There are 3 Chevrolet LS pilot bearings with different OUTSIDE dimensions. This guide will help determine the size for your engine. 

When using the TREMEC aluminum bellhousing or the Quicktime Chevy LS1-LS2-LS3-LS6 bell (RM-8020) with the 5.555" height, you will be using the "inner" pilot which is marked as "A" in the diagram above.

Pilot bearing “A” is the smallest outside diameter 1.094" outer diameter bearing that fits into the innermost pocket of the crankshaft flange. This bearing is typically with T56 and T56 magnum transmission combinations. NOTE: if using the small GM pilot bearing, the internal O-ring seal faces the transmission when installed properly. You can get it at local Autozone or Napa - Dorman part# 690-057 or ACDelco/GM 14061685

If using the Quicktime Small Block / Big Block bell housing (RM-6023) with the 5.950" height you will use the Pilot bearing "B".

Pilot bearing "B" is a 1.705" outer diameter bearing that fits into the outer pocket of the crankshaft flange. This bearing is used with Transmission/Bell housing combination from a 2003 to 2012 GM vehicle. NOTE: This is a double sealed bearing and it may be installed either direction into the crankshaft. You can find this local at NAPA / Autozone part #12557583

IMPORTANT NOTE: many GM LS engines use a press-in oil seal plug deep inside the crankshaft flange. DO NOT move or disturb this plug when removing or installing a pilot bearing. Your old pilot bearing must be removed by means of a mechanical pilot bearing puller. DO NOT attempt to use a “hydraulic” (grease or bread) method to push-out or remove your old pilot bearing! 

The best method to determine the correct pilot bearing position for your application is by using a straight edge or yard stick across the front of your transmission Bell housing and measure the distance that your transmission input shaft protrudes past the front of the bell housing. 

If your transmission input shaft dimension is between 3/4 to 1 inch (19mm to 25.4mm) use the SMALL diameter pilot bearing in crankshaft position "A".

If your transmission input shaft dimension is between 1/4 to 3/8 inch (6.3mm to 9.5mm) test fit the MEDIUM outside diameter pilot bearing into crankshaft position "B". The pilot bearing should be a slight press fit into the crankshaft. If the MEDIUM diameter bearing is a loose fit, please use the LARGE outside diameter bearing for a slight press fit into the crankshaft.

Be sure to install your pilot bearing into the crankshaft by driving or pressing on the outermost part of the bearing ONLY. The pilot bearing must be straight and fully seated into the crankshaft. DO NOT FORCE the installation. Once installed, the inner portion of the bearing must spin freely and smoothly.