“What I Learned Today” With Jeff Smith — Don’t Trust Timing Marks

Chevy guys all know that nearly everything in a small-block Chevy is interchangeable. In most cases, that is probably true – but that doesn’t mean that swapping parts around is a good idea unless you are aware of the hidden dangers. This episode occurred while we were building an early small-block for a ’64 El Camino. This required rebuilding the car’s original 283. The factory balancer had seen better days, so we ordered a new small diameter, stock replacement balancer and found that the timing marks didn’t line up.

The early 283 and 327 small-blocks used smaller diameter balancers – generally around 6-3/4-inch in diameter. We found a suitable replacement balancer that was very affordable. As small-block guys know, these engines use a timing tab that was welded to the timing chain cover. We wanted to use the same diameter balancer so that we could retain the stock timing tab. After installing the balancer, we decided it might be a good idea to double-check that the TDC mark on the balancer lined up with the 0 mark on the tag with the piston at TDC.

We had already installed the heads, so we used a screw-in piston stop in place of the spark plug. We placed a piece of tape over the balancer and marked where the piston stopped on both sides by turning the crank over by hand. We then compared the mid-point between those two marks to the original TDC mark on the balancer. We quickly discovered that the mid-point mark on the tape was 12 degrees After Top Dead Center compared to the 0 or TDC mark on the balancer. This was using the zero or TDC mark as reference from the timing chain cover tab. Clearly, the TDC mark on the balancer was off.

We used a spark plug style piston stop and marked the stop points compared to the 0 mark on the timing tab. We discovered that the new balancer’s timing marks were 12 degrees retarded when using the original tag.

The story behind this is that prior to 1969, Chevrolet aligned the woodruff key with the TDC mark on the balancer. But starting in 1969, they moved the mark roughly 12 degrees retarded from the woodruff key position. Now some 50 years later, most replacement balancer manufacturers just use the later model position for all balancers. We have not purchased all the different balancers to check this, but the point is that if you do not double-check TDC, it’s possible that you could get tripped up.

The result will be that what is indicated as 12 degrees initial timing will really be 0 degrees. This means the engine will always be lazy and will certainly run hot. The solution is very easy. We merely used a later model bolt-on timing tab with updated timing marks to replace the old tab. This relocates the TDC mark to align with the moved TDC mark on the balancer. Unfortunately, these tabs are intended for a larger, 8-inch diameter balancer, so there will be some space between the tab and the balancer that can cause some minor reading errors, but it’s not nearly as bad as being 12 degrees retarded!

This falls under the category of an important engine builder by-law — trust, but verify.

In this photo, we’ve placed the newer bolt-on timing tab over the original one. You can see that TDC has moved. The older tab shows 0 just below the bottom of the new tab, with the timing marks aligned. This is why verifying TDC is so important. You can’t trust that everything will always be as you expect.

About the author

Jeff Smith

Jeff Smith, a 35-year veteran of automotive journalism, comes to Power Automedia after serving as the senior technical editor at Car Craft magazine. An Iowa native, Smith served a variety of roles at Car Craft before moving to the senior editor role at Hot Rod and Chevy High Performance, and ultimately returning to Car Craft. An accomplished engine builder and technical expert, he will focus on the tech-heavy content that is the foundation of EngineLabs.
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