The bearing "Class Fit" information I use was compiled and supplied to me by Jack Shortland. Who was Jack Shortland?
Jack Shortland was the Export Spares Manager for Triumph. When Dennis Poore sold Factory #2, which held all of the spares for a production run of TR5T models, to the Danish Triumph Distributor, Buster Reinhardt. Buster set up Jack to become the Managing Director of a company they called PS Motors. The "S" was for Shortland and the "P" was for Price. Yes, the same Brenda Price that started in the factory office at 17 and ended up running the U.S. operations for the Triumph Co-operative.
As spares were always short it was important for a distributor to have more than a working relationship with the factory and Jack and Buster became close friends. Having a friend in the office never hurt and Brenda would be invited to spend the production break time in August staying with the Reinhardt family in Denmark.
Buster sold PS Motors, including the #2 factory site, to David Holder. David was the son of Matt Holder who was the Scott enthusiast and ran a company that made a lot of parts for the original W.E. Wassell called Aerco. Matt is the guy who went around buying up old motorcycle manufacturers like Scott, Velocette, Royal Enfield and Vincent. Along with the PS Motors stock and trade was Jack. He continued to work for David until his death some 10 odd years ago.
Jack was a man immersed in details and organization. His card index was the source of all sorts of information. He was one of the people behind the Wall Poster showing all of the gear interchanges, which he admitted, on the the day it was published, at best, still a work in progress. Jack had also developed a close working relationship with many suppliers. One was RHP and this relationship continues today.
It is correct to say that the bearings used in the engines of most old British motorcycles should have a 'C3' or as used to be known as '000' rating?
The simple answer is no. There are many factors that go into the selection of pre-fit bearing clearances. While we look at a bearing as being solid hunk of steel, under pressure bearing steel will expand or compress.
Take the MJ1 1 /8 ball bearing used both the timing side of a Triumph 650 (C3 condition) and the drive side of a T150/A75 (CN normal condition). On the 650 the bearing is a press fit in the crankcase AND on the crankshaft main shaft (also there can be alignment problems from production) so engineers allow extra clearance for compression and possible alignment issues.
On the T150 alignment isn't an issue as production tolerances have to be much better because of the two center main bearings, and while the bearing is still a press fit in the crankcase, it is a loose slip fit on the crankshaft main shaft.
With specifying pre-fit bearing clearances engineers are looking at getting specific fit running clearances once the bearing's inner race has expanded and the outer compressed.