Flywheel/Flex Plate

Every vehicle on the road has a coupler between the engine and transmission, for an automatic transmission this is called the flex plate, for a manual transmission it is called a flywheel. This article will explain the difference between the two, what they do and hopefully everything else you will need to know about them. A flex plate is what connects the engine to the automatic transmission in a vehicle. It is basically a piece of thick sheet metal that bolts to the end of the crankshaft and to the torque converter. They are made of a thinner material than a flywheel to allow for expansion of the torque converter at higher engine rpms, thus the name “flex plate”. Some flex plates have a toothed ring on the outer edge for the starter to engage with, others have the toothed ring on the torque converter. flex plates are engine specific items due to the fact that some engines require external balancing to smooth out vibrations, these flex plates will have a weight welded to it somewhere, others will not. If these are mixed up, the engine will have a serious vibration that could actually cause internal engine damage if left unchanged.

Engine Flex Plate
flex plates have been know to crack and or break for no particular reason, if your vehicle makes a chirping/light knocking noise just off idle in gear, you may have a cracked or broken flex plate. If you suspect this, some transmissions have a removable inspection cover that will allow enough access to see if there are any cracks (Look for rust, this is a sign of a crack), but unfortunately most of the time the transmission must be removed to gain access to see if it is indeed cracked/broken.
If your vehicles flex plate does need to be replaced, use a high quality OEM (Original Equipment Manufacturer) part or the equivalent, a cheap one may not be made of a high quality steel or could cause starter alignment issues. Always match up the old one to the new one before reassembly to assure they are the same diameter and that the torque converter and crankshaft bolt holes line up properly. The crankshaft mounting holes are usually “Timed” which means that they will all only line up in one spot, so you may have to rotate the flex plate a few times to get them all to line up to verify it is the correct flex plate for your vehicle

Engine Flywheel
If your vehicle has a manual transmission, then it uses a flywheel instead of a flex plate. Flywheels are much heavier than a flex plate, this is due to the lack of a torque converter which is what helps keep the engine from stalling or bogging down by using its weight to maintain the inertia of the engine at low rpms. Flywheels come in different weights depending on the application. A lighter flywheel would be used in high-performance applications where higher rpms are the norm, a heavier flywheel is used in trucks and other low rpm applications where you need the extra help just off idle. Heavier flywheels also make the transition between gear shifts smoother by maintaining the engines inertia which decreases the jerking motion felt when letting the clutch out.
The flywheel is where the clutch assembly mounts and is the contact/friction surface for the clutch disk. Whenever a clutch is replaced in a manual transmission vehicle the mating surface on the flywheel needs to be resurfaced and measured to be sure the thickness is within specification, otherwise the flywheel can crack and even break in extreme cases. Flywheels are where the toothed ring is mounted for the starter motor to engage. In certain applications this ring can be replaced instead of replacing the entire flywheel when the teeth become damaged. If you need to replace your flywheel, always use a high quality replacement part such as the OEM (Original Equipment Manufacturer) parts. They will fit right and perform better than less expensive aftermarket products.