Primary gear - bush replacement
With the cost of new primary gears spiralling ever upwards, and the availability of good, serviceable used ones, fitting new bushes to existing gears is becoming a more common solution. For some reasons for bush failures and rectification to stop it re-occurring, see article 'Primary gear - Bush problems'. Since there is no useful information given in any of the workshop or DIY manuals on this subject, following is my approach to dealing with the issue.
The first thing to get to grips with is which bush is which. This has added an element of confusion to many conversations I have had with folks on this subject, so - the FRONT bush is the one nearest the engine block, the REAR bush is the top-hat one nearest the flywheel. This is because the front of the engine is actually the radiator end - quoted in 'in-line' engine-speak. The REAR bush is the same on all primary gears - be they for small or large bore engines. The FRONT bush is different for each though; the small-bore one is smaller in diameter as there is no step in the crankshaft where the primary gear runs. The large-bore crankshaft steps up in size next to number three main journal for increased strength. Part numbers for each are displayed at the end of this article.
Getting the old bushes out isn't too much of a problem. The easiest way is to use a die-grinder or similar to carefully grind through the bushes at each end, 90-degrees to the crankshaft bore centreline. Take great care not to grind into the bush housings though! The bushes are then very easy to pull out. If the bushes fall out - this is bad news. It means they have been spinning in the gear, and will have damaged the bush housing size. It would be very foolish to re-use a gear that has suffered this failure. The only way to re-use the gear would be to have it accurately bore-ground then have a special bush made up. This would be very costly if you do not have a friend or connection to do it 'on the cheap'; however, don not despair if it's the only one you have or is a cherished 'straight-cut' one as I will look at a possible option a little later on.
Before fitting the new bushes, make sure the housings are clean, smooth, and free of any protruding marks. I use a fine grit flap wheel used VERY lightly. These things can re-bore the housing if used heavy-handedly! If not confident, simply use fine grit cloth/tape/Wet'N'Dry by hand. The bushes need to be PRESSED in squarely. Beating a bush to death in an attempt to fit it will neither ensure a square fit, nor be good for the bushes health. Especially the rear one; it is very brittle and is easy to break. And resist all temptation to use any bearing fit - it simply isn't necessary. The front bush should be eased into position, so the outer edge of the bush is just adjacent to the inner edge of the chamfer that can be seen around the outer housing edge. Pushing it in further will cover the oil holes that allow a little lubrication to reach the bush whilst the engine is static, and drain excess oil away when running to prevent oil getting through to the clutch.
So far so good. Now the hard bit. The bushes MUST be machined/reamed to the correct size. Theoretically they should be align-bored since they run concentrically on a common shaft centreline. In practice this is usually impossible because most machine shops simply don't have the equipment to do this. If your local one does - you're laughing. If not, then the only way to machine these bushes without incurring severe costs is to turn them in a lathe. Sounds simple, but the gear must be set up absolutely central and square in the chuck jaws. This is time consuming as it means using a DTI gauge on the clutch seal surface with the gear gripped in the chuck on the clutch plate splined area then using a long-shank tool holder to turn both bushes. Doing one bush then hoping to take it out and turn it around to do the other will cause heart-ache - the only place you can now grip it on is the drive teeth - setting up the gear square gripped on these is a very painfully slow and difficult process. In consequence it is advisable to get a quote from the machine shop before getting the work done! There is no simple way.
Once the bushes have been fitted and finish-machined, I always pre-lube them by heating the gear up in an oven at gas mark 5 for a half hour, then placing it into a pot of oil compatible with the oil you will be running. This is essentially any vegetable-based oil when using vegetable-based engine oils, and a good mineral oil for anything else. Don't use fully synthetic - it doesn't seem to have the same pre-oiling effect as mineral oil for some reason.
The finished sizes of the bushes are -
For road use;
Small-bore front bush 1.503" +0.002"/-0.0", same for rear bush.
Large-bore front bush 1.628" +0.002"/-0.0", rear bush 1.503" +0.002"/-0.0".
For race use;
Small-bore front bush 1.506" +0.002"/-0.0", rear bush 1.504" +0.001"/-0.0".
Large-bore front bush 1.631" +0.002"/-0.0", rear bush 1.504" +0.001"/-0.0".
NOTE; these figures are based on the crankshaft surfaces where the primary gear runs being 'as new' in sizing. If the crankshaft is well used and shows signs of wear in these areas - carefully measure the journal diameters with a micrometer (or some such suitably accurate device). To the figures gained allow a running tolerance of 0.003" +0.002"/-0.0" for road use on both bushes, and 0.006" +0.002"/-0.0" on the front bush, 0.004" +0.001"/-0.0" on the rear bush for race use.
Now, earlier I alluded to a way of salvaging gears that have experienced the 'spun bush syndrome'. What I have done successfully in many gears is so make a full-floating front bush. In fact this had been so successful, I have started using it on a number of race engines I have built to eradicate the possibility of the bush grabbing and spinning in the first place. Thus neatly circumventing all the hassles this causes - including loss of gear-change. I simply have the gear bore-ground to a bigger size, ending in a slight internal step near where the original bush ended to 'retain' the bush (stopping it floating out towards the flywheel end too far and covering the oil holes), then make a bronze bush up that leaves a light slide fit in the gear and a 0.003" clearance on the crankshaft journal. I have yet to have a gear prepared like this fail.
And again to emphasise - this is MY way of dealing with primary gear re-bushing. I have found the aforementioned methods to be entirely successful. Others will have alternative and equally successful ways of effecting primary gear bush replacement - I simply offer this information in the face of there being absolutely none offered by the regular manuals, and the plight of those that want to do something about their wobbly primary gears!