Repair - Oil Pump Shaft

This project originated with a Craigslist posting from an individual asking for help to repair the above parts of an oil pump. Per the owner: "This oil pump shaft is from a late 90's Simplicity Sovereign garden tractor engine. I believe they only used this engine in their 1997 to 2000 model years. It is a Kohler TH18S engine that was discontinued. I was told by a Simplicity dealer that Kohler had many problems with this engine so they discontinued it. He told me because they discontinued it so long ago many parts including the oil pump are no longer available."

The two parts are shown separated at the left - a shaft that fits into the oil pumo housing, and a toothed belt pulley with an integral key that matches a flat on the end of the shaft.

This photo shows part of the problem in more detail. The interior of the pulley is badly worn and the integral key has a deep groove worn into it.

The nominal shaft diameter is 12mm, but the hole in the pulley varies in ID from 12.09mm at one end to 12.43mm at the outer end, so that it is actually tapered.

Similarly, the end of the shaft (the left end in this picture) is also badly worn. The shaft diameter ranges from 11.85mm at the inner part to 11.75mm towards the outer end. In short, it is also worn in a taper in the opposite direction to the hole in the pulley.

In addition, the flat on the shaft has been badly word and rounded over.

Note also at the far left end of the shaft is a very small section (~1mm) of shaft that has not been worn because it protruded from the pulley hole; the diameter of the very short section is 12mm.

Here's and end view of the shaft, showing a threaded hole in the end (M6 x 1.0), and an end view of the flat on the shaft.

As a result of the extreme wear on the shaft and pulley, the pulley wobbles badly on the shaft (which is likely leading to even more wear at an accelerating pace). Probably the best way to repair this assembly would be to build up the shaft and pulley hole with welding, turn them back to size, and install a new key. However, as I do not have the required welding equipment I came up with another solution.

The proposed fix for these problems:

1. Mill the flat on the shaft to smooth it out
2. Turn the shaft to remove wear and obtain a contestant diameter
3. Bore the pulley to remove the integral key and slightly enlarge the hole to a constant diameter
4. Plug the pulley hole and then drill and bore it out to match the internal ID to the new shaft OD
5. Cut a key slot in the pulley and install a square key

1. Mill the flat

The shaft was clamped in the mill vise between two vee blocks. Using an indicator on the unworn end of the shaft, the flat of the shaft was oriented parallel to the mill table.

A total of 0.021" was milled from the top of the flat - this was sufficient to remove perhaps 90% of the wear. The remaining small amount of wear was left untouched so as not to reduce the shaft diameter too greatly.

Here is the completed repair of the flat.

2. Turn the shaft

In order to be able to support the end of the shaft for turning, a small fixture was made with an M6x1 thread on one end and a center hole on the other end.

Her you can see the above fixture between the shaft and the dead center; the section to be turned is the narrow section in the middle.

Note that an aluminum shim was used between the shaft and the chuck jaws to prevent marring the shaft.

Note the "frosted" band of wear on the large diameter of the shaft.

4. Plug the pulley hole and re-bore

First attempt

Note: This first attempt failed, as will be seen. A second attempt using a better approach succeed.

I turned a small section of rod from my scrap bin to a few thousands of an inch greater diameter than the new ID of the pulley. After allowing the piece to cool to room temperature, I took numerous spring passes on the piece until it was ~0.001" oversize. I then used abrasive cloth to further reduce the diameter until the "slug" jsut barely fit in the pulley.

After parting of the slug to slightly oversize length, I cleaned it with de-greaser and acetone (along with the pulley), then coated it with Loctite 263 (red Loctite) and pressed the slug into the pulley with an arbor press. The Loctite will need to cure overnight (24 hours, or a bit less if the piece is kept warm).

After allowing the Loctite to set for 24 hours, the plug was faced and center drilled.

Drilling out the hole started with a 1/8" drill, with additional drills in increasingly larger sizes. This was done in order to reduce the heat build-up from drilling, as well as to reduce the torsional force on the plug. After each drill size, the piece was allowed to cool down to room temperature before proceeding to the next size.

