Introduction: Lubricate the Gearhead on a Stihl FS56 Trimmer

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About: I miss the days when magazines like Popular Mechanics had all sorts of DIY projects for making and repairing just about everything. I am enjoying posting things I have learned and done since I got my first to…

The Stihl FS56 string trimmer is a popular model with homeowners. But, there is some frustration because the gearhead has no grease fitting or plug for adding lubricant, and owners have some anxiety about early failure of the gearhead. This Instructable shows a non-invasive way to add grease for the gears.

Step 1: Disassembly

(First photo) Use the TORX tool provided to remove the mounting screw for the shield. Lift the rear end of the shield and remove. (Second photo) Insert a rod into the hole on the gearhead so it goes down into the stringhead to lock the stringhead from turning. A medium Phillips screwdriver works well. Remember that the threads for the string head are left hand or reverse. (Third photo) Use the TORX tool to loosen the clamp screw on the gearhead. Twist and pull the gearhead from the drive tube.

Step 2: Dead End Not to Be Pursued

The photo shows a snap ring on the bearing that holds the vertical shaft in the gearhead. Removing it does nothing. The bearing does not fall out, even with tapping from a hammer. The splined shaft cannot be pulled out because its gear is above the gear on the angled shaft. Do not concern yourself with removing snap rings.

Step 3: A Discovery

The first photo looks into the gearhead’s collar for the drive shaft. Although difficult to see because of grease and problems with lighting the inside of the collar, at the center is a square socket to receive the square end of the driveshaft. There is a snap ring under the visible grease and I wanted to remove the snap ring. I think removing the socket and gear for the angled shaft would allow adding grease to the gearhead, but I found an easier way. (The snap ring under the grease would require a snap ring pliers with tongs at least two inches long. I contemplated making one in my shop, but noticed something before I could begin.)

See the second photo. As I was contemplating how I could remove the snap ring I turned the vertical shaft by hand. I noticed grease inside the collar for the drive tube was growing in volume. Somehow grease was migrating from the gears to the space inside the collar. It seemed to me that what comes out could also go in. I used some thin steel wire about 20 or 22 gauge in size to test the square opening for the driveshaft. It is open on its inner end. The second photo shows how much wire I was able to feed into the square socket. As I turned the splined shaft I could even feel the teeth from one of the gears tickling the end of the wire. I decided to try pushing new grease through the square opening for the driveshaft.

Step 4: Pushing Some Grease

This is admittedly a tedious process. Put some fresh grease onto the end of the driveshaft and carefully push the driveshaft into its square socket inside the gearhead. Then turn the splined shaft so the grease is pulled from the end of the shaft inside the gearhead and does not simply come back out with the shaft. You can also try using a finger to push some grease into the square socket. Then push the driveshaft into its socket with or without grease loaded onto it. Again, turn the splined shaft by hand before removing the shaft from its socket.

I have a piece of clear plastic tubing with an internal diameter of 3/8 inch. I removed the nozzle from a small grease gun I have and slipped the tubing onto the gun’s tube. The open end fits over the fitting for the square socket that receives the driveshaft, although just a little loosely. If I hold the plastic tubing firmly I can pump grease into the square shaft socket. Some grease does ooze out of the receiver for the square shaft around the bottom of it. Perhaps that means the gearhead is full of grease. (I did also notice quite a bit of grease inside the end of the driveshaft tube. I am not sure what that means.)

I do not know how often grease needs to be added. I trim a grassy area too rough for a mower and each time I do I am using the trimmer for about 30 minutes. I will likely try to add grease at least every other time I use the trimmer.

Step 5: Assembly

Assembly is mostly the reverse of disassembly. The photo shows the steel flange that goes onto the splined shaft before the string head. The side shown goes on toward the gearhead and away from the string head.

I want to make a mark on the drive tube so I can more easily align the gearhead in one attempt.