The rod/crank tolerance is about as critical as it gets in an internal combustion engine.
A couple of thousandths would all it would take I imagine.
Or, like you said, substandard materials.
A couple of thousandths is typical total rod bearing clearance on a sbc, so that would definitely be critical!
I’ve built a bunch of racing engines, drag raced for 20 years or so and now have an oval track car.
When talking about oil viscosity, the critical thing is bearing clearance, lack of volume of oil will be worse than oil pressure. You adjust your oil use to clearance, the one I just put together this year was slightly tighter than I really like, so I’m running 10W30 racing full synthetic. You have to have a sufficient volume of oil for cooling the bearing surfaces, or it’s all over. At 10,000 rpm, things need to be just so.
A couple of thousandths is typical total rod bearing clearance on a sbc, so that would definitely be critical!
I’ve built a bunch of racing engines, drag raced for 20 years or so and now have an oval track car.
When talking about oil viscosity, the critical thing is bearing clearance, lack of volume of oil will be worse than oil pressure. You adjust your oil use to clearance, the one I just put together this year was slightly tighter than I really like, so I’m running 10W30 racing full synthetic. You have to have a sufficient volume of oil for cooling the bearing surfaces, or it’s all over. At 10,000 rpm, things need to be just so.