Tap Drill Size Chart

M56 x 5.5 Tap Drill Size

Tap drill

50.5 mm 1.9882 in

Use a 50.5 mm drill (1.9882 in) for 77.0% thread.

The drill size for a M56 x 5.5 tap is 50.5 mm (1.9882 in). If you are wondering what size drill bit a M56 x 5.5 tap needs, that is the one: it leaves 77.0% of thread, which is the figure standard tap and drill charts are built around. Below are the other holes you might want for this size - a larger drill for tough material, the form-tap hole, and the clearance holes for the bolt that goes in.

Every drill for M56 x 5.5

Tap drills and clearance drills for M56 x 5.5 (Metric coarse, 5.5 mm pitch). Percent of thread is the figure stored for each drill, which reproduces from the major diameter and pitch of M56 x 5.5.
Use Drill Decimal in mm % thread
Standard tap drill Cut tap, general use (D - P) 50.5 mm 1.9882 50.50 77.0%
Inch substitute Closest inch drill if you have no metric set 1-63/64″ 1.9844 50.40 78.3%
Hard material Stainless, tool steel, anything that fights the tap 51.5 mm 2.0276 51.50 63.0%
Form (roll) tap Theoretical 53.57 mm - starting point, check your tap maker's chart 53.5 mm 2.1063 53.50 —

Clearance holes for a M56 x 5.5 bolt

A clearance hole is not tapped. It is the hole in the part the bolt passes through, so it is always larger than the thread.

Clearance holes for M56 x 5.5 per ISO 273. Clearance depends on the bolt diameter alone, so every M56 thread shares these figures.
Fit Hole mm Hole in
Fine 58.00 2.2835
Medium 62.00 2.4409
Coarse 66.00 2.5984

M56 x 5.5 dimensions

Nominal dimensions for M56 x 5.5. The basic internal minor diameter is the hole a 100% thread would start from, which is why no tap drill is ever that small.
Dimension Inch mm
Major diameter 2.2047 56.00
Basic internal minor diameter 1.9703 50.05
Pitch 0.2165 5.50
Thread designation M56 x 5.5

What the hole looks like

Cross-section of a M56 x 5.5 tapped hole, drawn to scale The drilled hole is 50.5 mm, 1.9882 inches or 50.50 millimetres across. The tap cuts outward from that wall to the major diameter of 2.2047 inches. A 100 percent thread would reach in to the basic minor diameter of 1.9703 inches, shown as a dashed line; the difference is why this drill gives 77.0% of thread. major 56 mm drill 50.5 mm · 50.5 mm dashed = 100% thread at 50.05 mm
M56 x 5.5 in section, to scale. The accent line is the thread the tap cuts from a 50.5 mm hole; the dashed line is where the crest would sit at 100% thread. The gap between them is the 23.0% of thread this drill leaves behind.

Work out a different hole for M56 x 5.5

Pre-filled with M56 x 5.5. Change the material, the tap type, or which drills you actually own, and it picks the closest real drill and tells you the percent of thread you end up with. The tables above are the published figures and do not depend on this.

The general-purpose target

Tap type
Drills you have

Best drill you have, cut tap, 70% target

51 mm2.0079 in70.0% thread

Theoretical hole 51 mm

One size down, deeper thread2″ 2.000 in · 72.8%

One size up, easier tapping51.5 mm 2.0276 in · 63.0%

For comparison, the published chart drill for M56 x 5.5 is 50.5 mm at 77.0% thread.

Notes on M56 x 5.5

Only pitch in the data
M56 x 5.5 is the only pitch this chart lists at 56 mm, so there is no fine or coarse alternative to confuse it with.
What the standard drill gives you
50.5 mm leaves 77.0% of thread (D - P). That is in the normal 65-80% band most shop charts aim for.
Stainless, tool steel and other tough material
51.5 mm opens the hole to 63.0% engagement. Less material for the tap to cut means less torque and less heat, at the cost of some thread depth.
Using a form (roll) tap
A form tap displaces metal instead of cutting it, so it needs a larger hole: 53.5 mm, against a theoretical 53.57 mm. Treat that as a starting point and check your tap maker's chart.
This size is usually bored, not drilled
At 56 mm, holes this big are normally bored or thread-milled rather than drilled and tapped. The 50.5 mm figure is here so the numbers are complete and so you can check a hole you already have; on a machine you would single-point or thread-mill the thread and bore the hole to size.
Closest inch size
1-1/2-6 is the nearest inch thread by diameter (38.1 mm against 56 mm). It is a planning reference only. The two are not interchangeable: the pitches differ, and a bolt from one will not run into a hole tapped for the other.

Related sizes

Next size up and down

Closest inch size

1-1/2-6 1-11/32″ · 1.3438 in · 34.13 mm · 72.1% 38.1 mm major against 56 mm

Not interchangeable. 1-1/2-6 is the nearest size by diameter, nothing more. The pitches differ, so a 1-1/2-6 fastener will not run into a M56 x 5.5 hole and forcing one will strip both.

Guides for tapping M56 x 5.5

Longer reads on the decisions behind the numbers above.

All guides →

M56 x 5.5 tap drill: common questions

What size drill bit do I need for a M56 x 5.5 tap?
A 50.5 mm drill, 1.9882 in. That leaves 77.0% of thread, which is the standard chart figure for M56 x 5.5.
What is the drill size for a M56 x 5.5 tap in inches?
The 50.5 mm tap drill measures 1.9882 in. If you only have inch drills, 1-63/64 in (1.9844 in) is the closest, giving 78.3% thread.
What drill should I use for M56 x 5.5 in stainless steel?
Use 51.5 mm instead of 50.5 mm. It leaves 63.0% of thread rather than 77.0%, so there is less material for the tap to cut and less chance of snapping it.
What is the clearance hole for an M56 x 5.5 bolt?
For a medium fit, 62 mm per ISO 273. A clearance hole is not tapped; the bolt passes straight through it.
A 4 × 6 in card with the drills for this size.