Style over function
Join Date: Nov 2010
Location: United Kingdom
County: West Yorkshire
Diveboy, I just got around to doing my "metal bolt" upgrade two weeks ago, and all is well.
If you are leaving the original cast pipe on that had a plastic plug, yes, the thread pitch is M10 X 1.25.
The underside of the bolt I fitted was nice and flat. I wrapped some PTFE tape onto the thread and also fitted the rubber "O" ring that would fit at the top of the plastic plug, under the bolt head. As I previously said, I also (the first time) had to run a tap down my thread to get the remnants of the plastic plug out.
With regard to the plastic plug at the rear, it is of a smaller diameter than the bleed screw at the front. I overtightened mine when replacing it after I'd taken it out to look at and discovered that it is much smaller in diameter, and damaged the threads in the rubber coolant pipe, so beware if you ever play with this. I have rethreaded the coolant pipe and lockwired the new bolt in position (as the threads are a bit suspect)!
Leave the pressure cap on with a cold engine as you do it, and very little if any coolant will escape. Then just as you start to put the bolt in, loosen the cap a bit and then you will get rid of any air ingress.