The early 350 used a 6306 bearing on the drive side (30mm bore 72mm diameter 19mm wide) and a 6206 on the timing side (30mm bore 62mm diameter 16mm wide). The timing side bearing was a sliding fit on a hardened sleeve pressed onto the timing side of the crank. The 250 twin also runs these bearings, but without the hardened sleeve and can wear their cranks relativly fast. The 478 engine also uses the same drive side bearing, but both it and later 350 engines (after 1976?) use the same plain bearing for the timing side main.
The use of a plain bearing on the timing side may explain why the 500 engines were never tuned to the same extent as the 350 Sport. There have been reports of premature failure on tuned engines.
There are roller bearings that are dimensionally the same as the original 6203 timing side ball race. One option would be a FAG NU206ETVP2.C3 (used by NLM I think) another would be N306ETVPC3 (available from
www.simplybearings.co.uk). Note that these bearings are the C3 (class three fit) versions. These will go straight onto an early 350 crank and 250 twins, but all other cranks will need the plain bearing hournal grinding down. I can see no point at all in fitting the sleeve, just have the crank ground to the correct size for the bearing to be a direct press fit.
The 501 engines gained an upgrade to the drive side bearing although the timing side bearing was the same. The bearing was a custom made (and now obsolete) SKF 633250C. NLM have some stock at about £120. It appears to be a 4 point angular contact bearing with a larger than standard bore. 4 point contact bearings are designed primarily for axial loads, but due to the larger balls made possible by the split inner race the radial loading is also higher than an equivilent ball race. They also (usually) have a solid bronze cage. There are standard 4 point contact bearings dimensionally identical to earlier 6306 drive side ball races, the SKF QJ306MA for example. I don't know if this bearing has the same performance as the custom SKF (the contact angle is unknown for the original), but it has higher ratings than the original 6306 ball race.
I don't think I'd be happy with a bronze timing side bearing, I'd go for ether a standard replacement, or the roller conversion. I've had this done to the otherwise perfect crank in the tuned 501 motor that's going into my Valentini replica. The cost was about £60 including the bearing.
(at one tenth of the price)
)