Reaming cam bushes on a single

[nevhunter]

I generally regard a slotted bush as not able to be reamed, as it will always move towards the slot when cutting. Align boring is the only real way and getting them in line is essential. If they aren't you have a problem as you have no idea what effective clearance you have. The bush in the crankcase must be tight in the case. IF it moves into the case the engine jambs. It has no flange to prevent this. Putting the oilway in after makes a kingpin reamer OK for the job with the case and cover tightened ( with or without gasket). You can check the fit of both journals with them together. Deburr the slot. Nev

[Simon.Gardiner]

I wonder why the bush in the crankcase doesn't have an oil hole

Some of them (late ones?) don't even have the oil hole for the timing cover bush....

SG

[cmfalco]

I generally regard a slotted bush as not able to be reamed, as it will always move towards the slot when cutting. ... you have no idea what effective clearance you have.

In principle this is the case, but with a spiral flute reamer only a small fraction of one edge is ever unsupported by the slot so the issue is negligible compared with the bellmouthing that will occur when doing this by hand absent a supporting jig.

The actual clearance for a shaft undergoing simple rotation isn't very critical, with anything between 0.0002" and ~0.004" (or more) providing the necessary support from the oil film. That said, if the clearance is small there is increased risk of galling by particles in the oil (especially if there is no filter) and if gets to be too large there can be excessive mechanical motion from sideways forces.

The consequence of increased clearance is obvious in something like a loose fitting gudgeon pin, but even with a camshaft there are periodic sideways forces as the lobes raise the lifters. While the shaft is supported by a relatively thick oil film when there is no sideways pressure on it, it is pushed into the film when there is pressure. If the clearance is double what it should be, the momentum gained by the shaft when it is pushed is doubled, potentially allowing it to overcome the hydrodynamic pressure and make physical contact with the bush.

A further reason for there to be a maximum allowed clearance is the ideal fit of the teeth of the gear on a cam is only ~0.002". However, if both ends of the cam are allowed to move by ~0.002" due to wear of the bush the fit of the teeth will increase to ~0.004" for part of the rotation, and can rebound to ~0", resulting in excess noise and wear.

To summarize, for camshaft bushes:

--Use an appropriate material, such as SAE 660 (C93200) bearing bronze.
--size the OD to be ~+0.002"-0.003" for press fitting in the cases
--After fitting the bushes in the cases the 0.001"-0.0015" range of clearances given by Waller for this application is reasonable, although maybe a bit on the low side. If it were mine I would aim for the high end of this range, and maybe even a bit more (but no more than 0.002").