What You Need to Know About Flexplates
Putting The Power to The Pavement
There are a lot of moving pieces (literally!) when it comes to your project’s engine and drivetrain enjoying a happy marriage when your fresh engine build is going down on the mounts for the last time. Get one part wrong and your engine and transmission might lock up tight and not even turn by hand if you’re lucky. If you’re not so lucky you’ll most likely tear up a bunch of expensive parts, damaging your engine and your transmission in the process. Therefore, knowing what does a flexplate do and how to properly choose one is imperative to a smooth running engine and proper connection to your transmission.
What Is an Engine Flexplate?
The engine’s flexplate (also commonly referred to as a flex plate) bolts to the rear of the engine’s crankshaft and connects to the automatic transmission’s torque converter’s outer shell. The flexplate, in being installed in this manner, provides a conduit for the engine’s output to transfer to the transmission’s input shaft (which is splined to the torque converter’s turbine found inside the torque converter’s shell). The flexplate, in most applications, features a welded on ring gear that interfaces with the engine’s starter to help rotate the engine during the starting process.
Depending upon the application, flex plates are often weighted with either a bolt-on or welded-on weight to properly balance the engine in conjunction with the engine damper on the front of the crankshaft. These flexplates are commonly used in externally balanced engine applications and must match the engine’s specific balance requirements. Furthermore, flexplates are generally measured in one of two ways, by diameter or by tooth count of the ring gear. We have some useful shopping tips for small block and big block Chevy flexplates, LS swap flexplates, Mopar flexplates, and small block and big block Ford flexplates currently in our Toolbox section.
Does a Flexplate Flex?
Indeed, an engine’s flexplate does flex during operation of the engine. Since the flexplate is made from a thin sheet of stamped steel it is allowed some flex along its side to side axis. This compensates for minor harmonics, torque converter movement and even torque converter mounting points to the flex plate itself. Though it is manufactured from thin steel (in most cases) the stamping creates strength in the manufacturing process, as does welding on the starter ring gear.
What's The Difference Between a Flywheel and A Flexplate?
Many enthusiasts will typically use the term “flywheel” in a bench racing session when in fact they are really referring to a flexplate. The main difference in a flexplate vs flywheel is that the flexplate is designed for automatic transmission applications and the flywheel is designed for manual transmission applications, and here’s why. With the automatic transmission drivetrain, the engine’s torque is transferred to the drivetrain through the hydraulic coupling that happens inside the torque converter. The flexplate is simply a way to attach the torque converter shell to the engine’s crankshaft.
When it comes to manual transmission applications a flywheel is required. The flywheel is a much thicker piece of steel (or aluminum) that is attached to the rear of the crankshaft like a flexplate and is what the clutch pressure plate bolts to. The clutch disc, which is splined to the manual transmission’s input shaft, is sandwiched in between. But instead of simply being a “coupler” like the flex plate the flywheel stores the engine’s kinetic energy and is used via mechanical coupling of the clutch to transfer the power form the engine to the transmission. So, you see it is more a matter of flywheel or flexplate for manual or automatic transmission usage, and not flywheel vs flexplate. You’re going to use one or the other and the proper terminology is important.
What Causes a Flexplate to Fail?
Due to the flexplate’s location in the drivetrain (hidden inside the transmission bellhousing) it is rare for the flexplate to receive any sort of damage from road debris, and if it does you most likely have bigger problems! As such, you’ll find that the flexplate will last the life of your car or engine in most cases. That’s not to say that the flexplate is bullet proof, however. It’s main weakness is an improperly installed or loose starter motor causing damage to the ring gear teeth of the flexplate. Proper starter indexing and shimming is critical to flexplate (and flywheel) ring gear longevity.
While not as prevalent, the flexplate can crack due to excessive vibration or harmonics issues. The wrong engine balance, incorrect installation, or the wrong type of mounting fastener at the crankshaft or the torque converter are all possible reasons for the flexplate to crack or break due to undue vibration. When this happens, the broken flexplate noise caused by the crack or breakage can mimic a rod knock-like sound or can also be a metallic rattle. These symptoms of a bad flexplate will be intermittent and most noticeable at idle and on coasting versus part to full throttle application.
How Do I Choose the Right Flexplate?
For the majority of those building a street oriented ride a stock flexplate will do just fine. For some custom applications, such as stroker engines or modern engine or transmission conversions, you may need a specific custom flexplate. This may come with your automatic transmission conversion kit, but quite often it is up to you to purchase the necessary flexplate.
When you’re planning higher horsepower engine builds for the street, or going to be hitting a race track, then a SFI certified flexplate is highly recommended. Like other SFI certified performance parts, the SFI certified flexplate must pass stringent safety testing to receive the SFI certification label. There are currently three certifications depending upon horsepower level or if a diesel engine.
- SFI 29.1 covers flexplates for engines up to 1,500 hp
- SFI 29.2 covers high horsepower applications up to 3,500 hp
- SFI 29.3 covers flexplates for diesel applications
What Should Flexplate Bolts Be Torqued To?
There are far too many engine combinations to provide an all inclusive list in this high level review of flexplates. We do recommend the use of a factory shop manual or aftermarket repair manual for OEM-level flexplates and their mounting fastener torque specs to the crankshaft and torque converter to flexplate torque specs. For performance flexplate bolts, such as those from ARP, it is best to consult the fastener manufacturer’s website or the fastener’s packaging/instructions for their torque spec and if it is based off dry or lubricated threads.
