If Atlas/P11/N15 timing covers and M3 E-start covers blow off oil from the bypass valve into the timing chest, this obviously will provide lubrication for the timing chain (and magneto chain). How does this find its way back to the sump and hence scavenged back to the tank? It appears that oil would accumulate to a level halfway up the oil pump before it could find its way past the crankshaft to the sump.

If all Commando's except Mk3 blow off back into the feed side of the pump, is there no oil provided to the cam chain on these engines?

Any ideas why the design was changed for Commandos, only to be changed back on the Mk3?

I think all of them have the timing chest drain ABOVE the level of the entire oil pump, including the camchain level (at the bottom of it's run).

I was told it was changed in an attempt to reduce oil temperature. Supposedly when the excess oil was returned to the inlet of the pump to be pressurized again [possibly several times] it made the oil temp high so they decided to dump it into the timing chest. Unfortunately it did not solve anything because excess oil in the timing chest just gets whipped into a froth in the timing gears which makes it hot also. There is too much oil in the timing chest on all the engines whether the pressure relief is dumped there or not. Enough oil returns from the head or leaks from the pump to flood the timing chest quickly. That is why I have been drilling a new hole down low in the chest to lower the oil to a reasonable level. Just enough to touch the crank gear is more than enough to thoughly lube everything very well. And lowering the oil level in the chest reduces the oil temperature measured in the tank considerably. Jim

That sounds like a worthwhile mod (lower drilling in timing chest).

Does it make any difference whether the bike is an around-town plodder or typically ridden at sustained relatively higher speeds?

How big a hole, and where to drill, approximately?

Hi there, and what"s about removing the stell disc between the bearing and the triangular spacer (next to the crankshaft pinion)
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In the late John Hudson's NOC Heavy Twins rebuild video he remarks that the reason for the Commando oil recirc. gallery was that: "...it thus reduces the rate of circulation of oil around the complete engine to the tank..."

Paul, I use a 1/4 in hole and you can see it here. Just don't plug the breather holes unless you install the reed kit. Jim

crankcase-breather-t3864.html

I would not remove the disk next to the bearing. You do not want to flood the bearing with oil as that can cause the rollers to 'hydroplane' and skid and fail. Jim

Excellent! Thanks, guys. I missed the new drainback hole in the thread on the reed breather. I was thinking along those lines but afraid to do it. I keep thinking it's too easy of an answer and why did the factory never do it?

I would think this would be a worthwhile modification on any Norton twin.