store bore gauges read in 0.0005 inch or 0.001 inch increments. For an engine cylinder that is fine, because the wear you are chasing is five or ten thousandths. Transmission bores are a different problem. Typical bushing-to-shaft running clearance sits around 0.0015 to 0.003 inch, and the whole reuse decision often hinges on whether a bore has grown 0.0015 or 0.0035. On a 0.001-inch gauge those two readings can look identical.

The other reason is out-of-round. You are not just looking for a big diameter, you are looking for the difference between the reading at 0 degrees and the reading at 90 degrees. That delta is frequently under a thousandth on a bore that will still leak. A tenths gauge shows it. A thousandths gauge does not.

Related: temperature is real at this resolution. Aluminum expands roughly 0.0000123 inch per inch per degree F. A 3-inch aluminum pump bore that is 15 degrees warmer than your micrometer standard reads about 0.00055 inch large. Let the part, the gauge, and the setting micrometer all sit in the same room for an hour before you trust a tenths reading.

The Bores That Actually Matter

Pump Body Gear Pocket

On a gear-type pump, the pocket the driven gear runs in wears at the crescent and on the pressure side. Measure the pocket diameter at three depths and two axes. Growth much past 0.002 inch over nominal, or visible ovality, means the pump will bleed volume at idle and you will chase a low-pressure complaint forever. Side clearance between gear face and cover gets checked separately with a straightedge and feeler, typically in the 0.0008 to 0.002 inch range depending on unit.

Converter Hub Bore / Pump Bushing

This is the one that kills pumps. The converter hub rides in a bushing pressed into the pump body. When that bushing wears egg-shaped, the hub cocks, the pump seal sees a moving sealing surface, and you get a front leak that comes back no matter how many seals you install. Measure the bushing bore before and after you install a new bushing, because bushings size down when pressed and a bushing installed into a distorted housing will not clean up. Typical hub-to-bushing clearance runs about 0.002 to 0.004 inch; well past that and you are replacing the bushing, and if the housing bore itself is oval you are replacing the pump body.

Servo Bores in the Case

Aluminum case, aluminum piston, cast-iron ring or lip seal. Servo bores wear and score along the direction of side load. An oval servo bore of even 0.002 inch lets the seal cross-leak on apply, and the symptom is a soft or flaring band shift that nobody can find with a scan tool. Measure at the top, middle, and bottom of the piston travel path, two axes each.

Case Bushing Bores and Extension Housing

The rear extension housing bushing controls driveshaft yoke alignment. When the bore behind the bushing is worn or the bushing itself is oval, you get a driveline vibration and a rear seal that keeps leaking. Same measurement approach as the pump bushing: check the housing bore, then check the bushing ID after installation.

Valve Bores

Valve body bores are usually too small and too deep for a standard bore gauge, and the honest answer is that you check those with the valve itself, by feel and by vacuum. A hand vacuum pump on the bore with the valve installed is the practical test, covered in the valve body vacuum testing guide. Use the bore gauge for the larger accumulator and servo bores where it fits.

Typical Wear Limits

Always defer to the ATSG or OEM manual for the specific unit. These are the working ranges most transmission bores fall into, useful when the book is not in front of you.

Setting the Gauge Correctly

A dial bore gauge is a comparator. It does not know a diameter; it only tells you how far the bore differs from whatever you zeroed it against. That means the accuracy of your measurement is exactly the accuracy of your zero, and this is where most people go wrong.

• Check the micrometer first. Verify the outside micrometer against its standard or a gauge block before you use it to set anything. An uncalibrated mic just moves your error downstream.
• Set the mic to nominal bore size and lock it. If the bore is 1.7500 nominal, lock the mic on 1.7500.
• Install the right anvil and spacer stack on the bore gauge so the plunger sits near the middle of its travel with the gauge in the mic. Center of travel is where the gauge is most linear.
• Rock the gauge in the mic jaws and find the reversal point. The needle sweeps to a minimum and comes back. That minimum is the true measurement plane. Zero the bezel there.
• Take the reading in the bore the same way, rocking to the reversal point. Anything you read without rocking is a chord, not a diameter.
• Six readings minimum: three depths, two axes 90 degrees apart. Write them down. The pattern tells you the story, not any single number.

You need a real outside micrometer set to do this properly. Our micrometer and caliper guide for transmission rebuilds covers which sets are worth the money. Setting rings are more accurate than a micrometer if you happen to have one in the size you need, but a good mic covers every size.

What to Buy

For most transmission work a 1.4 to 6 inch tenths gauge covers pump bores, servo bores, extension housing bores, and drum bores. Add a small-hole or telescoping set for anything under about three quarters of an inch, and read those with the micrometer.

Browse 0.0001 inch dial bore gauges

Check Mitutoyo 511 series →Check Fowler bore gauge sets →

Browse 0-6 inch outside micrometer sets →Telescoping and small-hole gauge sets →

Common Mistakes That Wreck the Reading

• Not rocking the gauge. Every reading taken static is a chord and reads small. Rock to the reversal point, always.
• Setting the gauge on a dirty mic anvil. A speck of lint is a tenth. Wipe both anvils and the gauge contacts with a clean rag before zeroing.
• Hand heat. Holding an aluminum pump body for two minutes while you fumble with the gauge adds real expansion. Handle by the edges, work quickly.
• Measuring a bore with the bushing already pressed in and calling it the housing bore. They are two different measurements and you need both.
• Wrong spacer stack. If the plunger is near the end of its travel, the gauge is nonlinear and your zero will not hold across the range.
• Skipping the mic verification. A mic that is off by 0.0004 makes every bore in the unit read the same wrong amount, which is worse than no measurement because you believe it.

After You Measure

A measurement is only worth what you do with it. If a bushing is out of spec, it comes out and a new one goes in with the correct driver so it does not go in cocked or shaved. Our bushing driver set guide covers sizing and pilot vs step drivers, and the shop press guide covers the press capacity you need for the bigger interference fits. Measure again after installation, because a bushing installed into a distorted housing will mirror the distortion.

For endplay and shaft float, which is a different measurement entirely, use a dial indicator on a magnetic base rather than a bore gauge. See the endplay dial indicator guide.

FAQ

Can I just use a caliper to check a transmission bore?

No. A caliper reads a chord at an unknown angle and depth, and its own repeatability is around 0.001 inch on a good day. The wear you are hunting is often smaller than the caliper's error.

Do I need a setting ring or is a micrometer enough?

A verified micrometer is enough for transmission work. Setting rings are more accurate but you would need one for every bore size. Check the mic against its standard first and you are fine.

What bore gauge range covers most automatic transmissions?

A 1.4 to 6 inch gauge handles pump bores, servo bores, drum bores, and extension housing bores. Add telescoping or small-hole gauges for anything under about 0.7 inch.

How much out-of-round is too much?

As a working rule, under 0.0005 inch is good, 0.001 inch is watch it, and over 0.002 inch means rework or replace. Sealing bores are less tolerant than support bores.

Why does my reading change every time I take it?

Almost always one of three things: you are not rocking to the reversal point, the plunger is near the end of its travel, or the part is changing temperature in your hands. Fix those and a good gauge repeats within a tenth or two.

Is a digital bore gauge better than a dial one?

Not inherently. A digital head is easier to read and can hold min values, but it will not fix a bad zero or a bad technique. Repeatability of the mechanism matters more than the display.