Mechbox wont cycle over????
 

OK so here is the deal

I took apart one of my MP5's to add/fix the following

nozzle, tappet plate, spring and piston head O ring

After I put everything back together it would turn over then jam up with the piston locked back.

I took it apart again (and again, and again), reshimmed it but it continues to do the same thing. Even tried putting the old spring back in thinking that might be in.

At this point I am at a bit of a loss as to what it might be so I am turning to the experts here for advice on what I should be looking at to solve this issue.

Thanks

did you try a different battery?

yes I have tried 4 different batteries

3 8.4v and 1 9.6v

Did you try putting the mechbox back together without the piston, tapit plate just to see if the gears turn smoothly. After it's together without the piston, spring and tapit plate, put your finger in the hole where the piston is supposed to be and turn the gears to see if your shims are too tight.

Of course I could be way off. I apologize if I am.

if you added a bearing spring guide and a bearing piston head try removing one of the bearings i had this happen to me as well after an upgrade turns out there isnt enuff space for two bearings and will jam in the fully compressed position.

@ FACE: yea I reshimmed it and made sure everything spun freely before I installed everything else and put it back together (have pulled it apart and put it back together 4 times now with same results)

@Spawn28: Spring guide on the spring guide but not the piston head as its still stock. Just added a new O ring to it to get the compression back.

I am starting to think it might be the plastic bushings as they are old and maybe they are being pulled out of shape after the tension from the spring and everything once back together

Is the piston lined up on both tracks/ledges of the gearbox when put together? This happened to me and ended up being my probablem, maybe its that.

There are your problems.

Change the piston head to a bearing one. The stock head uses a large (1/2") metal spacer as a nut to hold the head in place. An upgraded piston head with bearing is about half that.

When you upgrade any gun, the VERY FIRST part to be changed is the nylon bushings. You can't shim a gun properly with theses. And if your gun shot more than about 10 000 BBs, the gear axles won't be aligned properly anymore. You can't do any kind of decent shim job in that case.

When testing the shimming, did you screw down the mechbox?
If not, it means nothing (shim job isn't necessarily OK).