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Replacing a BRONZE BUSHING -- UT-104

Some uses of bronze bushings are discussed in the prior lesson. Many of the cast of sintered bushings are finished to size close enough to be installed and used with no machining after installation. This is usually true with parts that are not press fit in the housing. The small bushings in the MG windscreen wiper drive and heater motor are like this. They may be slip fit on the OD and held in place by a small metal clip, or by a thrust washer, or by a shoulder on the mating shaft. In the MG gearbox the cast bushings on the mainshaft are retained between a shaft shoulder and a thrust washer and keyed to the shaft to prevent rotation of the bushings. These parts are simply assembled into position and run as received.

When a bushing is held in the housing by a press fit, it may or may not require finish machining after installation. This depends on whether the bushing was finished to size to begin with, and how much it might be crushed on installation by the interference fit in the housing. The bushing in the tail end of the generator is a light press fit and will usually work okay as installed with no other attention. Bushings used in the starter motor may have similar fit and may not require post installation machining, but I have never had cause to repair a starter motor, so I couldn't verify that.

Solid cast bushings are usually not very tight in the housing, because the bronze material is not particularly strong and may be crushed a bit during installation. When these are installed in a blind hole, they can usually be removed by running in a thread tap slightly larger than the bore of the bushing, and then either pulling the bushing out with the tap, or screwing in a bolt to use as a puller. The tail bushing in the generator can be removed this way, as well as the spigot bushing in the crankshaft. Many slide hammers have a fixture piece to attach a ViceGrip that may be useful for pulling on a tap or a bolt, or a 2 or 3 jaw puller may be used to pull a bolt. There is a trick for removing the spigot bushing in the crankshaft if you don't have a tap of the proper size, and if the bushing is not too badly worn. You can pack the bore full of grease, then place a snug fitting flat nose punch in the bore and strike it with a heavy hammer. The hydraulic pressure created by the hammer blow may work against the back end of the bushing to push it out of the hole. The grease may spatter out of the impact zone with some velocity, so wear goggles and maybe cover the punch with a shop rag.

The spigot bushing in the tail end of the MG crankshaft might occasionally work as installed, but more often than not this one will get crushed a bit during installation and end up with a tight fit on the shaft. A clutch alignment tool should be an easy slip fit in the bushing. If you have a loose gearbox input shaft lying around, that may be the best measure of a running fit. If this bushing fits tight on the shaft it will disable the operation of the clutch, because the gearbox input shaft will remain turning with the crankshaft when the clutch is released. It is NOT a good time to discover this after the engine is installed, so it bears checking immediately any time a new spigot bushing is installed. The tradition way to finish the proper bore size of the spigot bushing would be to run in a reamer of the appropriate diameter. This would be 5/8" for the MGA and early MGB, which by coincidence is the same as the rocker shaft bushing finished size.

Bushings like this which are very close to but not exactly at the finished size might be burnished to the correct size by driving in a smooth surfaced finishing punch of the appropriate diameter. When you are concerned about the 4th decimal place in the diameter dimension, figure that the bushing bore will spring back (shrink) about 0.0003" when the burnishing tool is removed. Burnishing works well if the bushing bore is in the range of .001" to .004" undersize, which is often the case after installing a press fit bushing. The burnishing tool may be a smooth cylindrical punch. A ball bearing ball works well for a burnishing tool when you have a through hole where you can just push the ball through and out the other end of the bushing.

Bushings are typically finished to nominal diameter, so they can be finished with a standard diameter reamer or burnishing tool. The mating shaft will normally be a bit undersize to provide running clearance. The MGA and MGB rocker shaft and the pilot nose on the MGA and early MGB gearbox input shaft are about 0.623" diameter to give 0.002" running clearance in the bushing. So driving an old rocker shaft into the spigot bushing is not going to give the correct finished size to the bushing, and would leave it too tight on the gearbox shaft. To burnish that bushing you would have to us a true .625" diameter punch. And if you do that, you might expect the tool to be a tight fit and possibly be hard to extract after being driven into the bushing. If you finish this bushing with a reamer, you need to hold the tool perfectly straight.

For using reamers, lubricate the cutter edges with plenty of oil and turn in the forward direction only. Turning a reamer backward can dull the cutting edges.

The MGA and MGB rocker arm bushings are also to be .625" finished diameter, but these will generally need to be reamed to finished size. The lined bearings are commonly fabricated from flat stock which is a thick sheet of steel with a thinner layer of bronze. This is cut into a strip with width equal to the length of the bushing and length of the strip equal to the circumference of the bushing. It is then rolled up into a cylinder, and possibly burnished on the OD to assure that is round. These bushings will generally be made intentionally undersize on the ID to allow for final machining after installation. Rocker arm bushings are made in this manner, as well as the original type camshaft bearings, MGA and MGB front suspension trunnion bushings, MGB kingpin bushings, and the tail bearing in the MGA 1500 gearbox. All of these bushings require reaming to finished size after installation. See instructions for rebushing rocker arms and for rebushing front suspension trunnions.

For cam bearings and MGA 1500 gearbox tail bearings, I have a couple of suggestions to make life easier (and less expensive). The MG engines were originally mass produced with proper production tooling. These relatively cheap roll formed cam bushings were pressed into place and finish reamed with a long line reaming tool that had pilot guide diameters and multiple reaming cutters. This would quickly ream all three cam bearings to finished size in a single pass, and the tool was self aligning. This tool was similar to out current kingpin reamers (but larger). Unfortunately many engine machine shops (most) do not have a copy of this particular special stepped cam bearing reamer for every type of engine. So the shop has to resort to the more expensive process of line boring, or possibly assemble a guided cam bearing reamer from component parts (arbor and pilot and reamer shells), or maybe spend substantial time honing the cam bearings to finished size. None of this is cheap.

Fortunately there are available some aftermarket precision cam bearings which are made in the form of a closed ring, possibly cut to length from a piece of tubing with the bronze (and babbitt) lining. As received these will be pre-finished to the intended final size. With a little luck these may be pressed into place and still have the proper running fit after installation, not requiring any finish machining. At worst they may require just a little light honing or burnishing to return them to proper bore size after installation, so the labor charge may be much less. The machine shop should be able to procure this type of cam bearing for your engine. For the MGA and MGB B-series engine this 3-piece cam bearing set is Federal Mogul #1307M.

The MGA 1500 gearbox tail bearing has been made of "Unobtainium" for a long time. The original part is 1-3/8" ID x 1-1/2" OD x 2-3/4" length (1/16" wall thickness), and rolled up from bi-metal flat stock. If this part was available new (which it isn't), it would have to be pressed into place and reamed to final size. I have in recent years successfully replaced these bearings with solid cast bronze bushings (local bearing supply house). These are available in the proper ID and OD, but not in full length. So I will buy three pieces which are 1-3/8 x 1-1/2 x 2" long, then cut two 3/4" long pieces from one of the parts, and install one 2" long piece and one 3/4" long piece end to end to make up the required 2-3/4" length. The three 2" pieces will ultimately replace two original bushings. These cast and pre-finished bushings will be a light press fit in the housing, easily to install (after a fight to remove the original bushing), and may require only light honing to give the proper finished size in assembly.

Now you see you can do most of this work in your home workshop. If anyone has further suggestions for addition notes here, do tell.

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