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The flexplate is the thin steel disc that bolts to the back of the crankshaft on an automatic and carries the ring gear the starter spins. It is easy to ignore until it cracks. When one lets go, you get a bang, a horrible grinding, and sometimes a hole in the bellhousing. On engines that use it for external balance, the wrong plate will shake the whole car apart at idle.
Three questions decide which flexplate you buy: is your engine internally or externally balanced, how many teeth does your starter need, and how much power are you making. The first picks weighted vs neutral. The second picks the tooth count. The third decides whether you need an SFI-rated plate. Get those three right and the rest is bolt pattern.
Weighted vs Neutral Balance — The Decision Tree
Neutral (Internal) Balance Flexplates
A neutral-balance flexplate has no counterweight. It is used on engines that are internally balanced, meaning the rotating assembly is balanced on its own with heavy-metal in the crank counterweights. Small-block Chevy 350, LS engines, and most modern engines run neutral balance. Bolt a neutral plate to an externally balanced engine and it will shake violently.
Buy a neutral-balance plate if: your engine is internally balanced (LS, SBC 350, most late-model V8s). When in doubt, an LS is always neutral.
Weighted (External) Balance Flexplates
A weighted flexplate carries a cast or welded counterweight that offsets an externally balanced rotating assembly. The two classic offenders are the Chevy 400 small-block and the 454 big-block, plus many Ford and Chrysler applications. The weight amount and its clocking are specific to the engine. There is no universal weighted plate, you match it to the engine family.
Buy a weighted plate if: your engine is externally balanced (Chevy 400 SBC, 454 BBC, and various Ford/Mopar). Confirm the exact application before ordering.
Do not guess on balance. If a plate looks like it has a chunk of steel riveted or welded near the edge, it is a weighted plate for a specific engine. A smooth plate with no counterweight is neutral. Putting the wrong one on is the single most common flexplate mistake, and it shows up as a bad vibration the moment you start it.
Tooth Count — Match The Starter Or It Will Not Crank
The ring gear teeth on the outer edge are what the starter pinion engages. Get the count wrong and the starter either will not mesh at all or grinds because the pitch is off. The two most common GM counts are 153-tooth and 168-tooth, and they pair with a specific starter and, on GM, a specific starter bolt spacing. Match the plate to the starter you are running, not the other way around.
| Application | Common Tooth Count | Notes |
|---|---|---|
| GM small-block (small-diameter) | 153-tooth | Straight-across starter bolt pattern |
| GM small/big-block (large-diameter) | 168-tooth | Staggered / diagonal starter bolt pattern |
| GM LS series | 168-tooth (neutral) | Neutral balance, LS-specific bolt pattern |
| Ford small-block | 157 / 164-tooth | Match to 157 or 164 starter and balance weight |
If you are changing to a mini gear-reduction starter, confirm which tooth count and diameter that starter is built for before you order the plate. The starter and flexplate are a matched pair.
Signs Of A Cracked Flexplate — Catch It Before It Grenades
A cracked flexplate rarely fails all at once. It gives you warning noises first. Learn them, because the failure mode when it finally lets go can put a piece of steel through the bellhousing.
- Rhythmic clunk or knock at idle that changes or disappears when you shift into neutral or park. The crack opens and closes with load.
- Intermittent starter grind or no-crank because a cracked plate flexes and the ring gear no longer sits true to the starter pinion.
- Ticking or slapping that rises with rpm, often mistaken for a rod knock, but it is the cracked plate flexing against the converter.
- Vibration that was not there before, especially if a chunk of a weighted plate broke off and threw the balance out.
- Visible crack radiating from a bolt hole or the crank register when you pull the inspection cover. This is where they almost always start.
If you hear any of these, pull the lower inspection cover and inspect the plate before driving it. A flexplate that lets go can crack the bellhousing, wreck the starter, and strand you.
SFI-Rated Flexplates — When Stock Is Not Enough
Stock stamped-steel flexplates are fine for a stock or mildly modified engine. Once you are making real power, launching hard, or running at a track, a thin stamped plate can crack from the shock loads. That is where SFI comes in.
SFI 29.1 is the certification for flexplates built to survive high-rpm and high-shock use. These plates are thicker, made from better steel, and many tracks require an SFI-rated flexplate (and often a scattershield or SFI bellhousing) once you run a certain elapsed time or trap speed. If you are building a performance combination, buy the SFI plate up front. It is cheap insurance compared to a bellhousing repair.
Browse SFI-rated flexplates
How To Pick The Right Flexplate — The Short Version
- Confirm balance: internal (neutral) or external (weighted) for your exact engine.
- Match tooth count and diameter to your starter.
- Verify the crank bolt pattern and converter bolt pattern. LS, SBC, and BBC differ.
- Choose stamped for stock, SFI 29.1 for performance.
- Use new, correct-length flexplate bolts with thread locker, torqued to spec. Reused bolts are a common failure point.
Check flexplates
Need Parts? Ships Same-Day
Swapping the flexplate means the converter and trans come back. Our companion shop gearboxinsider.com stocks the flexplate bolts, converter seals, and rebuild parts you will want while it is apart. Same-day ship before 2pm Eastern.
Shop Gearbox Insider →FAQ
How do I know if my engine is internally or externally balanced?
Look up your exact engine family. As a rule, LS and SBC 350 are internally balanced (neutral plate), while the Chevy 400 small-block and 454 big-block are externally balanced (weighted plate). If a plate has an obvious counterweight, it is for an externally balanced engine.
Can a cracked flexplate cause a no-start?
Yes. A crack lets the plate flex so the ring gear no longer meshes cleanly with the starter, giving intermittent grinding or no-crank. Pull the inspection cover and check before you keep cranking on it.
Do I really need an SFI flexplate for a street car?
For a stock or mild street engine, a good stamped plate is fine. Once you are making serious power or running the track, an SFI 29.1 plate is worth it, and many tracks require one past a certain elapsed time.