Q&A

Ask Away! with Jeff Smith: Troubleshooting a Chevy 454 with a Bad Vibration at 2,100 RPM

I recently had a 1970s 454 Chevy big block rebuilt and assembled by my local machine shop for my 1971 Monte Carlo. The engine was assembled with new pistons, rings, and bearings and a nice hydraulic roller cam, and the assembly was fully balanced by a competent shop. They re-used the original balancer and the flexplate. We put the engine back in the car and it runs fine but it has a noticeable vibration at around 2,100 rpm. Above and below this rpm, the vibration goes away. It will do this even with the car sitting in Park so that rules out the driveshaft. Do you have any ideas on what could cause this?

D.B.

Let’s first look at the engine in question. The early 454 engines used a two-piece rear main seal and were externally balanced. This means that the harmonic balancer and flexplate are fitted with an external weight. The balancer weight is built into the inside diameter while on the flexplate it is generally welded in place near the outside diameter. The balance shop would have carefully inspected both of those components before reusing them so we will assume that they are in good condition and worthy of use.

We started thinking about what would affect the engine operation and it occurred to us that during the installation of the engine and transmission, it would be easy to position the flexplate in the incorrect orientation. Chevrolet places a dowel pin in the crankshaft that lines up with a hole in the flexplate (or flywheel if a manual application).

This dowel pin is crucial for external flexplate or flywheel applications because it only allows the flexplate or flywheel to be bolted on in only one position. In many cases, however, these dowel pins are removed, lost, or missing and are rarely replaced. This isn’t really critical in a neutral balance engine like the two-piece rear main seal small or big block Chevys where the position of the flexplate or flywheel isn’t important because each individual component is neutral balanced.

But with a 400ci small block or a 454 big block, the external balance factor demands that the flexplate or flywheel be positioned to line up with the dowel pin because this positions the external balance weight in the correct orientation. What allows this situation to occur is that the crankshaft bolt pattern for the small and big block Chevys is symmetrical. This means that if the crankshaft flange is not fitted with a dowel pin, the flexplate can be installed in six different positions—five of which are incorrect for an externally balanced 454.

Our theory on the source of your vibration is that the engine was assembled and balanced properly but during the installation there was no dowel pin in the crankshaft which allowed the assembler to inadvertently install the flexplate in an incorrect position because he only had a one in six chance to install it correctly if he wasn’t aware of this potential error. This is a very easy mistake to make but with serious consequences for externally balanced engines.

The fix is easy and does not cost anything except the work to remove the transmission and converter and look at the relationship of the flexplate to the crankshaft flange. What we would expect is that the flexplate is off by one or more bolt holes and that’s what is causing the vibration.

It’s worth noting that you should not drive the vehicle until this problem has been resolved. We had a similar experience where a 400ci small-block Chevy was balanced and the flexplate was improperly installed. This caused a minor imbalance that was ignored and after six passes down the drag strip and roughly 100 miles of street travel, the main bearings overheated and the block was ruined. Right at the end of its life, the engine started to squeak! Clearly, this is an issue that must be repaired before any miles are put on your 454.

Another item to watch out for is to always install the flexplate in the correct front-to-rear orientation. All flexplates offer a raised portion to mount the torque converter to the flexplate. It’s very easy to install the raised portion backwards or toward the engine and you may not notice the error until the transmission is completely installed. This will require removing the trans and converter in order to correctly install the flexplate.

This proper placement of the flexplate is doubly important on externally balanced engines like the 454. If the flexplate is installed backwards (with the raised portion of the flexplate pointed toward the engine) the external weight is relocated roughly 20 or 30 degrees from its intended location. That would also cause the vibration you mentioned.

Hopefully one of these solutions will solve your problem. If the dowel pin is not installed, merely line up the matching hole in the flexplate with the dowel pin location, make sure the raised converter mounting lugs are aimed at the converter and re-install the trans and your problem should be solved.

All flexplates for GM engines employ a raised portion on the flexplate to help locate the torque converter. Be sure this raised pad is pointed toward the converter. Especially with regard to externally balanced engines like the 454, if the flexplate is installed backwards, that error also relocates the external weight by roughly 20 degrees and that will create a vibration. (Image/Jeff Smith)
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2 Comments

  1. Bart Logan says:

    I’d also pull the belts and run the engine on the off chance that maybe the cooling fan isn’t an issue.

  2. Is there a possibility that a 400 flexplate was used. Some of the aftermarket flexplates have a weight that may appear to look like the 454. I’ve seen this a few times. For $50 its worth trying a new one. JMO

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