Lego Technic (/ Expert Builder) bricks have always had a very different design than the normal Lego brick, with peg holes lining the sides serving fairly obvious purposes, but they also have had (as long as I can remember) hollow studs on the top:

1x4 grey Technic brick viewed from the side

Now that these pieces are appearing in non-Technic sets I've seen them used often to hold things - they're about the same size as a minifigure's grip, so wrenches, guns, gems, and other decorative elements that need to stand are often stuck into them.

But Technic has no minifigures and features only sparse decoration. I don't recall ever seeing a Technic set that uses the holes. It seems like they would be much more difficult to mold especially as far back as Technic's introduction in the late 1970s.

So why were they introduced in Technic? (Or am I wrong, and they were not introduced in Technic / Expert Builder?) Did they serve a functional purpose, or were they just another visual marker to separate it from "regular" Lego during its introduction?

8 Answers 8


From principles of molding, you want a uniform part thickness throughout if possible. This facilitates plastic flow as well as dimensional stability (you want uniform shrinkage on cooling.) Removing a divot from the underside of a stud serves this purpose in a regular brick. The Technic brick with through holes would have had a large amount of solid plastic under the stud compared to regular bricks, which could create undesirable shrinkage and warping from stress in cooling. So removing the top divot helps there.

However, if you examine a 1x16 beam from Expert Builder, you can observe a plastic channel running along the underside between studs 4 to 13 (which are injection gates; also stud 8, or 9.) The 1x8 doesn't have this. This channel would let the plastic flow away from the injection gate without turning too many corners. It's also possible LEGO used a reciprocal molding action where plastic is pressed in one gate, then another, in a rocking action, to facilitate welding of the multiple injection streams. Solid studs might lessen the effectiveness of this process, but the channel helps it. If the channel was necessary to produce a 1x16 beam back in the day, and the channel forced hollow studs, then the hollow stud style was the only way to make a 1x16 beam. The technical difficulty of making a new, longer brick (beam) may have influenced the look of the whole Expert Builder system.

That's only my theory, but you can still see the channel for yourself.

My tests also indicate the hollow stud separates from another beam more easily than a common solid stud would.

It remains possible though that they were just maximizing the usefulness of the part by making the stud hollow. Though the fun part of being an engineer is maximizing more than one property at once.

So my thought is the hollow stud came from various molding principles. Cost of plastic comes after that (and yes, if you can save a bit, you do.)

Another interesting trivia: the Technic studs were each freehand drafted in the early Expert Builder instruction booklets. I joked about reproducing this while showing some Lego employees (in 2001) what could be done with non-representational rendering, for instance a pen-and-ink renderer with random jitter.

  • True, molding complexity does influence cost more than quantity of plastic.
    – Joubarc
    Commented Oct 28, 2011 at 6:43

I was very intrigued by this question. So I messed around with two LEGO Technic beams, a long one and a short one. The 'pins' at the bottom of the beams are made to fit in the hole on top of the studs.

enter image description here

When using the studs holes instead of the normal 'in between' way, the holes on the side of the beams are offset at mid distance. This could be very useful working with gears (I'll remember that one next time I have a complex gears ratio to create).

enter image description here

In the end the rules are:

  1. The bottom beam must be shorter than the top beam.
  2. The bottom beam cannot extend past the top beam ends.

The following picture shows why rule number 2 is important. The end of the top beam and the studs of the bottom beam interfere.

enter image description here

  • 1
    My question is about the holes in the studs, not the holes in the sides of the bricks and on beams.
    – user23
    Commented Oct 26, 2011 at 13:14
  • I tried stuff with the beams and changed my super bad answer for this one
    – pcantin
    Commented Nov 8, 2011 at 1:17

In my opinion, you don't need to look further than plastic quantity, and thus cost.

Take a normal brick or plate (easier) and look inside it. You'll notice that each stud is hollow, but from below. Why? Simply because there's no need to have it full with material as it serves no purpose. It's easy to make the mould that way, it doesn't remove any functionality of the brick, everybody is happy.

