1967brutus Thanks for your comments
When it starts to slow down and I stop it there is certainly a lot of drag but quickly disappears.
I have put shrouds against the fins to force air past cooling fins but still starts to bind up. Yes you are right the tests are all static as yet.
I was wondering if it could be the bearings, not enough clearance check if (C3)
I have two ASP 180 fs running perfect on gas no problems, thermisters on the heads with telemetry to monitor temp, these convert really good and also have bronze guides and seats from factory..
I did convert an OS200FS in the past but it didn't behave like this but the exhaust valve ate its way through the alloy head and that was that.