Right - a quick update on this.
Rebuild kit for my damaged gear diff arrived - the kit had 2 large shims and 4 small for the internal gears. Built it up exactly as per the instructions and it's cack - notchy and really not nice at all. I tried to squeeze another really thin shim in on the outdrive gear but there just wasn't room to get the drive pin in as well. I've now got 2 gear diffs installed in the Centro and SC10 that I don't 100% trust, and a spare in the box that feels like it will make a good paperweight
So I think it's time to throw in the towel and revert to ball diffs in the truck and Centro. I have enough B4 diffs laying around but obviously the B4 was Imperial and everything since went Metric.
Can I get away with just buying new SC10/B4.1 outdrives for the B4 diffs rather than buying complete new diffs? Will the drive rings and thriust race be a straight swap? I know I need the Metric outdrives as the SC10 and Centro use the 10x16mm bearings.
Thanks for any advice!