...also, and be reminded of the burnt exhaust valve on that same engine...I remember some questioning whether a crushed header 80% closed would do that...but then that was the cylinder in question...
If it is a spun crank bearing, it all goes back to a lubrication failure. All these other issues may be contributing, but that has to be the bottom line.
Plane bearings like on the rods and crank depend on a film of oil to carry the load. Modern plane bearings are made to be tolerant of dirt particles by allowing them to be embedded into the bearing. They are also 'self healing' to a point should they ever bottom out.
If you go back to the bad old days in the 80's when emission controls were grafted onto engines that were not intended for emissions some bad things would happen. Engines were set to run very lean (in place of efficient) which created hot running engines with hot spots in the combustion chamber which led to knocking and detonation. Each time a cylinder experienced an uncontrolled flame front (ping/knock/detonation) the piston experienced a sledge hammer blow which was transferred to the rod bearing on the crank and also shocked the crank. Under these conditions the hammering could thin the oil film on the plane bearings creating damage. The rods and pistons themselves could also be damaged. It was not unusual for the mid displacement engines to develop 'rod knock' or 'crank knock' upon first start-up due to knock/detonation pressure pulses in the combustion chamber.
Patriot mentions that his header was dented and had significant narrowing. [speculation] At higher RPMs only, the exhaust
might create pressure pulses and would not shed heat from the header the way it should. The pressure pulses would hammer on the rod & crank bearings as mild knock/detonation might. It could be that the #3 cylinder lost a long war of attrition as it got beat up by back pressure from the exhaust.[/speculation]
But, what about the small balancing passage that couples Cyl #2 & Cyl #3 together in the header? Wouldn't that have help alleviate the pressure pulses that some idiot speculated about? I dunno. It sure is suspicious that the cylinder that had the occluded pipe also had burned valves and then a spun bearing. I guess someone could put that header on their engine and see if it kills their valves and rod bearings
BTW, Patriot where is that dented header these days?