A Question of Balance
Craig Wilhelm; Powhatan, VA: I have a ’95 Formula Firebird with a 383 LT1. I used a 6-inch rod kit from Eagle (PN B13057L030). This was an internal balance stroker kit with a cast crank and forged Mahle pistons. The cam is the GMPP LT4 Hot hydraulic roller with Comp Cams High Energy hydraulic rollers and GMPP 1.6 rockers. I also used an ATI Super Damper with the ATI hub. When stock, these engines are neutral balanced in the front and externally balanced in the rear. I had my machine shop assemble the short-block and do all the clearances and balancing because I was in a time crunch to get the car done. I asked the machine shop if they could balance the stock flywheel to the rotator kit. They said they could, so I went with it.
The problem is that the engine vibrates from idle through about 4,000 rpm. While idling, the engine seems to oscillate up and down. Could it be possible that it’s not balanced correctly? While assembling the engine, I didn’t really notice any machine marks on either the flywheel or the crank. I have known these guys for years and they are one of the area’s best machine shops, and I trusted them. When I put the flywheel on the crank, I had to de-burr it with a flapper wheel so it would fit the rear of the crank. I would think that had they balanced it, they would have corrected that when they did the work. I know I indexed the flywheel to the crank correctly because I lined up the dowel holes. The dowel, however, was missing, so I left it out. Could that be it?
I suspect the flywheel wasn’t balanced at all. I want to either get the flywheel neutral balanced or buy a new zero-balance flywheel. Your suggestions and thoughts would be greatly appreciated.
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Jeff Smith: In 1986, Chevy changed the traditional small-block engine from the two-piece rear main seal to the one-piece style. The original two-piece rear main seal crankshaft flange used a small weight on the end of the crank. Adopting the one-piece rear main seal required moving the offset weight into the flywheel or flexplate, setting up an odd combination in which the harmonic balancer is still a neutral-balance unit, while the flywheel (or flexplate) still required offset weight. With the Eagle stroker packages, balance depends on the crank you order. Eagle’s forged cranks are designed as true “internal/internal,” which means the balance is all handled within the rotating assembly, and the engine requires a neutral balancer and a flywheel/flexplate. Eagle’s cast-crank stroker combinations are packaged both ways—you can get either a true internally balanced package or one that requires using a neutral harmonic balancer and an offset weight flywheel/flexplate. We called Eagle to verify the cast-crank stroker package you used. Eagle told us your package is internal/internal, which means it requires a harmonic balancer and flywheel/flexplate with a neutral balance. The stock LT1 flywheel you are using is an external-balance design with a weight offset and is therefore your vibration smoking gun. What is unclear is how your balance shop overlooked this fact. One clue that leads us to believe they missed this critical point is your observation that it looked as if they never bolted the flywheel onto the crankshaft. If the crank assembly was balanced separately from the flywheel at a different time, it’s possible they may not have made the connection that the flywheel should be neutral balanced. An offset weight flywheel will usually have holes drilled into the flywheel instead of an added offset weight. Flexplates typically have external weight welded to the face. Assuming your flywheel has the drilled holes, it will need to be neutral balanced. We’re betting that offset weight in the flywheel is the problem.
This is the crankshaft flange on a two-piece rear main seal small-block Chevy. Notice the
Looking into this a little further, we spoke with Danny O’Donnell at West Valley Balancing in Chatsworth, California, who has years of experience with engine balancing. He says the best way to balance an engine is to include the flywheel, clutch, and pressure plate together as a unit. He requires the customer to supply all the parts, including the fasteners. What many car crafters don’t realize is that older pressure plates require special shouldered, 3/8-inch NC bolts that not only attach the pressure plate but, more important, center it on the flywheel. If these shouldered bolts are not used, the pressure plate can move by as much as 0.030-0.040 inch, which is more than enough to create a vibration. Later-model vehicles use two or three small dowel pins to locate the pressure plate. This design does not require a shouldered bolt. If you think the wrong bolts may be the cause of your vibration, you could replace them with the proper fasteners (ARP and Mr. Gasket, for example) and start the engine with the pressure plate and clutch in place. You could also remove these components and start the engine with just the flywheel in place if the vibration persists. This would help isolate the problem.
Danny also mentioned that he’s run across poor surfacing work on a flywheel, in which case the friction surface is not parallel to the crank flange. This creates a thicker side to the flywheel that will cause an imbalance. The quick way to check for this is to measure the flywheel thickness with a caliper in four places around the circumference and compare the readings. He says as little as 0.020 inch can cause a vibration. This is rare, as most machine shops are competent when it comes to surfacing flywheels
It might also be helpful to note that when Chevy changed to a one-piece rear main seal, the flywheel bolt pattern also changed. The earlier two-piece rear main seal neutral-balance flywheels and flexplates will not bolt up to the one-piece rear main seal crank. I mention this in case someone might consider swapping flywheels or flexplates between these engines. (Editor’s Note: Craig subsequently contacted us and said that the balance shop indeed missed the fact that the flywheel should have been neutral balanced. The shop neutral balanced the flywheel, and the problem has been rectified.)
Eagle Specialty Products; Southaven, MS; 662/796-7373; EagleRod.com
West Valley Balancing; Chatsworth, CA; 818/341-9043
Did we run enough burnout pictures in this issue? The correct answer is “no.” So here’s another version of Ford’s Coyote V8 destroying the Nittos on Christian France’s ’86 Mustang.