Vanenkelen

Missed your reaction, sorry... You might be right about me disassembling the engine that much. I Just don't remember, although I like to take things apart as much as I can . But, I found another reason/solution. Just flip the black metal housing, which holds the magnets and surrounds the coil and shaft, up side down (180 degrees). This does the trick, the engine will run the other way around. Simple as that.

Although this topic is already 9 months old, it was for me the reason to subscribe to Eurobricks forum. I ran in to the same problem as @Reza. I had 2 defective 12 volt engines, 1 with a worn gear on the wheel shaft and 1 with a crack in the housing. Both were still running but with a lot of noise and very slow. I opened up both engines a couple of months ago to see what was needed to be replaced and I bought two defective engines on a dutch second hand website (marktplaats). Last week I finally started my repair project. After replacing the defective parts I glued them back together and to my surprise they both run in to the opposite direction of my other engines. Huh... what... I really didn't understand how this was possible. I did not take any pictures before (or during) disassembly (as suggested above), because to me it seemed pretty straight forward. The wormwheel on the engine shaft and the gears on the wheel shaft can not be assembled in a wrong manner. The metal structures also fit in only one way... So I started my search on internet and run into this topic. I signed up to ask if any progress has been made by the topic starter. But in the mean time (it takes some time before your account is accepted) I found out what went wrong and I thought it would be nice to share it here. It is possible to take the worm-wheel shaft (with the coil) out of the magnetic housing of the engine. If you put it back in the other way around, the engine runs the other way. I thought I didn't disassemble the engine this much, but probably I did. With the worm-wheel shaft in the other direction the engines now work like new (and in the right direction). I hope this will be of help to the topic starter or anyone else who faces this problem.