Counterbore Depth and Thread Engagement Left
How deep a counterbore goes, and what it leaves behind for the threads.
Example
You enter
- Screw nominal diameter (in) 0.5
- Head height (in) 0.5
- Clearance below flush (in) 0.015
- Plate thickness (in) 0.75
- Engagement multiplier (1.0 steel, 1.5 cast iron, 2.0 aluminum) 1
- The counterbore is in the tapped part yes
You get
- Counterbore depth 0.515 in
- Remaining thickness below the bore 0.235 in
- Required engagement (in) 0.5
- Shortfall (in) 0.265
- Minimum thickness (in) 1.015
No published worked example is on file for this calculation. It is checked against a hand-worked derivation of its method and an independent reference test.
Details, formula, and sources
ScopeThe bore itself is arithmetic: the head has to end up flush or a chosen amount below flush, so the counterbore goes as deep as the head is tall plus that clearance, and fifteen thousandths below flush is a common choice -- enough that the head is definitively under the surface, little enough that a driver still reaches it. The second half is where a design gets caught. Every thousandth of counterbore is a thousandth taken out of the material below, and when the counterbore is in the TAPPED part rather than in a clearance plate it comes straight out of the thread engagement. Steel wants about one diameter of engagement, cast iron about one and a half, and aluminum two or more, and a counterbore that leaves less than that has quietly turned a strong joint into one that strips its threads before the bolt yields. A half-inch socket head cap screw with a 0.500 in tall head, counterbored 0.015 in below flush in a 0.750 in steel plate, uses 0.515 in of the plate and leaves 0.235 in against a 0.500 in requirement -- three quarters of an inch of plate looked like plenty and the bore ate two thirds of it. The fixes are a thicker tapped part, or moving the counterbore into a separate clearance plate, which is what a properly detailed joint does. A design check; the fastener standard's head dimensions, the material's own thread strength, and the engineer of record govern.
Formula or tablecounterbore_depth_in = head_height_in + below_flush_in; remaining_thickness_in = plate_thickness_in - counterbore_depth_in; required_engagement_in = engagement_multiplier x screw_diameter_in; the available engagement is the remaining thickness when the bore is in the tapped part.
Source and editionCounterbore depth from head height plus the clearance below flush, checked against the thread engagement the material needs (about 1.0 diameter into steel, 1.5 into cast iron, 2.0 or more into aluminum), by name -- standard machine-design practice, with the multiplier entered rather than bundled. The fastener standard's head dimensions and the engineer of record govern.
Free accessTwo subtractions and a comparison on the fastener's published head dimensions and the designer's own plate thickness.
What governsEstimate. AHJ and licensed professional govern.
API field namesscrew_diameter_in, head_height_in, below_flush_in, plate_thickness_in, engagement_multiplier, tapped_part, counterbore_depth_in, remaining_thickness_in, required_engagement_in, shortfall_in, minimum_thickness_in
- Engagement by material about 1.0 diameter into steel, 1.5 into cast iron, 2.0 or more into aluminum; entered, not bundledmachine design practice
- Where the bore is a counterbore in the TAPPED part eats engagement; one in a clearance plate does notmachine design practice
- Failure mode too little engagement strips the threads before the bolt yieldsfastener practice