How To Machine Crankcases for Ball-races
The reason for supporting crankshafts in ball-races is to reduce friction. If the alignment of the ballraces is not perfect, friction will be worse than that of a plain bearing engine, so all advantage is lost. Even if the alignment is perfect, if the bearing is distorted by a too-tight fit the crankcase, or loose in the recess, the advantage will be diminished. The correct fit of a ball-race into the crankcase is a complex subject and the reader is directed to the FMV Story website for greater detail. On this page, we present a method devised by engine designer, Gordon Cornell, for an effective way of machining the ball race recesses that ensures accurate axial and radial alignment.
Gordon recommends that the recess for ball races in aluminium crankcases be bored 0.0008" to 0.0016" smaller than the measured diameter of the outer ball-race cage. It should be obvious that you can't use the race itself as a gauge and inside measuring equipment is really not up to the precision required. As described in his series on Model Engine Development, taking the time to machine a Go-NoGo gauge is the best and surest way of assuring the correct fit is obtained. you will need gauges for the ball-races to be used before commencing machining.
To actually machine the recesses, the case will have to be flipped end for end, so we need a way of mounting for the second operation that aligns the case so that the second recess is perfectly axial with the first. If the crankcase in question is of the type which is internally threaded for a screw-in backplate, there is a simple solution. It requires that the rear face of the case be machined at the same setting as the recess for the ball race, but this would be standard practice anyway.
What is needed is a fixture that mounts the case using the ballrace recess and rear face of the crankcase for alignment. The fixture is threaded so the case can be mounted using the backplate thread. Such a fixture is shown in the picture here and is obviously well suited to efficient quantity prodiction. It has been turned on a Morse Taper (MT) blank so that it can be removed and replaced in the lathe headstock with precision. An alternate would be a precision circular spigot that can be gripped in a collet, but the MT version is probably better. The stub at the front aligns the next section for entry into the ball-race recess. This needs to be a close fit in the recess, but not as tight as the ball-race itself as we don't want to gaul the recess as the case is screwed onto the fixture. Having positioned the case, final alignment is assured as the rear of the case registers against the rear flange of the fixture which was turned to be perfectly at right angles to the axis of the lathe when the fixture was made.
A precision taper fitted using a draw-bar should always result in repeatable positional accuracy. If there is any doubt, fter mounting the fixture, the front can be clocked with a sensitive dial test indicator (DTI). If it does not run true, disassemble and clean the mandrel and headstock bore. With the case screwed onto the fixture, the front recess can be bored out using the Go-NoGo gauge for the front ball-race to achieve the correct size and perfect alignment.
In this How-to, we've skipped over how the case was held for the first boring operation. Some sand-cast cases have a "chucking spud" at the front for workholding which is parted off after the interior of the case is fully machined. Die-cast cases will need some other mounting means. This might take the form of a fixture that allows the case to be mounted using the engine lugs—provided thay are flat and true enough. Such a fixture also aids production machining allowing swift change of work pieces.
While this is fine for production work, you might be thinking that it sounds like a lot of work to go to for a single crankcase. Remember that unless the ball-races are perfectly aligned and do not distort the ball-races, you will probably have a crankshaft that has more running friction than a plain bearing engine, so the effort is not just worthwhile, it is mandatory! You can however, for a one-of job, machine the fixture from scrap barstock held in the 3-jaw chuck after the first operation on the case is complete. That way, it will be in perfect alignment until removed from the chuck, but after its single use, you won't need it again and it can go in the offcuts bin for turning into something else, or for mystifying the person who inherits your rusting bin of off-cuts.