How to Measure Deep Hole Diameter: Tools and Techniques

I use bore gauges for most deep hole measurements. I have measured thousands of holes across dozens of shops, and the right tool always depends on the tolerance and the hole depth.

Measuring the diameter of a deep hole is not the same as checking a conventional bore. Access is limited. The bore is long and narrow. Standard calipers and bore mikes will not reach past the first 100mm. Over the years, I have settled on five main methods that cover the full range of what I encounter on the shop floor.

Measurement Tools: Accuracy and Limits

The accuracy numbers above are manufacturer specs. In real-world shop conditions, I see bore gauges hold ±0.005mm on a typical day. Air gauges deliver ±0.002mm consistently when the jets are clean. CMM accuracy depends on the machine and probe — I have seen results drift by ±0.010mm between different CMMs on the same part.

Bore Gauge: My Primary Method

A three-point bore gauge with extension rods is my go-to for holes deeper than 200mm. Here is my procedure:

1. Set the gauge with a master ring at the nominal diameter. I use a ring calibrated to ±0.001mm of the target size.
2. Insert the gauge into the hole. I rock it gently side to side to find the true bore center. The gauge reads the minimum when centered.
3. Take readings at multiple depths. I check every 50mm for the first 200mm, then every 100mm after that. This catches taper and bellmouthing.
4. Record the max and min readings at each depth. The difference tells me ovality. The trend across depths tells me taper.

For a 25mm hole at 500mm depth, setup takes 2 minutes and measurement takes 3 minutes. That is faster than sending the part to CMM and waiting in the queue.

Temperature is a hidden variable. A part fresh off the machine can measure 0.005-0.010mm smaller once it cools to 20°C. I let parts sit for 30 minutes on a granite plate before final inspection. I have scrapped parts that were actually in tolerance simply because I measured them hot.

Air Gauging for Tight Tolerances

I reach for air gauges when the tolerance is ±0.010mm or tighter. The gauge nozzle is small enough to fit deep holes, and it measures without contacting the surface. No contact means no wear on the gauge and no marking of the bore.

The main limitation is depth. Most air gauge probes max out around 500mm. Beyond that, the air pressure drop makes readings unreliable. I learned this the hard way on a 700mm deep hydraulic cylinder bore.

Coolant residue is another problem. A blocked air jet reads 0.010-0.020mm undersize. I now wipe the bore with a clean rag and blow it out with compressed air before using the air gauge. This adds 30 seconds to the inspection but saves me from false rejects.

CMM and Ultrasonic Methods

I reserve CMM for first article inspection and qualification. The output is comprehensive — a full profile report showing diameter at 10mm intervals, ovality, and taper over the full length. The downside is time. A deep hole scan takes 20-30 minutes of machine time plus setup and programming.

Ultrasonic gauges serve one purpose in my work: wall thickness measurement. The resolution is ±0.05mm, which is too coarse for bore diameter verification. But when I need to check concentricity between a deep bore and the outer diameter, ultrasonic is the only option short of cutting the part in half.

For more on how measurement fits into the overall drilling process, see [/process-deep-dives/deep-hole-drilling-cycle-time](cycle time estimation). The method you choose directly affects your inspection cycle time.

Plug Gauges for Production Inspection

I use plug gauges for high-volume inspection where the tolerance is ±0.050mm or wider. A plug gauge is a simple go/no-go tool. If the go end passes through the hole and the no-go end does not, the diameter is within tolerance.

The advantage is speed. I can check a hole in 5 seconds with a plug gauge. The disadvantage is that I get no numerical data — just a pass/fail result. I cannot see taper, ovality, or diameter trend over the tool life.

I keep a set of plug gauges for each size I run regularly. They cost $50-$200 each depending on diameter and tolerance class. I send them out for calibration every 12 months.

Choosing the Right Measurement Method

I match the measurement method to the tolerance and production volume using this guide:

I use this guide as a starting point. The final decision depends on how deep the hole is and whether I can get a CMM probe into it.

Common Measurement Errors

I see the same mistakes repeatedly in shops I visit. Here are the ones that cost the most time and money:

• Measuring on the bellmouth. The first 10-20mm of a deep hole is often slightly larger than the rest of the bore. I always measure at least 30mm from the entrance.
• Using a damaged bore gauge. The contact points wear after about 5000 measurements. I recalibrate every 6 months or 2000 measurements, whichever comes first.
• Assuming the master ring is accurate. I send my master rings out for calibration annually. A 0.002mm error in the ring becomes a 0.002mm error in every measurement.
• Measuring a hot part. A 10°C temperature swing changes a 50mm steel bore by about 0.006mm. Cool the part first.

Key Takeaways

• Bore gauges handle 80% of my deep hole measurement work. I recommend them as the primary tool for any shop doing precision deep hole drilling.
• Air gauges deliver the best accuracy, ±0.001mm under ideal conditions, but they max out at 500mm depth.
• CMM is the gold standard for documentation but not for speed. Use it for first articles and customer qualification.
• Temperature control is critical. A 10°C swing changes a 50mm steel bore by 0.006mm. Let parts stabilize before measuring.
• Match your measurement method to the tolerance band. ±0.050mm calls for a plug gauge. ±0.005mm calls for a bore gauge or air gauge.
• Recalibrate your bore gauge and master rings on a fixed schedule. Small errors compound fast.