How do the gears bind? Oval gears should cause binding in a part of the gears' rotation. If it binds throughout the entire rotation your gears are binding on the casing.
I had a lot of issues with this myself, perhaps they're worth checking? They ranged from:
- A slightly too long layshaft spacer. The binding was solved with 0.1mm shims between the gear casing and the extension piece (312057-3, 310459-3 on Type B casing) that holds the larger of the two layshaft bearings.
- Too long screws for the motor plate. This caused them to extend outside of the motor plate, which caused them to hit the slipper plate. It was frustating and hard to find, but easily solved with an extra 0.5mm shim/ring just behind the screw head (so they don't extend as far).
- Too wide gear diff casings. The solution here is definetely
NOT to tighten the casings further, they are extremely sensitive to overtightening! Instead, Dremel/shave off some material where the bearing sits against and shim away the slop you might have created by excess Dremel work. You can also contact Durango for replacement parts, but my replacement parts actually had a worse fit than the originals I had!
- Too large bearing seat on ball diff outdrive. This caused the bearing not to slide completely onto the outdrive (against the vertical face), which in term caused it to be too wide for the gear casings. When the bearing was pressed further onto it, it cracked. If this is the case, contact Durango: these parts should fit, and imo at 20GBP it's even more important that Durango cover this for you!
In addition, watch out for the following things:
- That you haven't overtightened the casings: They're quite sensitive to overtightening.
- That you haven't stretched the casings because gearbox parts were too wide.