Post-war Vincents are generally accepted as needing C3 fit main bearings so as to allow for the radial clamping action of the crankcase on the bearing outer ring - causing a slight tightening of the bearing clearances. The C3 looser fit takes this into account and minimises the chance of early bearing destruction.
However, what is the general view regarding the need for C3 fit bearings in the Series A?
Don't forget we have the same basic principle on the timing-side of the 'A'; that is the bearings press-in to the alloy of the crankcase (although in my experience there appears to be less interference here than with post-war Vincents).
On the drive side, the 2 x bearings are housed in a fairly thick steel sleeve that is a push-fit in to the crankcase. Again, the bearing fit in to this sleeve is little more than a firm push-fit. Moreover, the drive side roller (MRJA1) has a thrust face on the outer sleeve making it an L shape (in section) which would better resist any tendency to closing-up radially. So I contend that the need for C3 clearance bearings in the Series A, particularly those with original 80 year old crankcases, is not essential. But I would welcome other views.
Peter B
However, what is the general view regarding the need for C3 fit bearings in the Series A?
Don't forget we have the same basic principle on the timing-side of the 'A'; that is the bearings press-in to the alloy of the crankcase (although in my experience there appears to be less interference here than with post-war Vincents).
On the drive side, the 2 x bearings are housed in a fairly thick steel sleeve that is a push-fit in to the crankcase. Again, the bearing fit in to this sleeve is little more than a firm push-fit. Moreover, the drive side roller (MRJA1) has a thrust face on the outer sleeve making it an L shape (in section) which would better resist any tendency to closing-up radially. So I contend that the need for C3 clearance bearings in the Series A, particularly those with original 80 year old crankcases, is not essential. But I would welcome other views.
Peter B