Managed to get hold of the bike, and although im going to need to confirm tomorrow morning im pretty sure ive got to the bottom of it, and as usual its pretty much what tester said.
I put wheel in with the 42c tyre, and as @Saffronspokes had the displeasure of discovering it was indeed very much in contact with the frame. Next step was to check alignment of rear end on surface plate using BB face as datum, everything ok. I then put dummy wheel in with nominal 650bx42 tyre, and everything was ok there too. Next step was to put new stock wheel without tyre in, which was centred. Ive attached photos of the dummy wheel on surface plate for people as dull as me who find this stuff interesting.
After waving a vernier at the wtb tyre, it appeared to be not only width wise oversized, but also very tall, even without taking knobbles into consideration. This causes us issues as after the tyre the stays need to bend in very aggressively to clear the road q factor cranks/road chain line, so if the tyre goes beyond where we expected it to be, it very quickly gets boxed in by chainstays darting inwards, away from the cranks.
On their own these issues wouldn't be enough to cause any grief, but together they compound very quickly to what sam bumped up against this weekend.
Nomimal clearance and real world clearance often dont have a lot in common, just compare the nominal 42c tyre guide in the picture attached versus Sam's photos and you'll see what i mean. He was sold a bike that said it cleared a 42c tyre, bought a 42c tyre and somehow found himself with a new permanent braking system instead of fashionable mixed terrain rubber attached to a boutique artisan frame.
We have actually put a whole section on our (soon to go live)new website about this exact problem, and are looking at ways of mechanically squeezing a little more room if we can, but while narrow cranks, short stays and fat tyres are being used its always going to be a squeeze.
Once im sure that this is the issue, im going to increase dimple size to match these tyres, and going forward im going to change dimpling process to move the dimple centre slightly further up to avoid this from happening in the future.
Im also going to post some parts of a (unused) check list to show how many times we check rear end alignment throughout the build process. This isn't for any defensive reasons, but solely so that everyone who had clearance issues with the protoype frames knows we took that incredibly seriously and put a lot of checks in place to avoid it happening again.
@Saffronspokes ive dropped a message on email, and will be sending proper report tomorrow, thanks again for spotting this. We sell a lot of frames abroad now so incredibly grateful this didnt happen in Malaysia! 😂