Thread Engagement Chart: Minimum Length Table by Material (Free PDF)

To read it, find your nut, tapped hole, or whatever has the internal thread on it. Read down that column until you get to the material. Then read across the chart to find the recommended minimum engagement (depth of thread contact needed for the joint) relative to the nominal bolt diameter d.

For instance: A M10 bolt into a cast aluminum part should of had around 25 mm of thread engaged. Into a steel block? About 10 mm.

Why does that happen? The design range show how much the range might vary from one reference to another. If you’re not sure what’s going on, always go with the high end. If you know the application, you know the load case, and you understand the materials involved, then go with the low end.

The next two columns calculates the multiplier for a 3/8 in bolt and an M10 so you can check your math against their results.

One rule governs everything on the entire table: don't let the internal threads strip before the bolt breaks. Why? Because a broken bolt is obvious, cheap to replace, and doesn't damage anything but itself.

Internal thread stripping is the reverse problem. The problem isn't obvious when the joint looks OK. It fails gradually under load. Fixing it involves replacing the expensive housing by drilling out the threads instead of just swapping fasteners.

How does this happen? Internal thread failure is all about engagement length. When the bolt pulls against the threaded hole, it fail by shearing across the area where the threads meet (the bit inside the two part where they touch). The more there is to engage, the more there is for that shear area to grow across. By doubling the engagement length you double that shear area; a piece of soft material will always be able to last longer then the bolt if you go deeper.

In fact, it’s the ratio between the tensile strength of the bolt vs. The shear strength of the material it’s threaded into drives the multiplier. That explains why a Grade 8 or Class 10.9 bolt require more depth in a given aluminum block than would be assumed from published values for aluminum (or most other materials) when using ordinary bolts.

And that’s why the references differ about aluminum. With a Grade 5 bolt and a 6061-T6 alloy, it calculates out close to 1.5 x d. The safe number everyone learns is 2 x d, and even as high as 2.5 x d for castings.

Upgrading bolt grade doesn’t make the hole any stronger if you have a tapped hole already. Ensure that the new bolt will remain protected by the engagement, otherwise the stronger bolt becomes a better thread stripper.

These are common pitfalls. For engagement, the full threads must be in contact. Subtract any counter sink at the top (and the chamfer threads on a blind hole bottom) from the engagement length used. In all, usually lose 2 to 3 pitches of thread.

Bottoming out the bolt in the hole before the head can clamp counts for nada. Drill and tap further then the engagement figure above. Common shop practice is to add drill point clearance, then add an additional 4 pitches of tap length beyond that. Adding more helps to a certain extent.

For steel, about 1.5 x d is when the first engaged threads does most of the work, and adding more doesn’t help much. The solution to a soft material might be tapping a threaded insert instead of going ever deeper.

Note that this table doesn't apply if your screw type is a self tapping (thread forming) screw in sheet metal or plastic. In that case the data will be provided by the maker of the screws in terms of boss and hole sizes. Nor does it apply to Helicoils and threaded inserts. Use the data from the insert manufacturer.

And it does not apply to standard nuts. Standard nuts are already sized for the bolt breaking before the nut does (around 0.8 to 0.9 x d for moddern ASME and ISO hex nuts). So don't stack up thin nuts to pretend like there is more engagement.

It's a rule of thumb chart for statically loaded joints. If the joint is important, safety related or subject to fatigue loading, make the FED-STD-H28/2B or VDI 2230 calculation using actual material properties rather than a factor.

The first column shows your thread size and the next one give you the coarse pitch. The engagement length you actually need sits under the material columns, read across to the material your hole is tapped in.

The pitch column is the ISO 261 pitch (millimetre distance between each thread crest) for coarse thread; this is the default pitch for any given size.

If there’s no pitch specified, it's a coarse pitch, like in a callout of M10 or whatever. These last five columns is just the first table’s rule of thumb multipliers times the nominal diameter.

