Flex Plate Or Flywheel

kingleodiorio said:

hello, I'm getting ready to put a new motor in my 67 mustang. It's my first project and I've ran into a small problem. I'm mating the engine (1978 5.0) to a c4 automatic transmission. Do I need a flex plate, or flyeheel? do I have to get it balanced for my engine, or are they generic in that regard. Why is the number of teeth important? How do I choose the right part for this job?

Thanks,
Leo

Click to expand...

First of all, lets discuss the engine as you need to identify it correctly to get the correct parts. The engine known as a 5.0 didn't first come out until approximately 1985. Those motors use a 50oz imbalance crankshaft/balancer/flexplate or flywheel.

A 1978 motor is properly called a 302 (even though it works out to 5.0 liters.) Unless it has been modified internally (unlikely) it will use a 28 oz imbalance crankshaft/balancer/flexplate or flywheel.

Obviously that distinction between the 2 motors is very relevant to getting the correct parts for your motor. You cannot intermix those parts without having severe vibration problems. Other parts (heads/intakes/etc) can be swapped without issues, but not the a fore mentioned crank related parts.

Automatic cars use a flexplate. You will need to use the correct balanced flex plate or else you will have a vibration. You do not always need to have the flexplate balanced for the motor, but that is the best way to do it. If you are just building a daily driver, installing a different flexplate than the original is generally not an issue, as long as it has the correct imbalance. A 1978 engine with an automatic needs a 28 oz imbalance flexplate. It shouldn't be too hard to find one.

The # of teeth relates to the outer diameter of the flywheel. A 157 tooth flexplate is smaller than a 164 tooth flexplate (the teeth spacing is equal though.) The tooth count is real important in determining which flexplate you need and it is specific to the bellhousing of the transmission you want to use. A 164 tooth flexplate will not fit inside a bellhousing designed for a 157 tooth flexplate. Although the smaller diameter 157 could physically fit inside the 164 tooth bellhousing, the location of the starter would be incorrect as the starter location of the 164 tooth bell is located further away from the crank centerline (to compensate for the larger diameter.)

I am not fully up on automatic starters and there interchangeability but once you figure out which size flexplate you should have enough info to ID the proper starter if you need to purchase one.

Right around 1981 is when they changed the crankshaft imbalance of the 302 from 28 ounces to the ""5.0" style 50 ounce imbalance. That imbalance is critical to flexplate selections. (Flywheels don't flex and are used with clutches, but most folks know what you mean either way.) Pre-1981 302 is the same as a 289 imbalance. A stock flexplate for a 1967 289 C4 Mustang should do you just fine. Although the 289 and 302 are slightly different internally, for the most part they are the same externally as far as bolting stuff to them. With only a few cosmetic differences, 1967's and 1968's are almost identical. Early 1968's got 289's, a few in the middle got sort of 289/302 hybrids, and late 1968's got 302's. So it might make it easier on you to consider your car a late 1968 302 when you look to buy parts. And lie to the parts counter people.
Ford guys go by the number of flexplate/flywheel teeth but the real difference is the diameter. The teeth are the same size so logically to fit more teeth, the circle has to be bigger. The bellhousing is sized to accommodate a certain sized flexplate and places the starter accordingly. So if you have a "big" bellhousing and a small flexplate the starter will likely completely miss the teeth. Generally big flexplates just don't fit into small bellhousings. Big meaning 157 teeth and small 164. Though there are vintage 160 tooth ones, they are considered interchangeable with 157 teeth. Though a little rare, there exist "big bell" 164 tooth flexplate C4's that were used in trucks. Just in case you accidentally got hold of one. You can use one, they're just not common.
When you are spending a lot of money on an expensive engine rebuild, there is a balancing process towards the end where they will want to have your intended flexplate/flywheel at the shop with the engine. Other than that you just buy the correct parts at the store and bolt them on. Building your drivetrain as a 1968 302/C4, I would simply use the correct starter for that car.

Gypsy, you have your flexplate sizes backwards. 164 tooth plate is larger diameter than the 157 tooth plate\flywheel.