An M18-1.5 screw takes a Ø20 mm normal clearance hole, a Ø16.5 mm tap drill, and a Ø21 mm printed free-fit hole.
M18-1.51.5 mm pitch
Printed free-fit clearance hole — interactive in the app
Clearance hole (3D printed)
same grades as the drilled hole
Close fit
snug press fit · = 0.748 in
Ø19 mm
Free fit
easy assembly · = 0.8268 in
Ø21 mm
Slicer hole compensation
typical FDM starting point — not included above
0.2 mm
Head recess (3D printed)
Socket head cap
Recess bore
clears the head
Ø27.5 mm
Head height
18 mm
Tap drill
drill 16.5mm
Tap drill
for a tapped thread · = 0.6496 in
Ø16.5 mm
Clearance hole (drilled)
Close fit
drill 19.0mm · = 0.748 in
Ø19 mm
Normal fit
drill 20.0mm · = 0.7874 in
Ø20 mm
Loose fit
drill 21.0mm · = 0.8268 in
Ø21 mm
Counterbore
Counterbore
clears a socket-cap head
Ø30 mm
Counterbore depth
18 mm
Min thread engagement
depth of thread to hold
Steel / hard
18 mm
Aluminum / soft
27 mm
Plastic / print
36 mm
Identify an unknown screw
Major Ø range
caliper across the thread crests
17.85–18 mm
Pitch check
threads per inch equivalent
16.93 TPI
These are standard reference numbers. Every machine prints and cuts a little differently — set a hole to Ø5.00 and it might land at Ø4.95. In the FitCheck app you calibrate your machine once, and every dimension here auto-corrects to match it — plus tolerance bands, more insert series, and full offline use.
Clearance holes come in three graded sizes. Close locates the part precisely and forgives nothing; normal is the default for most assemblies; loose buys alignment slop across multi-hole patterns. (The standards name the same three grades differently — ISO 273 calls them Fine/Medium/Coarse, ASME B18.2.8 Close/Normal/Loose; the plain-English set is used throughout this page.) The tap drill is a different animal: it leaves material for the thread, so it is always smaller than any clearance hole.
Why your printed hole comes out small
An FDM hole is walked out by the extrusion width and pulled in as the plastic cools, so a commanded Ø5.00 typically measures under. The printed clearances above are the same standard grades as the drilled ones — they are not pre-compensated. Close that gap either with your slicer's hole-compensation setting (the starting value is listed above) or, properly, per machine: print one test coupon, measure it, and the app applies your own measured error to every dimension on this page.
Heat-set inserts
The pocket bore is the hole the insert melts into; it is smaller than the insert so the displaced plastic fills the knurling. Too tight and the boss splits, too loose and the insert spins under torque. Keep the wall around it at least as thick as the value above.
Metric clearances per ISO 273 · imperial per ASME B18.2.8 · heat-set pockets per CNC Kitchen · bearing housing seats per ISO 286. Reference only — verify before you cut.