While dramatic parts failures can result in some pretty spectacular photos, they’re definitely not fun for whoever gets stuck footing the bill to repair the damage. And, while many failures are caused by engine components, an improperly spec’d flexplate can also occasionally be the culprit.

An engine can make all the power in the world, but if that power isn’t properly delivered to the transmission, it’s essentially useless. The flexplate plays a critical role in this process, so it needs to be the right fit for the application. There’s a lot that goes into selecting the right flexplate than just making sure it fits on the back of your engine.

Finding The Right Flexplate For Your Build

The ultimate goal for racers, of course, is to transmit every bit of power from the engine to the transmission, but it’s absolutely imperative to select the correct flexplate to ensure proper performance as well as safety.

“First, you’ll want to make sure the flexplate is rated to support the horsepower you’re expecting to make,” said Don Meziere, co-owner of Meziere Enterprises and a multi-time NHRA national event winner. “The flexplate also needs to be centered and balanced correctly, and run true so that it’s safe. Having a well-machined, precision flexplate is a big part of that equation.”

Additionally, the weight of the flexplate is also important to consider, as a lighter flexplate can help the engine accelerate quicker, but it might not be strong enough to withstand the severe demands of a higher-horsepower application. “Meziere offers several grades of flexplates from our standard line to our low-inertia products, as well as our true billet flexplates that can transmit thousands of horsepower for Pro Mod-style applications,” added Meziere.

Flexplates can have different attributes based on the engine they’re bolted to. They might have areas where weight is strategically removed. A flexplate could also have additional material added to deal with high-horsepower engines. That’s why there are numerous options available for the same type of engine.

For most bracket or index racers, consistency and reliability are vastly more important than shaving off a few thousandths of a second. In those cases, the extra cost of a low-inertia flexplate most likely wouldn’t be money well spent, and a standard flexplate would be better suited to the racer’s needs. For heads-up racers whose engines produce up to about 1,200 horsepower at the crank and where every little bit counts, a low-inertia plate may work well to help improve elapsed times. Above 1,700 hp, Meziere recommends the Pro Mod line of billet steel flexplates.

Both the vehicle’s weight and its engine’s displacement also have a lot to do with selecting the right flexplate, too, and need to be included as factors in the selection process. “The low-inertia plates are excellent solutions, but on heavier vehicles, even with a smaller displacement engine, it needs to do more work to accelerate the heavier vehicle and that’s a lot more strenuous,” cautioned Meziere.

As engines have gotten larger over the years and RPM ranges regularly eclipse 7,000 or even 9,000-plus, safety concerns have become more and more important. With welded flexplates, which Meziere does not offer, the high rotational speeds can put excessive stress on the welds and cause them to break; the resulting failures are often catastrophic — so keeping safety in mind when making a purchase can also tangentially keep the racer’s bank account “safe,” too.

Considering the outer ring gear of the flexplate, too, is also an important factor in the first steps of selecting a proper flexplate. When the engine’s starter is engaged, it needs to mesh correctly and hold a proper tolerance to do its job — just like the gearset in a transmission or rearend.

Chevrolet and Ford engines generally use a 12-pitch gear form in their factory configuration.. “It’s the form of the gear that fits on a certain diameter,” clarified Meziere, who doesn’t recommend a 12-pitch gear form for larger higher-compression engines as it has inherent weaknesses. “Years ago, Meziere took a larger, stronger 10-pitch gear from Chrysler and grafted it onto the Chevrolet and Ford plates. It’s stronger and a much better long-term solution, but you have to match that ring gear with a starter that will mesh properly with it.”

The flexplate you choose needs to have the correct SFI rating for your application and to be compliant with what’s in the rule book. This is important not only for safety, but to ensure you won’t experience an expensive parts failure.

Meanwhile, many higher-horsepower drag classes have a rule that requires the use of a secondary liner inside the transmission bellhousing. As a result of the reduced internal space, the maximum potential diameter of the flexplate is also reduced. So, a standard 14.125-inch Chevrolet diameter can’t be used. Fortunately, Meziere has accounted for this, too, and the company produces its Pro Mod flexplates with a 136-tooth,10-pitch gear form that fits on a 13.8-inch diameter.

“Beyond basic functionality and safety, the flexplate has to have the right strength properties for how it will be used, and the right bolt pattern to connect the components,” Meziere shared in response to common ways to avoid flexplate installation issues. “You’ll need to know both your crankshaft bolt pattern and your torque converter bolt pattern. Fortunately, most of our customers will use a 6-bolt pattern, and most of our flexplates are designed to accept the three most common: 6 on 10.75 inches, 6 on 11.50 inches and the ‘split’ pattern which is a combination of the two bolt circles with three on the 10.75-inch and three on the 11.50-inch pattern.”

The Importance Of Propper Installation Of A Flexplate

Once all of the specs have been identified and a flexplate has been chosen and purchased, it’s on to the fun part — installing it and going racing. But here, too, a little prior planning can prevent some poor performance later on.

First, make sure that all mating surfaces — including the crankshaft flange where it meets the flexplate — are clean. Bolt holes and threads should also be checked, and it’s wise to test-fit all the components first to ensure everything matches up as expected so there aren’t any frustrating surprises later on. “Make sure all your fasteners and hardware match up, and that the radiuses on the parts won’t prevent things from sitting flat and being secured tightly,” noted Meziere.

Although most manufacturers (including Meziere) mark their flexplates with an “engine side” or “transmission side” to make things easy, it’s important to make sure the flexplate is positioned correctly as the ring gear is typically biased either towards or away from the engine.

There is a wrong way and a right way to install a flexplate. Make sure the engine side is facing the correct direction, it may sound simple, but people still try to install a flexplate the wrong way.

“For Chevrolets, the crankshaft or engine side of the flexplate will have a larger chamfer on the center bore to avoid any large radius on the crankshaft,” he clarified. The centering feature, such as a boss sticking out from the crankshaft or a flexplate with a counterbore to fit around the crank, should provide accurate radial positioning. “In some cases, like the Ford small-block engine or the GM LS engine, the flexplate isn’t flat — it has a slight dish or bell shape to it. This will also be a big clue to the user when determining engine side versus transmission side.”

Make sure to check all clearances during pre-assembly as well, and finger-tighten the bolts once everything is in place. “With the flexplate installed, the torque converter should generally pull up 1/8 to 3/16 inch,” Meziere continued. “The converter boss should slip into the crankshaft or centering adapter with minimal clearance. However, you don’t want it too tight, because they can weld themselves together due to the engine’s harmonics. Be mindful of the clearance for the starter pinion, too, which should be about 1/8 inch off the ring gear at rest.”

Before you go through the final toque sequence on the fasteners, make sure everything is lined up correctly.

When bolting up the flexplate, it’s also essential to make sure the hardware selected is appropriate for the application, that the quality of the threads receiving the bolts is up to par. “It’s important to torque the bolts correctly so they don’t loosen and fail, but it’s equally important to not overstress them,” affirmed Meziere.

However, depending on whether the bolts go in dry, oiled, or with locking compound can impact the required torque rating — so it’s imperative to double-check and confirm the figures. For example, a standard 7/16-inch ARP bolt typically has a torque rating of around 70 lb-ft, while Pro Mod teams often use half-inch bolts torqued up to 120 lb-ft or more.

Once everything has been checked for correct clearances and flawless fitment, it’s time to do the final assembly. And, if all of the proper considerations have been made when selecting the flexplate in the first place, it should be a relatively easy process with minimal issues that ends up with more time at the track and better performance overall — and without those fiasco photos — which is exactly what every racer wants.