Ensuring the proper mesh is...
Ensuring the proper mesh is placed between the distributor and driveshaft gears won't make more power, but it will help your engine live longer.
There's no doubt that plenty of racers feel the ignition system is the area of the engine that gives them the most trouble. Is the timing right? Am I getting enough spark to the combustion chambers at high rpm? Is everything wired correctly?
Yes, the electrical part of the ignition system can be difficult and confusing at times, but don't allow your focus on the electrical component to cause you to ignore the mechanical component when it comes to keeping your distributor working the way it should. In many cases, quality racing distributors come from their manufacturers practically "plug and play." For example, the internal bushings that hold the shaft steady are already lubricated. But one thing that no distributor manufacturer can do for you is make sure you have the proper mesh between the distributor and cam gears. Racers spend a lot of time and money ensuring their ring-and-pinion gears mesh as perfectly as possible, so doesn't your distributor gear deserve a little more respect than simply slamming the unit in place and tightening up the hold-down clamp?
Because the distributor mounts...
Because the distributor mounts to the intake manifold in Chevrolet engines, machining processes that can change the height of the intake relative to the camshaft can cause the distributor gear to bottom out on the oil pump driveshaft and cause wear problems.
Wear Versus Destruction
The gear that drives the distributor lives in a very difficult environment. First, in stock form, the only oil that lubricates the spinning cam and distributor gears gets there by the practically random splash effect. Second, in a wet-sump engine, the oil pump is driven off the distributor shaft. That means the resistance that the distributor gear applies as the cam gear tries to turn it comes not only from spinning the distributor shaft, but also the oil pump. If you are running a high-volume oil pump or racing on cold motor oil, this can cause tremendous pressure on the system. Even if you don't run a high-volume pump, the tight bearing tolerances used to increase oil control and high rpm levels seen in modern racing engines still cause the pump to work very hard and put extra resistance on the distributor gear.
Finally, a distributor gear has a tough life because it's designed to be sacrificed if things go wrong. It's a lot easier to pull the distributor and replace a worn gear than it is to replace the camshaft, so the distributor gear should always be made of a softer material than the cam gear. If you are racing a hardened flat-tappet camshaft, an iron distributor gear should be fine, but if you are racing a roller cam, you need to go with a bronze gear. Whichever type you run, the softer distributor gear also wears some during engine break-in to help the two gears mesh properly. Some racers fear that a bronze gear will wear too quickly and is undependable, but if the distributor is installed correctly, it will last a full season under even the worst racing abuse, and the particles that are worn off the gear are so fine that they do not hurt the engine.
An excessively worn distributor...
An excessively worn distributor gear such as this one will display razor-like gear teeth. Photo courtesy of Performance Distributors
The Chevy V-8 is, by all accounts, a fantastic engine design, but it also has its quirks. One is that the distributor mounts to the intake manifold. This means the distributor's installed height is determined by the height of the intake's distributor mounting boss. In stock form, this isn't a problem, but many modifications can be made to the engine in search of power-such as decking the block for a higher compression ratio, as well as decking or angle milling the heads-and all can require cutting the intake to fit and change the height relative to the camshaft. This isn't as much of a problem on Ford engines, where the distributor shaft collar seats against the engine block and not the intake.
When this happens, it raises the possibility that the distributor will bottom out on the camshaft gear and oil pump driveshaft. The same problem can also arise (no matter what type of engine you are building) if you use a replacement oil pump driveshaft that's longer than necessary. This puts extra pressure on the entire system and causes destructive wear on the cam and distributor gears, and the pressure can even be transmitted down to the oil pump and bind that up, too.
Before checking for free play...
Before checking for free play in the distributor shaft, remove the cap and rotor so you can reach the top plate of the shaft. Since this is securely fixed to the shaft, it won't give you false movement.
Steve Davis of Performance Distributors says checking for proper distributor gear depth is critical, especially on Chevy engines, but it isn't that difficult to do. "When you install a distributor, you need to check to make sure it isn't bottomed out," he says. "You can do this by installing the distributor in the engine without any gaskets. If you haven't already, pull the cap and rotor so you can reach the top plate for the main shaft where the advance weights connect.
"With one hand, hold the distributor firmly against the intake and use the other hand on the top plate to see if there is any up-and-down movement. Make sure you do not grab the reluctor because this slides on the shaft and will always have movement. If you can move the distributor shaft up and down a few thousandths, you should be OK. Put the gasket on, set your timing, and you are good to go. If you don't have any movement, you need to shim the distributor to get it up some so it won't be bottomed out."
Performance Distributors-and most reputable distributor manufacturers, for that matter-sells a nylon shim kit to help you achieve the correct distributor height for your engine. Davis warns against simply stacking gaskets on top of each other to get the correct height. Over time, the gaskets will compress and the gear mesh problem you thought you had fixed will return. Also, as the distributor gear wears, you want to see a nice even wear pattern centered vertically on the gear.