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Small Block and Big Block Chevy Engine Flexplate Guide

1/11/2023
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Small Block or Big Block Chevy, Using the Correct Flexplate Is Paramount

Enthusiasts have their favorite car models, engines, and more that they’ve spent countless hours working on and learning about. For many, tearing apart a small block Chevy, knowing its firing order, distributor rotation, and bolt torque specs are second nature and no shop manual or research is needed. It’s great to have friends with that kind of knowledge to pass along or to help in the garage or shop. However, for those just getting into hobby with their own hot rod or muscle car build it is important to educate yourself on the chassis or engine you’re working on. Knowledge is power, as the saying goes, and to that end we’ve put together this flexplate guide on SBC and BBC engines to help ensure you choose the correct flex plate for your build or swap.

How Do I Know What Flexplate I Need?

There are four main factors to consider when determining the correct SBC flexplate or BBC flexplate 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?
  • The torque converter bolt pattern: Do you need the large 11-1/2” or small 10-3/4” pattern
  • Internal or external balance: Are you building a 400 or 383 stroker, or using an ’86 up block?

Let’s look at each one of these sub-topics and dig into them further so you have the correct information you need to make the right product decision based upon your engine, and if you’re not exactly sure just what you have, see our Chevy Small Block ID Guide for some assistance.

What Chevy Flexplate Ring Gear Tooth Count Is Better?
Many GM applications can use either 153 or 168 tooth flexplates, so it is critical to understand what block, crank, and torque converter you have.

To be honest, it isn’t so much which one is better, but which one is right for your application. A lot of people get hung up on the tooth count and while the smaller flexplate requires less room (smaller bellhousing, more header clearance to the starter) that is pretty much its only advantage. The more important questions will be type of block/crank, what transmission/torque converter you’re bolting up to said block, and more. Then you can find the correct flex plate, either in a stock application or a swap application fitment. Lastly, for performance or motorsports use we highly recommend SFI flexplates for their extra strength and safety.

This comparison photo shows the straight (153-tooth) small block Chevy starter bolt pattern compared to the angled (168-tooth) pattern.

For Chevy engines, both small and big block, the 153 tooth and 168 tooth flexplates 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 flexplate, which is 12-3/4-inches in diameter, and the 168 tooth flexplate, which is 14-inches in diameter. The critical item to match up here will be the starter. A 153 tooth flex plate always uses the straight bolt pattern starter, while the 168 tooth flexplate 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 flexplate of choice. Aftermarket starters often have two sets of straight bolt holes to work with both flexplate sizes as well.

Some, but not all, GM blocks are drilled for both straight and angled starter bolt patterns. If your block only has the straight pattern you can drill and tap the block, or use an aftermarket mini starter that supports both flexplate diameters.

Note:

  • The only engines that never used 153 tooth flexplates from the factory are the 400 small block and 454 big block engines. Both used the 168 tooth flex plate 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 flexplate.
How Does the One Piece vs Two Piece Rear Main Seal Affect Flexplate Choice?
You cannot use a flexplate 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 and were later updated to a better one piece rear main seal design. Knowing what seal type your block/crank uses will determine flex plate fitment, as the crankshaft to flexplate bolt circle pattern changed, as did the ID of the flexplate’s center 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 2.5” ID center hole. (Crankshaft register diameter is 2.485” OD so the hole size in the flexplate is approximately 2.5”)
  • Small blocks with one piece rear main seal engines have a 3” diameter bolt circle and a 2-1/16” diameter center hole. (Crankshaft register diameter is 2.067” OD so the hole size in the flex plate is approximately 2.0625”)

See our small block flexplate dimensions diagram below in full size for easy viewing.

Small Block Chevy Flexplate ID Measurements
  • 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 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

See our big block flexplate dimensions diagram below in full size for easy viewing.

Big Block Chevy Flexplate ID Measurements
How Do I Know If My Engine Is Internally or Externally Balanced?
On external balance applications you will find a welded (or bolt on) weight affixed to the flexplate.

Save for a few unique applications, such as the 400 small block, the Chevy small block and big block engines are easy to determine balance if you confirm production year. For a small block Chevy flexplate balance the production year break between internal and external balance would be 1986. For the BBC flexplate balance that year break comes a bit later in 1991. We’ve broken it down in our notes below, but we also have a previous Toolbox guide on flexplate internal/external balance that lists small and big Chevy engines (along with Pontiac and Oldsmobile as well) that may be of interest to you. Lastly, remember that external balance engines will have an offset flexplate mounting pattern (not six identically spaced bolt holes) so that the crankshaft and flexplate line up properly to maintain engine balance.

Small Block Chevy Notes:

  • All engines 1985 and older (two piece rear main) are internally balanced except for the 400 engine.
  • 400 engine requires an externally balanced small block Chevy flexplate
  • 383 stroker engines are frequently externally balanced as well and would require the same SBC flexplate as a 400 if the engine is externally balanced
  • All engines 1986 and newer require an externally balanced SBC flexplate
  • The earlier 400 external balanced SBC flexplates have a different counterweight and crankshaft bolt pattern, so they are not compatible with 1986 and up one piece rear main seal engines

Big Block Chevy Notes:

  • All Mark IV engines 1990 and older are internally balanced except for the 454 engines
  • 454 engines require an externally balanced 454 flexplate
  • All Gen V and VI big block engines 1991 and newer are externally balanced
  • The Mark IV 454 flexplate has a different counterweight and crankshaft bolt pattern and is not compatible with the Gen V and VI engines
Is There More Than One Torque Converter Bolt Pattern?
This SFI rated flexplate from B&M features both Chevy torque converter bolt patterns, providing a stress free installation.

Just like you’ll find both 153 and 168 tooth flexplates in the same engine family, you will find two different torque converter bolt patterns were used by GM in the same engine family as well. This is because the torque converter bolt pattern was dictated by the transmission the torque converter was designed for. There is a small and large pattern (four hole), but you’ll find some transmissions used 3- and 6-bolt converters as well. While this makes things fun if you’re trying to match up a stock flex plate you might have on the shelf, know that most aftermarket SFI flexplates carry both torque converter bolt pattern diameters.

In some cases one of these dual pattern torque converters from B&M could be a life saver if you already have a nice new flexplate on a crate engine for example.

We offer dual pattern torque converters for popular GM “turbo” transmissions as well if your flexplate only has one pattern and it doesn’t match your current converter. With a new flexplate ordered for the proper balance, tooth count, and converter pattern, you’ll not have to worry about any issues bolting your engine and transmission together.

Notes: Small and big block engines used two different torque converter bolt patterns

  • small pattern of 10.75”
  • large pattern of 11.5”

Most Chevy transmissions such as TH350, 700R4, Powerglide, and TH250 used the smaller bolt pattern, while the TH400 and 4L80E transmissions use the larger pattern

  • Most TH400 converters are 3-bolt
  • Most 4L80E converters are 6-bolt

4L60E transmissions use the small pattern

  • The exception here is that the later model 4L60E transmissions, which were originally used behind LS engines, have an “LS specific” bolt pattern which is 11.08” and comes in both 3- and 6-bolt versions

To measure and confirm your torque converter bolt pattern on either a flexplate or torque converter, measure from a bolt hole or converter stud to the center of the flex plate or converter, then take that measurement and multiply it times two.

We hope explaining these Chevy 350 flexplate differences (and those relevant to big blocks too) has made your selection of the proper small block or big block Chevy flexplate a little easier and provides you the confidence to order just what your combination needs to get your project on the road.

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