Building LEGO Technic 42009 Mobile Crane MK II

Something Smaller After the AE86

After finishing the DeAgostini 1/8 Toyota Sprinter Trueno AE86, I wanted something a little smaller and more relaxing for my next build.

Somehow, I ended up with LEGO Technic 42009 Mobile Crane MK II.

Smaller than the AE86, yes. But still 2,606 pieces. 😅

It's been quite a while since I last built a large LEGO Technic set, and this one was released back in 2013. I thought it would be interesting to build an older Technic model again, especially after spending so much time working on the AE86.

Let's see how relaxing this turns out to be.

A Little Background on LEGO Technic 42009

LEGO Technic 42009 Mobile Crane MK II was released in 2013. With 2,606 pieces, it was LEGO Technic's biggest and most complex model at the time.

The crane features 10 wheels on five axles, with 8-wheel steering, a rotating superstructure and a LEGO Power Functions motor that operates several crane mechanisms.

The motorised functions include extending the outriggers, lowering the outrigger jacks, raising and extending the main boom, and operating the winch and hook.

The set also includes a V8 engine with moving pistons.

LEGO retired the set in November 2015, so by the time I started this project, it had already been out of production for more than ten years.

Rather than going through every specification here, I think it's more interesting to see how these mechanisms come together during the actual build.

Starting the Build — Sorting 2,606 Pieces

Instead of following the numbered bags, I decided to open everything and pour all the parts onto my workbench.

Honestly, that was quite a lot of parts. 

I started sorting everything by part type and colour, using separate trays to keep the different pieces organised.

It took quite a while. In fact, most of my first build session was spent sorting rather than assembling anything.

After all that, I finally started building the centre section of the chassis, where the front and rear sections would eventually connect.

Not much progress for the first session, but it was nice to be building LEGO again.

One thing I noticed was how different this felt compared with the AE86. There were no screws or glue involved, and I could simply concentrate on finding the correct pieces and putting them together.

At least, that was how it felt at the beginning. lol.

The Chassis, Steering and All Those Gears

As the chassis started taking shape, more wheel axles and steering linkages were added.

By the second session, I could already see the steering mechanism working. Turning the steering system caused the front and rear steerable wheels to move in opposite directions.

It was quite satisfying to see some movement after spending so much time connecting beams and pins.

However, things became more complicated as additional gears were fitted.

The LEGO instructions show where everything goes, but they don't really explain the purpose of each mechanism. I had to follow the assembly steps carefully and check the movement whenever possible.

A few times, I fitted parts in the wrong positions and only realised the mistakes later. That meant taking some pieces apart and doing them again.

By the fourth session, I was working on the most complicated LEGO Technic assembly I'd built so far.

There were gears connected to more gears, with different axles and linkages running through the chassis. A lot more concentration was needed to make sure everything was fitted correctly.

The outrigger mechanisms were also assembled during this stage.

Both the front and rear outrigger beams are designed to extend symmetrically when their gears are turned. The outrigger jacks are then lowered to provide additional stability for the crane.

At this point, I was quite curious about how well those small jacks would support the completed model.

Another thing I noticed was how tiring it could be to connect so many Technic pins and beams. After several hours, my fingers were actually getting painful.

Still fun, though. Just not quite as effortless as I originally imagined. 😅

The V8 Engine, Driver Cabin and Wheels

After the complicated gear assemblies, the next stage felt much more relaxing.

Most of the internal mechanisms were already in place, so I could start covering the chassis, building the driver cabin and installing the wheels.

I also assembled the V8 engine.

This is one of the little details I really enjoyed. When the crane rolls, the pistons inside the engine move as well.

The steering mechanism was also working, controlled by a knob at the rear of the crane.

With all ten wheels installed, the model was finally starting to look like a proper heavy mobile crane rather than a collection of gears and beams.

It was a nice change of pace after the previous sessions.

The Counterweight, Hoist Unit and Operator Cabin

I originally thought I could complete the counterweight and operator cabin in one build session.

I was wrong. lol.

The counterweight section turned out to be another complicated assembly, with so many gears packed into a relatively small space.

I eventually lost count of how many gears I'd installed. Definitely more than twenty.

