Here at McLeod Racing we field tech and application fitment questions from all walks of life. Some call in because the information on the internet is confusing, and we hope with this tech article to clarify the confusion.
 
There are four main factors to consider when determining the correct SBC or BBC flywheel for your application. Be it a new crate engine, or an old 350 SBC you’ve pulled out of the barn to rebuild for a project.

• Ring gear tooth count: Do you need a 153 or 168 tooth version?
• The crank/block style: Does your block use a one- or two-piece rear main seal?
• Large Crank Register or Small Crank Register: Is the bolt circle on your crank 3.580” or 3.00”?
• Internal or external balance: Are you building a 400 or 383 stroker, or using an ’86 up block?

For Chevrolet engines, both small and big block, the 153 tooth and 168 tooth flywheels have been used in both applications at various times. So, you can’t go by any sort of hard and fast rule that all small blocks used a specific tooth count for example. You will find that transmission bellhousings will fit both the 153 tooth flywheels which is 12.835” in diameter, and the 168-tooth flywheel, which is 14.086” in diameter. The critical item to match up here will be the starter A 153-tooth flywheel always uses the straight bolt pattern starter, while the 168-tooth flywheel will use the angle bolt pattern starter. Some blocks come with both patterns, or you can drill and tap the block to change starter bolt pattern to match your flywheel of choice. Aftermarket starters (Such as Powermaster) often have two sets of straight bolt holes to work with both flywheel sizes.
 

• The only engines that never used 153 tooth flywheel from the factory are the 400 SBC and 454 BBC engines. Both used the 168-tooth flywheels with an external balance (welded on weight).
• It is common, however, for an aftermarket performance crankshaft for 400 and 454 engines to be an internal balance version, which would then allow the possibility of using a 153-tooth flywheel

 
How Does the One-Piece vs Two Piece Rear Main Seal Affect Flywheel Choice?
 
You cannot use a flywheel designed for a one-piece rear main seal with a two-piece seal, and vice versa.
 
Both small block and big block GM engines initially launched with a two-piece rear main seal (1955-1985 SBC & 1970-1990 BBC) and were later updated to a better one-piece rear main seal design. Knowing what seal type your block/crank uses will determine flywheel fitment, as the crankshaft to flywheel bolt circle pattern changed, as did the ID of the flywheel center hole (Register Hole).
 

• SBC 1985 and older (302, 305, 327, 350, 400) are two piece rear main seal
• SBC 1986 and newer (305/5.0L, 350/5.7L) are one piece rear main seal
• Small blocks with two-piece rear main seal have a 3.58” diameter bolt circle and a 2.485”- 2.487” register hole.
• Small blocks with one piece rear main seal engines have a 3.00” diameter bolt circle and a 2.068”- 2.071” register hole.
• BBC Mark IV engines 1990 and older (396, 427, 454) are two piece rear main seal
• BBC 454/7.4L Gen V and VI engines 1991 and newer (427,454, 502, 572, 632) are one piece rear main seal
• All Big Block Chevy engines, no matter the type of rear main seal uses a 3.58” diameter bolt circle and a 2.485”- 2.487” register hole.