353BTB Oil Loss Problem.

The problem that prompted this total engine re-build was oil passing through the drive-side main bearing at a rate averaging about a pint every 150 miles.
It varied a little with speed and revs, but not with temperature nor oil type nor viscosity.
Nor did it vary with the many attempts to cure it by Ralph Seymour and others since 1983.

The problem started in 1983 after Ralph did "everything possible" to my Venom engine to make it go faster while sticking to standard Venom bore & stroke.
(He fitted Fath coil valve springs & rocker return springs for a safe 7000rpm, lightened all the valve gear, narrowed the valve seats, re-shaped the ports, etc.)
One minor item that concerned Ralph was oil drag and so he worked on the inside of the crankcases........

Over the years since then everything apart from the flywheels and timing-side shaft has been changed, with no affect on the oil-loss.
Different oil-pumps, all possible combinations of breathers, oil-scrapers, drive-side shaft....and finally both crankcases.
Nothing made any difference. I had long-ago fitted a drain and catch-tank to cope with the problem, but pushed by Bob's certainty that there must be a root cause I agreed to one last attempt at identifying and correcting it, or at least fitting an oil-seal. We agreed that the he would either find the specific cause or he'd fit a proper lip oil-seal to the drive-shaft.

This winter Bob did what I think modern engine builders call  "blueprinting".
He checked and re-set every dimension, and throughout the lubrication system in particular. Minor examples were the sizes of holes in oil-unions and galleries, including even the clearances between oil bolts and the insides of the cavities of the unions. More intricately the eccentricities and diametric clearances of the oil pump in its crankcase housing. But most effort went in to re-setting the radial clearances between the drive-side sprocket boss and the crankcase opening in the main bearing housing.
But he concluded that none of the previous variations in these dimensions alone could be expected to lead to such an oil loss.

One thing did concern him - the gap between the drive side flywheel and the crankcase, which measured around 45 thou compared with other examples previously observed at up to 90 thou. Also noticed was the lack of any raised lip around the main bearing boss on the inside of the crankcase. Research of available drawings and other crankcases showed this lip as, quite possibly, an intentional feature in the design.
I had provided not just the recent new crankcase pair, but also the previous original pair as modified by Ralph.
He noted that this also did not have a significant lip around the inside of the drive-side main-bearing housing.
The original appeared to have been machined off, almost certainly by Ralph when reducing oil-drag.
So, by complete coincidence the replacement drive-side case (one of the stronger, thicker-walled ones) and the original cases appeared similar and effectively without the raised lip. Though we knew of no documented reason for this lip, it could conceivably be to divert oil flowing down the inside of the case (or even the face of the flywheel) before it reached the drive-side shaft and escaped. It looks just a surplus part of the casting, not a machined component. A typical lip is about 65 thou proud of the crankcase wall, with a clearance of about 25 thou from the flywheel. Clearly this isn't an "oil-tight" device, but it could be a "diverter". 
Also we conjectured that if the centre portion of the flywheel is to centrifuge oil away from the main bearing (where the lip is closest to the flywheel) then there should be extra clearance to the outside. 

So Bob machined extra side clearance into the extra thickness crankcase wall leaving a protruding lip around the bearing.

Click HERE to see a photo and Veloce drawing of the lip as in an original crankcase.
Click HERE to see photos of the original one from VM1309 with it turned off (top), and an original (Viper) crankcase with its lip (bottom).  
Click HERE to see photos of two MSS cases that show it even more clearly

The rebuilt engine now appears to pass zero oil through the drive-side bearing or shaft.
Over a measured 1000 miles the level of oil in the primary chaincase dropped from its initial 70cc to 40cc while leaving only an appropriate number of drips after a run, so it wasn't leaking and re-filling.
This problem may well be specific to these flywheels, which are the originals from 1957 engine VM1309 and we know there may have been differences over time. But we think not.

Incidentally the new engine is a revelation: Bob had also trued-up everything (the fly-wheels were apparently 4 thou out....), had reduced valve-to-guide etc clearances, and a new 60g lighter piston means the balance factor is probably back to how it was when Velocette first built the engine with a split-skirt one. So it's now smooth, eager to rev, oil-tight and with low oil consumption; like having a brand-new Venom. I've never had a new Velocette, though I did re-build a MAC to as new specification and that too was different from all the well-worn Velos I had experienced.  
Result happiness; except for 25 years of wasted Castrol.