For example, a tapped hole in medium strength steel is 1.0 x d. Grey iron and copper alloys are 1.5 x d. Common wrought alloys like 6061-T6 are 2.0 x d. Aluminum castings and pure aluminum and other soft metals is 2.5 x d. Unreinforced thermoplastics is 3.0 x d. All the numbers are millimetres of actual full thread that will be in contact.

The diameter divided by the course pitch. How many turns around will the thread buy you. This number go up as the size gets bigger. It's how per diamater works.

In industry there is a sanity check that a good joint doesn't depend on less than roughly 5 engaged thread. The rules work because the number of threads grows with size. This is also where your fine pitch to coarse pitch conversion is used in this column.

Fine thread don't change engagement length from the rule. It is a function of diameter and material, not pitch. So basically each fine thread are shallower. Fine threads strip easier in soft materials, so coarse threads is the default choice when tapping things like aluminum, cast iron and plastic.

So I get two common errors on that chart. One is thinking it’s a length for a screw. Nope. That’s how far into whatever gets clamped that you are engaged. Long enough to come out the other side and leave this much buried in what you have tapped.

Second error is giving yourself credit for bad threads. Blind holes can give you 1.5 to 3 pitches of partial thread at the bottom. Countersink or chamfering the entry take off contact at the top. So you’re measuring from the first full to the last full threads engaged.

In reality, a good way to drill a blind hole is to set the thread depth as the engagement plus about 4 pitches. Drill depth is thread depth + drill point (roughly 0.3 x drill diameter for a 118 degree point). Lastly you should of make sure there’s no way the screw bottoms out.

Beyond M39, it's mostly machines, fixtures and structurally important stuff where values matter. Anything else beyond that size tend to have tapped joints only in steel.

The table does show cast iron up to plastic columns for completeness but really, you don't tap 3.0 x d into plastic at M64 (that is 192 mm of hole) unless you want to send a message to re-design using inserts or through bolts instead!

Below M3, it's mostly about tap chamfer allowance. On an M2 tap, two pitches lost is 0.8mm out of a target of 2. So miniature work has thread depth specified generously.

The above table is for a typical bolt of class 8.8 strength (or equivalent) threaded to standard 6H/6g fit. Move one column to the right of the material column when using 10.9 and 12.9 bolts on anything less hard than hardened steel, because the harder bolt increase the load on the internal threads.

It doesn’t apply to nuts as they are already standardised for height. It does not apply to thread-forming screws either, their makers publish their own boss and hole data. And skip it for holes with poorly cut, oversized, or worn-out threads, as there is no actual contact anywhere near what might be theoreticaly expected.

The Size column contain number sizes starting at #0 up to #12 plus the fractional size and decimal diameter from 1/4 in to 4 in (example: 3/8 or .375). It includes all those fractional and decimal numbers. It also serves as a series lookup, since the next two columns contains the threads per inch for the UNC TPI and UNF TPI for that diameter. That means that 1/4-20 is UNC and 1/4-28 is UNF. These use the same multipliers as the metric table.

The multipliers is 1.0 x d, 1.5 x d, 2.0 x d, 2.5 x d, and 3.0 x d. The values is rounded to three decimal places. The same multipliers used in the metric table are applied to the decimal diameter.

The formula: Multiply the diameter by the UNC TPI (the number of turns). The result is 1.0 x d, or the number of complete turns within a depth of one diameter in steel. For fine threads, this also converts the length rule to a turns rule; therefore, a 3/8-24 UNF fastener gets 9 threads in the same 0.375 in that a 3/8-16 gets 6 threads.

Since each UNF thread is shallower then an equivalent UNC thread, use UNC when tapping plastic, cast iron or aluminum. Save UNF for steel parts, where the finer adjustment plus higher tensile area will pay off.

There is no such thing as a #0 UNC thread, so the row (#0) in the UNC columns is blank. Above 1-1/2 in the UNF series they just end, thus the dash in the corresponding cells.

Know your thread sizes; for example, #10 (0.190 in) is nearly 3/16 in (0.1875 in) but it is not the same thread. Same story with #12 (0.216 in), which is nearly 7/32 in. Metric threads like an M5 (0.197 in) will start in a #10-32 hole and then strip it. These are all engagements assuming a class 2A/2B fit between them and the right series.

