25-09-2020, 08:58 PM
Ivor has alluded to it, but no one has said it.
In a non pressure fed engine like a seven, a bigger working clearance in a big end is not only acceptable, its a benefit! One of the functions of the oil is to cool the bearing, as well as lubricating it, this is a function of flow, which is increased with a greater clearance. I would suggest that the "falls under its own weight" test is the very tightest a splash fed A7 big end should ever be, a little looser will not hurt. Remember, the oil is arriving at the big end through centrifugal force, the oil needs to escape to allow new cooler oil in. Tight big ends will over heat and run, due to excess friction, and lack of cooling. signs of squidgeing white metal show that it was very close to doing so!
Pressure fed engines are different, very little of the above applies to these.
In a non pressure fed engine like a seven, a bigger working clearance in a big end is not only acceptable, its a benefit! One of the functions of the oil is to cool the bearing, as well as lubricating it, this is a function of flow, which is increased with a greater clearance. I would suggest that the "falls under its own weight" test is the very tightest a splash fed A7 big end should ever be, a little looser will not hurt. Remember, the oil is arriving at the big end through centrifugal force, the oil needs to escape to allow new cooler oil in. Tight big ends will over heat and run, due to excess friction, and lack of cooling. signs of squidgeing white metal show that it was very close to doing so!
Pressure fed engines are different, very little of the above applies to these.