Custom Pinion Gear Replacements

Making patterns for casting a new pinion gear.


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Have you ever been out on a sunny day, the steam is up on your engine, you're lined into the belt, the block is set, and you see an older gentleman looking over your engine like a jeweler in a penny-ante pawnshop? You try as hard as you can not to let him distract you, because what can mess up a perfect day like this? After he has opened the smokebox door he walks around your engine several times. With much deliberation he finally steps up to you, sucks the saliva out of his top dentures and says, "Well sonny, you got yourself a nice engine, but she's shot in the gearing."

Well, he is probably right, but then show me a piece of old farm machinery that is almost 100 years old without a little wear and tear. Nevertheless, what do you do about it?

There are many things that you can do, one of which is smile and agree with him, and hope that you out live him so you don't hear that kind of stuff anymore. But this article is for those of you who want to repair the gearing on your engine. Keep in mind this is only one of several gear solutions. It is meant to give you some ideas and hopefully help in some small way. The information is based on my triumphs and tragedies, none of which is etched in stone. I have to say the best time to address the gearing is when you are doing a complete restoration of the engine, because you have the engine torn apart anyway. But if your intent is to just repair gearing then any time is fine. I probably don't have to tell you that no matter when you do it, it's work!


The gearing that generally shows the greatest amount of wear would be the pinions gears. Most all transmission of power is transferred through a pinion gear. They are smaller gears, and over the years have made a lot more revolutions than their counterparts. There are other reasons why they have more wear on them, none of which are important to this article. They can show plenty of wear, causing any old codger to call you out on them. This is what I will address in this article, and how to replace them.

My engine is a 22 HP Wood Bros. It was a complete restoration, and the hind wheels had to be removed to get to the pinion gears and differential. The hind wheels are a little wider than most engines. Taking them off and moving them around wasn't too bad, but if you can get help, do it.

To get to the crank pinion, the flywheel had to come off, along with the clutch. Because of the flanges on the crank pinion, the bearing caps on the crank journals had to be loosened enough to raise the flywheel end of the crankshaft. This would allow enough room to slide the crank pinion with its flanges out over the intermediate gear and off the end of the crankshaft.

Once the pinion gears were out and off the engine, the hard work was over. The two pinions at the ends of the counter shaft are the ones I will address first, because these gears are the simplest to correct. The objective is to use the gears that were taken off the engine and use them as patterns for new ones. Now keep in mind that you can do this with the smaller pinion gears, but you can't do this with the larger bull gears. Cast iron shrinks 1/8-inch per foot and the shrinkage for the bull gears would be too much. The problem is that you would lose the proper mesh between the pinion gear and the bull gear. The bull gear would be a different animal. To replace them, a free pattern would have to be made using a shrink rule. However, most pinion gears are less than one foot in diameter and the shrinkage would be somewhat minimal. So using them as patterns is doable.

The first thing these gears need is a good cleaning - they have to be free of grease. I cleaned my gears with a wire cup brush and a 4-inch grinder. You can use a sand blaster if you have one. It will be a lot easier, but the wire brush works fine.

You'll use automotive body putty to reshape the gears, and the putty has to stick to the teeth that you are going to build up. The cleaner the gear, the better, but the main thing is to get the rust, dust and grease off. Each tooth will have to be brought back to its original shape before the gear can be used as a pattern. You might be wondering how you can get the original shape of the tooth back. Depending on where the mating gear rode on the pinion, the original shape of the tooth should be either out at the face of the pinion gear or at the hind end.

The photo above shows what is called a tin cut. A tin cut matches the shape of the front of one tooth and the backside of the adjoining tooth. This is important to have, and important to make. You will be using this tin cut to pull between the gear teeth. This will help you shape the body putty as you are building up the teeth. This is important because it will eliminate a lot of sanding and filing. Remember, it is easier to take the body putty off and shape the tooth than it is to shape the tooth after it has been cast.

In my case, I plugged the gear's bore. I did this with wood I turned on a lathe, then stuck in the bore, covering the ends with body putty. Once you have the pinion back on the engine, assemble the adjoining gear and turn both gears by hand, one full revolution of the larger gear in both directions slowly. If there is any interference it will show up at this time. If there is, you will have to die grind clearance on the high spots of the new gear only.


Now it is time to address the crank pinion, and as Art Carney would say, "Hello crank pinion." The same method can't be used for this gear because of the flanges on either side of the gear. This gear has to be made in three parts: The two wooden flanges and the wooden gear body. The final gear body had to be made from foam because the foundry I was dealing with couldn't draw a wood body from the core sand. The main reason was they were using an oil sand core with a cereal binder. I think if they would have gone to an air-set, and waited until it turned somewhat plastic before they pulled the gear body, a wooden gear body would have drawn from the core. The foundry I was dealing with didn't have air-set, which is why I had to make the gear body out of foam. In all fairness, the wooden gear body didn't have any draft so it would have been a hard draw anyway. Nonetheless, I found another foundry that embossed the foam gear in a tapered block of air-set core sand as shown (Figure 1).

Once the foam gear body was embossed in a tapered block of air-set sand, the block of sand was placed upside down with the taper up. The wooden flange pattern clutch side was centered over the foam gear body. Then the drag flask was set over all and rammed with molding sand and struck off so when the drag flask was turned over, it would lay flat. Now the drag flask was upright (Figure 1). At this point the engine side of the wooden flange pattern was centered over the foam gear body. Then the cope flask was set over the drag flask (Figure 1) and the cope half was rammed with molding sand.

The process is reversed to retrieve the wooden flange patterns out of the mold. Gates and runner are cut in at this time. The tapered air-set core with the foam gear body is placed back into the drag. Both flasks are placed back together at the parting line and poured off. The foam gear will rapidly burn up when it comes in contact with hot iron. The end result is a new pinion gear on the end of the crankshaft.

These gears were poured with gray iron known in the foundry as C-30. This means it is a gray iron or cast iron with a 30,000-pound tensile strength. It has good machine-ability and good strength. This is a good mix for the pinion gear over the bull gear and a good mix for the crank pinion if you are just going to run your engine around the fairgrounds. But for you plow jockeys, I would recommend the crank pinion to be made out of something a little harder, maybe adding a little chrome to the melt. A good foundry man can help you with your mix to give you the properties you are looking for.

Well that is all I have for you fellows in Steam Engine Land. Until we talk again about triumphs, tragedies and pitfalls, remember to keep your steam and water up, your hand on the throttle, your eye on the gauge and Jesus Christ close to your heart!

Contact steam enthusiast Mike Murphy at 7115 W. Bleck Road, Michigan City, IN 46360.