EDIT- Megatron trailer mod in this first post, below.
Stumbled on this today, a simple way to let the trailer for the Op Action Set turn. Look at the 3 cylinders underneath the front of the trailer. Cut down the center one as shown. Find yourself a 3mm clip system ball-jointed peg that fits into that cylinder and cut the ball end off. (Can't remember where I got the one I used, sorry.) Verify it will clip onto OP's hitch as shown, trim as necessary. (EDIT- Duh, I had mistakenly said to glue to post into the trailer, but that would ruin the whole thing, now wouldn't it.

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Done correctly, it will let hte trailer sit at the original height and rotate:


Comments please!
EDIT- Megatron and his trailer were a little more involved than OP’s, but it’s done.
You’ll need to install a peg just in front of the C-clip on the underside of the trailer, on that leg that folds out. I used the skinny end of a TF missile, from ROTF Mixmaster, I think; it has a thinner section at the end, which is the part I inserted into the hole I drilled in front of the clip on the trailer part. Work carefully so you don’t break through the side of that slender part. Once you’ve drilled it, just glue the peg in place.

Now look at the clip post on Megatron’s truck chassis. Cut off the post and the sides that hold it, so you’re left with a flat square. Now remove the screw that holds that part onto his leg, but leave the part in place. Drill a hole in the center, slightly larger than the free end of the peg you just installed on the trailer. You’ll also have to cut down the post that fits into that square piece from the opposite leg, as it will interfere with the peg; don’t cut it all the way, just leave a little bit. In my case, it became hard to close the legs up for alt mode after cutting down that post, so I glued the square piece onto the opposite leg. The alt mode still holds together perfectly, nice and tight.


Now just plop the trailer’s new peg into the new hole on the back of the cab section and you’re done. See below.
