The good aftermarket aluminum timing covers are fitted with a small tapped hole that can b
Matt Kopocs, Akron, Ohio: I hope you can help me solve a timing problem. I have a '73 Chevelle SS with a 0.030-over 454, a 200-4R trans, a 2,400-stall-speed converter, headers, an intake, stock oval ports, and a Comp XE274H cam. I have 38 degrees total timing in by 3,000 rpm. When I rev the engine to 3,200 or higher, the timing retards by several degrees and it jumps erratically. I have tried several different distributors, coils, and timing lights with and without the 6AL box with the same result.
I currently have a new 8361 MSD distributor installed, and the problem persists. The MSD website states that this could be caused by cam walk. I do have the stock timing cover installed. I can turn the crank 21/2 degrees before the rotor starts to move. The timing chain was not very tight when I put together the motor. I have since discovered that timing chain slop can be caused by align-boring, and my engine was align-bored. Do I need to get a non-tin timing cover, as the MSD website suggests? What is your suggestion for a timing chain? The car is e.t. unresponsive to different carbs, jetting, and so on. I am convinced the erratic timing is why. Hope you can help.
Jeff Smith: Because you included enough information in your letter, Matt, you've really answered your own question. We called our pals at Jim Grubbs Motorsports, the shop that does all my engine machining. Machinist Ryan Peart performs the align-honing jobs, and it seems they align-hone just about every big-block Chevy that comes in because the main bore saddles are off.
This is Cloyes' suggestion for measuring the center-to-center distance for any engine. The
Align-honing requires cutting the main caps slightly to create an undersize condition, and a long honing bar is used to ensure that all five main caps are in line with the same inside diameter (id). The same procedure is used for align-boring, which is mainly done when adding new main caps. In this case, more material is removed from the block and the caps. Peart says that most of the big-blocks he does require the block to be machined around 0.004 inch. This means the main bore centerline has been moved by half the distance, which would be 0.002 inch that also reduces the centerline distance between the crankshaft and the camshaft by the same 0.002 inch. As you can probably guess, this will create a loose chain condition.
You might think that once the engine is running, the chain will be loaded and even if it is slightly loose, the constant load will not affect the timing. The reality is that even at a constant rpm, the crankshaft is subjected to instantaneous accelerations and decelerations as each cylinder fires and pressure pushes on the crank. This is one reason all engines require a harmonic damper. Now add a loose chain between the crank and the cam, and it's easy to see how the cam and ignition timing will be especially erratic. It's also worth emphasizing that for every degree of engine timing evidenced with a timing light (caused by a loose timing chain), the cam timing is also moving around a similar amount. This means the cam timing is also retarding- this is not a good thing.
The solution is easy, however. We'll assume the machine shop took a significant amount out of the main saddle, requiring a shorter chain. The part number for a typical Cloyes Original True Roller timing set for the big-block Chevy is 9-3110. If you already have this timing set, you can order a 0.005-inch-shorter chain, which will compensate for the machining operation. It would be best to first determine how much material the machine shop removed from the main saddle area. If the shop can't supply that information, you can measure the center-to-center distance. We've included a simple drawing from the Cloyes website on the previous page that shows how to measure this distance. Measure the diameters of the crank snout and the camshaft journal. Then measure the overall distance of those two installed components (called measure over shaft or MOS) in the engine as shown. The formula is: center distance = MOS - (crank diameter/2) + (cam diameter/2).