Mines the same as Wrightpaynes's - brass and so is the thread its screwing into - would make sense to be the same material so it has the same expansion properties when it gets hot - I wonder whether that;'s why yours was such a sod to get out. I've also got a a fibre washer as well like SMT25 Mine looks like the link below, although I'm pretty sure last time I had it off, my thread went to the top https://caterhamparts.co.uk/radiators/1800-bleed-screw-brass-radiator.html?search_query=radiator+bleed&results=72 Washer I would guess its this (no image to check against https://caterhamparts.co.uk/fittings/3359-fibre-washer-for-radiator-bleed-plug.html When filling and bleeding I've always got the front of the car as high as possible (my drives on a slope so arse down the drive, chock the back wheels and then front as high as I can get it) and final fill at the radiator Expansion tank reservoir up to top, and then bleed first at heater elbow (I've no heater fitted), then at radiator and then on occasions, refill at radiator again and repeated the same process, effectively getting coolant level in expansion tank down to the required level through the bleeding process - problem I've found doing it any other way you can end up with an air lock at top of the radiator and then risk the fan switch being in the air lock and not kicking the fan in at the right point. No doubt others have other methods which equally work for them, but this is what I do. P.S. mine is a 92 1400 K series supersport, so don't know if its different on other models. I know when I was short of time once, just before the Taffia fish and chip run a few years ago, due to work pressure I had to let a local garage fit new hoses and bleed the system, and they used the standard vacuum pump professional system used on modern cars, and that made a right horlicks of it - fan kick in switch was in an airlock and the engine was running well hot circa 5 degrees above normal in all conditions - fortunately found a welsh farm track on a 1 in 2 or 3 slope and stuck the nose up there and did a roadside bleed good enough to finish the run and get home