Typically what are you gaining on a two stroke with engine braking... It's not like a four stroke. When down shifting I always blip the throttle, to both rev match and maybe give it a squirt of oil. I certainly wouldn't be going down a big hill engine braking the whole way at high rpm on a two stroke. Nice blips on the throttle and a high gear to keep the RPMs down. When it comes to competition bikes, well reliability is all out the question and if engine braking does even a fraction of slowing down then it's worth it, but for road going use I only engine brake in an emergency.

@Calum I think it's something like that yes, that part of the crankcase is just an empty space.

@mbmO8 Head’s still leaking bud. It’s probably best to replace the base/barrel gasket as well. Myself, I remove the studs from the barrel an lap the top in with a square of plate glass and some 240-ish wet and dry sand paper. Then do the same with the cylinder head. Just to remove the crap. Doing up the head nuts really needs a little 1/4 in drive torque wrench to get it right. I do it in three stages… I know that it’s no consolation but I’ve just had to put a newly rebored barrel and head on and had to go through all the fuxin hassle of it myself 🫤 …

I crawled around the bike today trying to locate where the clatter came from, paying extra attention to the powervalve cap. Holding a screwdriver against various covers to feel vibrations. I am far from certain, but I think it is the water pump, actually. I might disassemble that and have a look later this week. Thank you both for the input!