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Design and Operation


Probe: The probe is typically manufactured to the lower end of the tolerance range from hardened steel (~68Rc) and chrome plated for wear resistance.

Tapered Needle: Manufactured of hardened steel (~63Rc) or carbide (~76Rc), the shank is ground and the taper is lapped to ensure smooth, accurate movement with the probe.

Indicator Holder: This standard holder connects the probe/needle assembly to the indicator and consists of a plastic shrouded steel body that comes in a variety of clamp sizes to fit virtually any indicator. Retracting holders are also available for easy entry into the bore.

Indicating Unit: Virtually any indicating device, either mechanical or electrical, can be used with Diatest Split-ball Probes.  Stark Industrial has a variety of indicating units available, many with SPC output.

Setting Ring: Setting rings are available in both inch and metric sizes to fit any BMD Split-ball gage. Rings up to 1.5mm have a synthetic sapphire insert, while rings over 1.5mm are manufactured from aged and hardened steel. Stark Industrial can provide rings with or without certification in many different styles.


A radius on the leading edge of the split-ball allows the gage to enter the bore smoothly.  As the split-ball expands and contracts, the movement is transferred to the needle in a 1:1 ratio. When the desired gaging depth has been reached, the gage should be rocked back and forth until the maximum value is found. This is called the reversal point and is the correct measured value. When a checking stand is used, the rocking movement is not required.