extra clearance

why would someone want an extra .001 for oil for example

If we are talking for big end and mains probably aiming for higher revs?

yeah big and mains am sure as you say its for some performance issue more power giving a little more clearance just wanted to know for sure

This is discussed in the course, however to reiterate, at higher power and torque levels the engine components tend to flex and this can result in metal to metal contact if your clearances are tight. It's relatively typical top build a high performance engine with a slightly looser clearance to prevent this and then use a heaver weight oil.

thanks Andre for clarifying i do remember from the course but could not quite remember the effects so that's understood so what would the opposite effect be on say loose clearances, thin oil and not using the engine as a high performance loose clearances when no need to be i assume low oil pressure for one thing? and any other down sides?