Distributor Installation - Don't Just Drop It In

There are pitfalls in distributor installation that may cause problems somewhere else. Many years ago I had a Chevy short-block. Deep in the cast-iron caverns of the engine there lived an evil wizard. I never saw him, but I could often find a trail of distributor gear dust where he had finished his last meal. I never found the wizard, but I know that he went through a relocation process involving intense heat and a total transformation.

All this transpired some time ago. It inspired me to learn about all those things I didn't know about distributor installation. Specifically, I'm referring to the use of HEI distributors in Chevy V-8s. However, some of the following information applies to other ignitions.

In the beginning, the vertical clearance between the gear and the bottom of the distributor housing must be within certain limits. When holding the distributor housing in one hand and moving the gear vertically, there should be 0.025-0.030 inch of clearance. Steve Davis of Performance Distributors assured me this was the correct amount. Steve says more problems occur when trying to adjust this clearance tighter rather than leaving it in the above range.

At this clearance, the reluctor teeth will stay in close alignment. This wasn't true on older point-type ignitions that had to run much closer tolerances to open the points at the proper time.

Notice on the bottom of the housing that there are some steel shims/washers. If the internal clearance needs adjusting, this is where you do it. Keep the thinnest shims in the middle of the shim pack. When changing shims, the gear will need to be removed. Do this by driving out the pin holding the gear to the shaft. The best way is to close the jaws of a vise to about 1/2 inch. Then lay the smooth part of the gear on this. Don't allow the gear teeth to rest on the vice. This can easily break a tooth, or more. It is helpful to have an assistant to support the big end of the distributor while you drive the pin out with a small-diameter punch. The gear and shaft need not be marked for reinstallation. During reinstallation you can visually detect if the pinhole in the gear will only line up one way with the hole in the shaft.

Next, the vertical clearance of the installed distributor must be checked. Check the position of the oil pump driveshaft slot. This must line up with the distributor shaft tang. A long screwdriver can be used to rotate the oil pump drive. Since the gear teeth on the distributor are spiraled, you will need to align the tang and slot, then back the distributor up one tooth. This will allow you to drop the distributor in where you want it.

Assuming you put on one gasket, install the clamp. Snug the clamp, but don't tighten it. Grasp the plate on top where the advance weights are. There should be some vertical slack here. If there is no vertical movement in the shaft, then the shaft is bottomed out against the oil pump drive. This can cause a number of problems, including distributor gear wear and oil pump wear. When heads or manifolds have been milled or replaced, it is imperative that this vertical clearance be checked. Resist the temptation to stack on more gaskets. According to Davis, these will collapse in time and allow the clamp to loosen. Since a distributor has no memory of where it should be, the distributor base will rotate and change the timing when the clamp loosens. A creeping distributor can be tantalizingly difficult to diagnose. The solution is to get nylon distributor spacers. They are made by Moroso and are available through dealers. They come in a package of three in thicknesses of 0.030 inch, 0.060 inch, and 0.100 inch. Use the spacers as needed to get the correct vertical clearance.