Commando vs. Atlas timing cover

I know that Atlas/G15/P11 timing covers and corresponding crankcases are missing the oil drillway found between the pump boss and the pressure relief on early (20M3) Commando covers. Where does this oil way go and what is its purpose?

It seems I knew this at one time, but I've slept since then.

Does it provide a path for the oil when the pressure relief valve opens? If so, where does the oil go in the Atlas style motor?

I have a Fastback motor on the bench and tonight when I picked up the cover that was supplied with it discovered it was an Atlas cover and not a Fastback!

Ron, the cover with the passage drilled to the extra bolt hole returns the oil from the pressure relief valve to the inlet side of the oil pump. Covers without this passage drilled have a hole near the pressure relief valve that just dumps the excess oil into the timing case. James

So, theoretically, if you block off the hole in the crankcase you can use an Atlas timing cover on an early Commando (20M3) motor with no ill effect?

Ron L said:

So, theoretically, if you block off the hole in the crankcase you can use an Atlas timing cover on an early Commando (20M3) motor with no ill effect?

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There's no actual need to block it off, if there's no corresponding hole in the timing cover. [Edit] Thinking about it, the gasket face of an Atlas or Dominator timing cover may not blank off the drilling in the Commando case, so as you said, you may have to blank off the hole in the case?

The 850 MkIII timing covers do not have the drilling either, as the MkIII models also vent their relief valve oil directly into the timing chest, however the MkIII T/S case still has the earlier drilling, which isn't blocked off.

Thanks, guys. Les, I never noticed that the Mk3 covers weren't drilled.

I wonder what was their reasoning for adding the oilway? Was there an issue with excessive oil in the timing chest of the Atlas? If so then why go back with Mk3?

Ron L said:

I wonder what was their reasoning for adding the oilway? Was there an issue with excessive oil in the timing chest of the Atlas? If so then why go back with Mk3?

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I don't know why they adopted that idea for the (pre-MkIII) Commando?

In theory I would guess that recirculating the OPRV oil back to the oil pump inlet allowed the scavenge to empty any excess oil from the sump more rapidly when the engine was started, instead of that oil being dumped straight into the timing chest where it would quickly drain to the sump and increase the oil level, but that's the only actual benefit I can think of?
I did hear the reason it was discontinued on the MkIIIs is because it was found that the OPRV could cause frothing of the recirculated oil, but I have no idea if that is actually true or not?