How to check the bottom end bearing clearance?

To check the bottom end bearing clearance.
• Turn the engine to BDC.
• Isolate the turning gear.
• Measure the clearance using feelers from within the crankcase.
The clearance is measured on the bottom half of the bearing
because the mass of the running gear acting downwards means
that the top half of the bearing is sitting on the journal. Check your manual for acceptable clearance range. On a
large engine this clearance can be 0.8 mm

In some small engines shells is incorporated into the bottom ends. When the clearance has reached a maximum as stated in the instruction manual the bearing is changed.
Thick wall shell bearings fitted on some engines have the clearance adjusted by fitting shims between the bearing halves. The shims
 are of equal thickness on both sides of the bearing housing.