This process was repeated until the drill size was just slightly smaller (7/16") than the required final diameter.

The final required diameter was achieved using a boring bar. As the final diameter was approached, cuts were limited to 5 tenths (0.0005"),and multiple spring passes were made to ensure a constant ID.

Once the final ID was reached, the small extra length of the plug on each end was removed by facing off with the boring bar.

5. Cutting a key slot

The final step is to cut a key slot in the pulley.

Two key options were considered as shown. On the left is a standard 1/4"key; this option leaves small gaps on each side of the shaft flat.

On the right is a custom key made the same width as the flat width; this key leaves no gaps.

The choice made was to use a standard key for these reasons:

• The small gaps left by a 1/4" key are insignificant
• Using a standard key size makes key replacement easy if the key is lost or damaged

The goal will be to cut a key slot which provides for a very tight fit, in order to eliminate any wobble of the pulley, as well as to help counter any starting torque. The pulley is held onto the shaft by a screw and washers - this will also serve to keep the key in place.

I set the pulley up on my metal shaper to cut the slot for the key. A purpose built tool made for cutting 1/4" slots was used, and the pulley was set up to cut the slot upwards.

I was concerned that the force of the shaper cutter might push out the new bushing, even though I was taking very shallow cuts (around 0.002" per cut). The cutter was almost through the bushing and sure enough, it got pushed out (likely because cutting through the bushing relieved some of the pressure holding it in).

Fortunately it should be possible to reinstall the bushing and Loctite it back into place.

In the interim, machining of the key slot was completed.

In fact, I was able to re-install the bushing. However, I then found that the key slot was not deep enough so I had to re-mount the piece in the shaper and cut the slot deeper.

Unfortunately, in cutting the slot deeper, the bushing was pushed out again with even worse consequences - see the photo at left. Very annoying, but fixable. Of course the fist bushing is a total loss, but it will be easy enough to make a new one.

4. Plug the pulley hole and re-bore

Second attempt

Note: This second attempt was improved in these ways:

• I used O-1 tool steel for the plug/bushing
• A tool steel rod was turned to the required OD and the pulley was mounted on it using JB Weld epoxy
• The ID was drilled and bored while the pulley was mounted on the rod - this made for simpler work holding and better concentricity

In fact, it only took about 15 minutes to turn a new metal piece down to size.This time, I am using JB Weld epoxy instead of Loctite. In addition, I will bore out the busing with the pulley epoxied in place. This will not only make it easier to hold the assembly for boring, it will also ensure maximum concentricity.

The final step will be to remove the section of bushing blocking the key seat. This will be done on the mill as will be seen later.

The ID was drilled a bored as previously.

This produced a the piece as shown at left. The next step is to remove the "web" blocking the key slot.

An aluminum plug was inserted into the ID for reinforcement, and the pulley was mounted on a fixture plate on the mill. A 1/8" OD end mill was used to remove the web - thise required cutting into the aluminum plug bt since this was a sacrificial piece it didn't matter.

Since this was a fairly deep cut for such a small diameter end mill, successive cuts were made with a 0.020" depth of cut so as to avoid too much side loading on the end mill.

This method succeeded, but it did result in loosening the bushing. Therefore I removed the bushing, cleaned off the old epoxy, and re-cemented it back into place using the axle to hold it while the epoxy set.

The new key slot was intentionally cut undersize - with a bit of hand filing I was able to get a good tight final fit.

To the right of the pulley you can also see a new washer I made.

The problem with this old washer set is that the end of the shaft is about 0.085" proud of the side of the pulley, so the small bolt tightens the washers against the end of the axle, but does not tighten the pulley onto the shaft. The spring washer might help somewhat, but would also allow the pulley to wobble on the shaft once it started to wear.

I made a thick cup washer which straddles the end of the axle and presses the pulley tightly against the shaft. I couldn't tell how worn the old bolt was, so to be on the safe side I replaced it with a new socket head cap screw and small washer.

Note that I did not use a split "lock" washer as studies have shown that these actually don't do a very good job of preventing loosening. I will advise the owner to use blue Loctite instead, which will do a better job of keeping the screw in place but also allow for removal if necessary.