[Andrew Rowe]

Good explanation Lynn.
Having removed many of these plugs on our production line, they all have holes in them, just off the top of my head the diameter is about 5mm, but can check.
It sits in the middle of the horizontal gallery when it bottoms out on the thread, between the two vertical galleries.
I think the reason that it had the hole was that if the cooler did get blocked there was at least some oil returning through the bearings.
One motor I checked recently was not running a cooler and it had a vibration when revved up, upon checking this engine it still had the plug with the hole in it.
( so was only getting small amount of oil to the bearings )
The vibration went away when the plug was removed, this carrier does not run a cooler but if it were fitted the plug would then be reinstated.
You could run cooler without this plug , but I presume it would not be that efficient as the cooler would only act as a bypass and the oil would not be forced through it to cool. Cheers