Now take a Technic beam and look at it from under. The Technic holes make it way more difficult to have a mould which has cavities under the studs. So the easy solution is to have the cavity in the stud itself, which has the added benefit of adding functionality to the stud itself. Also, I suspect that the sturdiness of a Technic beam is better if the upper side is full.

Note it's just speculation; but most of the times you need to analyze a part with the eyes of a LEGO producer (use as less plastic as possible), and of a designer (have a part which fits with as many other parts as possible). Nowadays, it's likely they would use hollow studs more and more, and I wouldn't be surprised if they end up on regular bricks some day — but that's going to take a while as there is a third factor in play: brand recognition. regular LEGO bricks have full studs, with the LEGO engraving, and that's not seomthing which LEGO will easily accept to replace.

  • Curiously, plastics texts suggest placing a logo to hide the injection gate, which is typically on a stud. LEGO does this (look for one or more logos with a blob in the center), but I have no idea if they pioneered it or just jumped on board. It is certainly part of their trade dress now.
    – Erik Olson
    Commented Oct 28, 2011 at 5:02
  • 2
    Oops, the gate was on a corner of the brick in the 1960s when it was made of cellulose acetate. So the stud logo came before the stud gate.
    – Erik Olson
    Commented Oct 28, 2011 at 5:52

Most 1×2 (and longer) plates and bricks have a pin at the bottom right in the middle of studs that perfectly fits this hole.

Plate 1×2

Connecting this way gives you half-stud shift.


After getting a great answer to my question: When did Lego decide that it was okay to put a stud into a technic hole? and looking at the pdf slides linked there: http://bramlambrecht.com/tmp/jamieberard-brickstress-bf06.pdf, it's stated in one of the slides that the words "LEGO" on the top of the brick adds a little height, .14mm to be exact, to the brick. This extra height interferes with the Lego dimensions for Technic elements. So the hole'd studs was likely a way to still have the stud, but shave off that top .14mm.


In answer to part of your question, Expert Builder sets were not the first appearance of the hollow stud. I believe the old-style windows were the first, starting in the mid 60s:

enter image description here

The reason for the hollow studs here seems to be related to the exposed underside of the stud face. LEGO seems to have wanted to have the inner frame of the window smooth rather than have the divots that regular studs would create. So they shifted the divots to the upper surface where they would be hidden in most uses.

There is another functional reason some modern pieces have hollow studs: child safety. Around the same time the Expert Builder sets were introduced, LEGO started using hollow studs on small cylindrical pieces like 1x1 round bricks and cones that went clear through the brick. This was to provide an air hole should one (or more) of them be stuffed in a nostril or ear canal, or be swallowed or inhaled. In some cases, like with 2x2 round plates and bricks, LEGO uses a through axle hole for the same purpose.


The 1x2 plate with 1 knob (Design ID 3794) has the same design for it's top stud - I assume for the same reason Joubarc gives - you can't create a hollow stud from underneath as the peg's in the way.

1x2 W. 1 Knob 3794

  • Sorry, this is a correct point, but -1 because it's not really an answer to anything - it would be better merged into someone else's answer as another example of the same design principle.
    – user23
    Commented Oct 27, 2011 at 0:19
  • @JoeWreschnig Possibly. However, it does answer the part that in the question that says "(Or am I wrong, and they were not introduced in Technic / Expert Builder?)" - all of these that I have from my youth are like this, but I can't prove which came first. Commented Oct 27, 2011 at 6:13
  • 1
    The jumper brick was a contemporary of Expert Builder, and space sets used the hole by plugging a torch into it.
    – Erik Olson
    Commented Oct 28, 2011 at 5:07

Hollow studs fit the bars on the underside of 1×x bricks. The same hollow studs are found on Lego windows and Minitalia pieces (Lego made in Italy), also 1×1 with holder and 1×2 tile with 1 stud, 1×2×5, 1×2×3 slope.

Other brands, such as Montini and Tente have hollow studs and unknown on 1×1 plate. So maybe should ask 'why aren't studs hollow on all Lego pieces?' So Technic beams were not the first.

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