Oil leak

Oil return from the rear main depends on bearing fit and the reverse thread on the crank to draw oil back into the sump. Would seem you have excess flow or something is restricting the return oil flow.
These are known to leak excessive oil into the flywheel housing if you run for extended periods of time on steep incline with nose up.

Crankcase pressure? Wondering if your crankcase breather is clean and unrestricted. New overhaul could have higher than normal blow-by gasses and even a normally OK breather might be enough to increase pressure.

too much oil pressure?

Hope you saved your old rear bearing s ,so you can mike the the thickness next to the new bearings that were installed .sometimes wrong bearing s appear in the wrong boxes , Also may pay to use plasta gage in the new bearing and torque up the bearing and cap to see what clearance you have between the crank and bearing .
Check the new bearing to see if the drain hole in the bearing is unrestricted ,and matches the cap drain hole

Hi Team,
the oil scroll (coarse thread) machined into the rear crank journal is a designed depth or volume to work properly.

If the crank was ground in the scroll area the scroll may now be too shallow to draw away the designed quantity of oil to stop oil transferring into the master clutch compartment.

Also, from memory, the main bearing shells are usually bored around 0.005" larger in the scroll area and the crank should not contact this part of the bearing. This clearance allows the scroll to work as designed to move the oil back into the crankcase.

Edit, I cannot recall if the undersized bearing shells have a standard diameter in the scroll area. If so and the crank was machined undersize in the scroll area then the scroll clearance would be too great to work as designed.

As mentioned by others check that the oil drain hole in the lower shell that is in the groove between the scroll area and the actual main bearing running area has free draining ability, ie, check that the drain holes in the rear main bearing cap and its shell align. Double check that the crankcase breather is free flowing and not part blocked in the filter medium.

Cheers,
Eddie B.

That's good to know Eddie - I had my 5U crank ground 030 but didn't do anything to the oil scroll. What does the depth need to be? I haven't fitted the crank yet so I can still send it away to be milled.

Hi Neil,
sorry I do not have any specs. for the scroll.
I just know that the things mentioned are traps for the un-enlightened and need to be taken into account.
I guess most of the old school crank grinder operators are now gone and this info is being lost at a fast rate.

An old school crank grinder shop that did cranks for a Dealer may have the spes. and procedures relating to the Cat cranks.
My guess would be to pre measure the existing scroll BEFORE grinding if that is the way the grinder recommends.

From memory someone from Downunder in West Australia did an RD6 crank recently and it came back with the scroll area unmolested. This required the bearing halves, in the scroll run area, to be machined back to spec. diameter to give the 0.004" to 0.005" clearance in the scroll area.
Needless to say a suitable fillet radius needs to be ground into the crank at the diametrical change step on the journal to stop a stress raiser sharp step that would weaken the shaft.

The main bearings for the above example were made elsewhere so were left the same diameter right thru to allow for any variances in ideas, and if the crank was ground undersize for the full length of the rear journal.

Hope this helps.

Cheers,
Eddie B.

No worries Eddie - logic would tell me that if I removed 030 off the journal, then removing 030 from the groove would get me pretty close (plus verifying that 005 from the bearing - I'll remember to check that)