Why you should index your tractor’s spark plugs.

Why you should index your tractor’s spark plugs.

February 24, 2021 0 By Ray Bohacz

Regardless of if it has shiny paint or is still in its work clothes, a collector tractor is living a life of leisure today compared to what it was designed for.

In some ways, that is good, but in others, it is not particularly good. A machine like a person needs to keep moving to remain healthy. And just like some things can be done to help people be healthier when less mobile, there are protocols for tractors.

Many cold starts (this defines any start with the engine at ambient temperature), short-run times and use under minimal load, puts in place a series of events that are less than desirable.

Any engine, when first started, requires an extremely rich mixture due to the reduced rate of vaporization of the fuel. This leaves fuel in a liquid form to wash the oil from the cylinder walls and collect in the crankcase, destroying the lubricity.

It also wets and eventually fouls the spark plugs, leading to poor running and hard starting.

Long periods of storage, even in a heated environment, allows moisture to wick into the oil. When it mixes with raw fuel, it creates acids and sludge.

When the tractor is eventually run, the short time and light-duty use do not allow the oil to get hot enough to evaporate off the moisture along with the collected gasoline in the oil pan.

Put these dynamics together, and it is not a pretty picture, engine-wise.

We can limit cold starting and short cycle, light- load use, but we cannot do much about the physics in play for gasoline to vaporize. But there is something that we can employ to hasten the time the engine is running with inefficient combustion. How may you ask? By indexing the spark plugs.

Taking a cue from early hot rodders

As much as we love and honor old iron, there is no denying that the combustion chamber design and technology leave much to be desired. The same holds true for early automotive engines that spawned the post-WWII era hot rodders.

It was recognized that if you could position the spark plug electrode in a specific rather than random orientation, two things would happen: each cylinder would experience closer to the same ignition event, and it would be easier for the mixture to ignite.

The process of controlling the clock position of the gap of the spark plug concerning the valves is called indexing.

It has been well documented that an older combustion chamber design that does not evoke a high level of mixture motion and has a slow fame speed can benefit from indexing the spark plugs.

Since the early racers were looking for power anywhere they could, indexing spark plugs became a common practice.

On the farm, this was not embraced since a tractor engine is a low rpm design that has the benefit of a high level of torque multiplication through gearing. Also, it was designed to run under load for long periods. Loading an engine creates heat.

Put all this together and the effort to index the spark plugs, though in theory increases efficiency, empirically was not worth the effort.

Plus, there was no practical way to accomplish repositioning the electrode in the bore since it was a function of the threads on the plug (called the reach) and their interaction with the companion threads in the cylinder head.

It was the early drag racers that applied American ingenuity and came up with the indexing washer.

These are application-specific washers of specified thicknesses that are placed on the spark plug threads which effectively alter the ending orientation of the spark plug electrode by acting as a spacer. They are used in conjunction with the spark plug sealing washer.

Remarkably simple to use but can be time-consuming and frustrating, just like connecting a three-point hitch.

Which way to face?

There are two distinctly different but accurate schools of thought on which way the spark plug electrode should face. The one you choose depends on what is trying to be accomplished.

Each theory references the gap between the side and center electrode, the discussion is, if it should face the intake or the exhaust valve.

One thought is that since the mixture enters the bore via the intake valve, it is best to steer the charge to the gap. By doing this increases the chances of igniting a lean mixture more efficiently, which is identified as the ionization process.

The other theory is founded on positioning the gap facing the exhaust valve, which is the hottest part of the combustion chamber. This is rationalized as the higher temperature will further vaporize the incoming charge and will improve flame speed.

In any engine, gas or diesel, a reaction zone needs to be established where the heat from the burned mixture travels to and raises the temperature of the surrounding area as the flame propagates across the bore.

With this established, the camp that favors the electrode bias toward the exhaust valve is concerned with the reaction zone.

Though the practice of indexing spark plugs is not that common today with race engines since the modern combustion chamber would benefit little if at all from it.

For those that compete with older engines, it is a routine procedure, and I feel that it should be embraced by the readers.

Before I explain how to identify and index the spark plugs, I will apply my thought to where the gap should face on a collector tractor.

If the tractor is used as described in the opening paragraphs of this article, then I would bias the gap toward the exhaust valve.

I would want the additional heat to help establish the reaction zone, be easier to create ionization, and to deter fuel fouling.

A spark plug once wet from a cold start will not begin to burn the electrode clean until it reaches 500 F degrees, and then the process is only initiated.

By placing the gap toward the exhaust valve, as soon as the engine fires, the temperature there will be higher than at the intake valve.

Identifying the orientation

The first step is to look at the cylinder head while on the engine and see where the exhaust port is concerning the spark plug hole. Your goal is to get the gap to point as much as possible to the exhaust valve.

The next step is to take the spark plug and using a black marker, make a line on the porcelain insulator straight down from the side electrode. The gap will be 180 rotational degrees from the mark you drew.

Choose an indexing washer and thread it down on the spark plug’s reach until it is seated. Thread the spark plug into the head and snug it.

The goal is to have the line you made to face the exhaust valve so that you can see it. If it does, you are done with that cylinder. Tighten the spark plug, install the wire, and go to the next cylinder.

If the orientation is incorrect, remove the spark plug and index washer and try one of a different thickness. Be patient and get it as good as you can.

Spark plugs, as is life, are always the most productive when they are pointed in the proper direction. Have fun!