After fitting the Power Functions motor, I performed a quick test to make sure the connected mechanisms were moving correctly.

Everything seemed fine at that stage.

I continued with the hoist unit, operator cabin and exterior covers in the following session.

This part was much more straightforward, as most of the complicated gear assemblies had already been completed.

Once the counterweight and operator cabin were finished, I fitted the entire upper assembly onto the crane chassis.

The model was getting quite large now, and it was finally possible to see how the upper and lower sections came together.

However, there was still one mechanism that needed attention.

The Outriggers — The Most Troublesome Part of My Build

The outriggers were probably the most troublesome part of my entire 42009 build.

During testing, I discovered that the front and rear outrigger jacks were operating in opposite directions.

When one end was lowering, the other was raising.

Obviously, that wasn't right.

I went back through the instructions and checked the gear arrangements until I managed to correct the direction of movement.

But after solving that problem, another issue appeared.

The motor started turning the gears, but it struggled and stopped after a short moment when operating the outrigger jacks.

The batteries were new, so I suspected that the problem was related to resistance somewhere in the mechanism.

There is quite a long gear train involved in operating the jacks, and resistance at different points can add up.

I considered several possible causes, including friction at the moving linkages and the positioning of the gears. Rather than replacing parts immediately, I decided to revisit the gear assemblies and check their movement.

Eventually, I managed to resolve the friction problem after going back through the gears.

I can't point to one particular component as the confirmed cause, so I won't claim there was a single faulty part.

What I learned from this was the importance of testing the mechanisms during assembly, especially before covering them with more Technic beams and panels.

Something may appear to be assembled correctly, but that doesn't necessarily mean the entire mechanism will operate smoothly once everything is connected.

Thankfully, the outriggers were eventually working properly.

Final Assembly — Main Boom and Hook Block

The final build session was mainly about assembling the main boom and hook block.

Compared with the earlier stages, these assemblies were quite straightforward. There weren't as many gears to deal with, apart from the mechanisms connecting the boom to the upper assembly.

Once everything was fitted, I tested the motorised boom.

The main boom could be raised and extended, and both functions appeared to be working properly.

And with that, LEGO Technic 42009 Mobile Crane MK II was finally assembled after eight build sessions!

Completed Model and Function Testing

After completing the assembly, I spent another session photographing the model and testing its functions.

I started with the smaller details, including the opening doors, steering movement and V8 engine with moving pistons.

Then I tested the motorised mechanisms.

The outrigger beams extended and retracted, the jacks raised and lowered, and the main boom could be lifted and extended. The hoist and hook mechanisms were working as well.

It was particularly satisfying to see the outrigger jacks operating correctly after the problems I'd experienced earlier.

Everything was working properly by the end of the project. 

The completed crane is quite a substantial model, especially with the main boom fully extended.

Looking at the finished model, it's interesting to think about how all those separate gears, axles, beams and linkages eventually became one working crane.

For anyone interested in seeing the mechanisms in motion, I also recorded a short video of the completed model and its functions.

LEGO Technic 42009 — Completed Build & Function Test

Watch the completed build video on YouTube

Final Thoughts — Eight Build Sessions Later

When I chose LEGO 42009 after the AE86, I wanted something smaller and more relaxing.

Was it relaxing?

Yes, definitely more relaxing than the AE86. 😁

But that doesn't mean it was a simple build.

Some sections, particularly the chassis and counterweight, required quite a lot of concentration. The number of interconnected gears was greater than I'd expected, and the outrigger mechanism gave me some trouble before I managed to get everything working properly.

The difficulty also wasn't consistent throughout the project. Some sessions involved complicated mechanical assemblies, while others were mainly about fitting body panels, cabins and larger structural components.

That variation actually made the building process quite enjoyable.

One thing I particularly liked about LEGO Technic was seeing the mechanisms come together. Sometimes a section didn't look like much during assembly, but once more components were connected, it became a working part of the model.

Overall, LEGO Technic 42009 Mobile Crane MK II was a very enjoyable build, even though it required more focus than I originally expected.

Eight build sessions, 2,606 pieces, quite a lot of gears, and a few moments of taking things apart to put them right again. lol.

Build Complete.

Ah Chong — DeToyz