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The 331 Kit Assembly

Our project will utilize a complete Scat 331 rotating assembly. We evaluated several 331 manufacturers on the basis of cost and quality of the components. We ultimately decided on Scat because their kit offered forged rods and pistons.

The crank is a Series 9000 cast steel crank, which will provide a significant increase in strength over cast iron, but without the expense of forged steel. Since we do not foresee power over 500 HP, or a poweradder, the cast steel will be more than adequate.

Scat offers either I beam or H beam 4340 forged rods. We opted for the I beam rods to keep cost down. Rod length is 5.400" for both their 331 or 347 kits, making it nice if we decide for more cubes in the future. The rods feature ARP 7/16" cap screws for superior strength.

Scat kits are offered with either hypereutectic KB pistons, or forged SRP pistons .The price difference is only $150, making it a worthwhile investment to step up to the stronger slugs. The pistons feature spirolocks for retaining the pins rather than a press fit.

Rings and Bearings
We will round out the bottom end with Clevite rod, cam, and main bearings. To control blow by and insure good compression, we went with Total Seal performance file-fit rings. We'll discuss the gaps later in the article series.

Most 331 and 347 kits on the market are recommended for 28oz (early Ford) balance factor. While some kits do come as 50oz, consensus is the lighter imbalance factor is better for bearing life.

In general stroker kits DO NOT come balanced, unless you specifically asked and paid for a balanced assembly. Because companies like Scat are manufacturing new cranks and rods, they simply are not going to be within production tolerances for a 28oz or 50oz rotating assembly.

Putting together a stroker kit without having it balanced is guaranteed to result in vibration and accelerated bearing wear and failure.

We recommend that you take your entire rotating assembly to a machine shop that is equipped to perform balancing. They should ask for your crank, rods, pistons, flywheel, balancer, pressure plate, and also one set of rings, and main and rod bearings.

The shop will weigh the ends of the rods and the pistons, and then modify them to match the lightest one in the set. Next they will calculate a "bob weight", the effective weight of the rod, piston, rings and bearings. From this the rotating assembly is mocked up and spun on a machine. If the shop is equipped with a electronic balancing machine it will pin point the out-of-balance location, and the machinist will modify the crank or flywheel to compensate.

We took our kit, as well as the harmonic balancer, flywheel and pressure plate, to Bay Area Balancing for an electronic balancing to within 3 grams. Factory Ford engines are allowed up to 5 grams variance.

We obtained a 28oz hub (right) for our Fluidamper harmonic balancer, which came originally with a 50oz balance (left). When balancing a Fluidamper, the hub must be separated from the outer ring.
It is typical to find slight deviance's in weight even when the piston manufacturer includes a sheet claiming all pistons in the set are within one-half gram. Our balance shop found a 3 gram difference from the highest to lowest in the set. A ball mill is used to remove a slight amount under the piston.
Bob-weights -the calculated sum of the piston, rods, and rings- are attached to the crank, along with the flywheel and balancer, to simulate a rotating assembly. This particular electronic balancer (Hines HC500) spins the crank at a specified rpm, and has a microcomputer which pin points the out of balance to within 1/8th of a gram. A drill press directly over the unit can immediately remove weight off the crank.
Nearly 70 grams had to be removed from the Scat cast steel crank in order to balance the assembly. (Note the large holes drilled in the counterweight.) That's a huge amount and goes to show you cannot just drop these kits in a motor and run.
We had just recently invested in a new billet steel 11" flywheel and clutch for our 50oz 302. We surely did not want to buy a new 28oz flywheel. Fortunately our balance shop could modify the flywheel to bring it from 50oz to 28oz.
In order to bring the flywheel to 28oz imbalance, material was removed from the opposite side of the 50oz counter weight. Compare to photo above.
Many balance shops don't even ask for the pressure plate. Good balance shops know they can be significantly out of balance (all clutches are supposed to come neutral balance.) Ours was five or six grams out, enough to cause a vibration. This was remedied by adding some weight. The plate is then indexed to the flywheel.
Many companies, including Scat, do offer balancing services at an additional cost. If you have them balance your stroker kit you MUST insist on sending them the harmonic balancer, flywheel and pressure plate you plan to use, or buy a new set from them. Do not allow them to balance your kit to their "shop standard" flywheel and balancer, since not all 50oz or 28oz balancers and flywheels measure the same. Continue


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