The main bearing cups in the crankcase were specially designed by Scott as no standard bearing existed at that time which fulfilled their requirements. These special components were fitted to a warm crankcase and then a heated steel ring was dropped into a face groove above the main bearing cup location diameter. This technique is similar to that traditionally used to fit steel bands to wooden cart wheels. In this case, the steel ring restricts the aluminium housing for the cup from growing with temperature at the higher rate of aluminium compared to steel.. The housing does not grow with heat to the degree that would allow the cup to come loose.
The Scott works then finally ground the internal diameter and gland thrust face of the cup in situ, on a special grinding machine. The internal bores of the cups are thus made in line with each other, but not necessarily concentric with their mounting bores or outside diameter. If a cup has been removed from a crankcase and it is measured, it will usually be found that the bore and outside diameter are eccentric to each other.
Many years have passed from the date of the manufacture of these crankcases with no facilities available to regrind cups in situ. This has forced some rebuilders in the past, to use good cups from a crankcase that maybe was severely damaged by a crank breakage and refit them into other crankcases. In such cases, the cups can be significantly out of correct alignment and with locations that come loose when warm. While such examples may be the exception rather than the norm, it is better to be aware of this possibility. The cups should be carefully scrutinised to see that the bearing rollers have been running evenly and that the spring loaded gland has left an even wear pattern on the thrust face.
The best answer is to have the cups inspected for alignment on a measuring machine, but this is not available to all.
For those who wish to take extra care that nothing is amiss, it is suggested that the case is warmed to a temperature that is the limit that can be tolerated with the bare hands, and the cups checked to ensure that they have not become loose.
For those interested in having the main bearings of crankcases renewed, please see our product list.
For measurement purposes, the thrust faces of cups should be in plane within 0.002
The clearance for the total roller assembly should be min. 0.0005 max. 0.0015
Ovality and taper to be within 0.001
Note. Where cups are in good condition but oversize, special oversize inner rings can be supplied to match.
Where cups are good but unevenly worn, they can be re ground to restore truth and special inner rings made to suit.
Our usual practice if cups have been incorrectly replaced is to fit a new housing to accommodate standard ball races and synthetic seals.