Note that these are engaged lengths, not screw lengths. Add up the thicknesses of all washers and other clamped parts. Then if it’s a blind hole, you’ll have to add the tap chamfer allowance of 2 to 4 pitches. That’s a full inch of extra depth at 4 TPI, which is why big tapped holes is generally specified through.

Bolt grade shifts the picture exactly as in metric. So a 1.0 x d column with medium strength steel (steel column) is using a Grade 5 bolt according to SAE. Step up one column to the right if you’re using Grade 8 in mild steel or any bolt into an unknown material type.

Connections such as structural ones like A325/F3125 and similar is another world entirely. Those aren’t threaded into a hole. Instead, a nut is used, and the engagement depends on the bolt end being at least flush with the nut face. It isn't based off this table.

Not all screw types fits this chart. Examples include sheet metal screws and wood screws. It also does not include certain tapered threads, like pipe threads that seal on a taper, spark plugs, or other threads sealed with gaskets.

Series like 8UN and 12UN (the constant pitch ones) and sizes greater than 4 in follow the same multiplier logic. However, check FED-STD-H28/2B for the shear area calculation, as they are outside the UNC/UNF charts in ASME B1.1.

The table provides examples for depth or thread engagement, which measures how much of the full thread height an internal thread actualy has. That number is determined by your choice of tap drill prior to tapping. The depth is expressed as the percent of full thread and represents amount of space used compared to the total possible.

The following section explains major diameter offsets. The offset refers to how far down the thread you need to place the drill. Again it's measured in pitches, but this time in multiples of the pitch itself. So by knowing your offset you can simply deduct from your major diameter to get the drill size. The two example columns works this out for an M10 x 1.5 metric coarse thread and a 1/4-20 UNC thread.

Typical use Column: This column says where each percentage of thread height is considered normal practice.

With inch threads, the pitch is 1 divided by TPI, so the offsets is all the same.

Strength is a result of multiplying the two engagements. A strong joint occurs when you have tapped the hole at 75 percent thread and then filled it up to the minimum engagement length of the material being joined.

The 75 percent line duplicates published tap drill charts. On a 1/4-20 thread, it stops on 0.2013 in, or the #7 drill. If you want to use the shortcut that most people know (drill major diameter minus pitch), it’s in the 77 percent row. That results in 8.5 mm for the M10 x 1.5 thread. It’s what your tap drill chart is already assuming. The length tables above assumes about 75 percent thread in the hole.

You don’t get much strength in the final quarter of thread height. Shear across the engagement cylinder cause internal thread failure. Adding a couple percent of joint strength at greater than about 65 to 75 percent thread height is pure vanity.

Typical handbooks suggest single digit improvements from 75 to 100 percent. Driving the tap requires steeply increasing torque. This also increases the risk of breaking the tap. Breaking a tap on a finished part generaly ruins the whole part.

Therefore, increasing beyond 75 to 83 percent are rarely the solution for a weak joint. Make the joint stronger or increase the engagement length. Don’t reduce the tap drill.

With hard material it's always the reverse. Tapping into stainless, titanium and even tough steels is much easier with 50 to 65 percent thread. The loss of strength is roughly less than 10 percent with enough engagement length.

Don't let this one get you; there are some pitfalls. This applies to conventional cut taps on 60 degree Unified or ISO threads. For roll form taps, the tool doesn't cut the metal but displaces it instead. Use the tap maker's drill chart for these, as they uses bigger drills to achieve the same percentage.

Below 50 percent, the formula fails. The threads simply aren't deep enough to be trusted at that point. It doesn't apply to sheet metal extrusions or Acme, buttress, and tapered pipe threads either.

There’s one caveat to how we name things here, both for drawing readers and buyers: If you see 75 percent thread engagement on a drawing, that refers to the height of the threads on this table. For example, if it says 1.5 x d engagement, then that's referring to the length of the tables above.

If someone mentions engagement in a forum thread or from a supplier, ask them whether they mean the height or the length before you cut any metal. You should of checked the dimension first.