oil pressure with Commando oil pump

If the new bigger capacity pump is being fitted to a 99 or 88  it will pass more oil than the system was designed for ,certainly will not need a 6 start gear set.  You may find rockers  run with less noise and  a few seepages appear. Cavitation could be a possibilty as the oilways and pipes may not cope as well. Exhausts may smoke a little from extra valve lubrication, Mag or Distributor may protest as the points get a bit too much oil. Oil may still drain to the sump,  But also everything may be just fine!! , Give it a try. We will all be interested in what happens .

Not quite, the pump is the same. The speed at which it is driven is what makes it high capacity. If you use a new pump, use your old worm drive and gear. If you use the 6 start gear it will deliver too much oil.

The oil pressure is determined from the spring and number of shims fitted into the pressure relief valve in your timing cover.  The flow rate of the oil is set by the speed of the pump, viscosity of the oil, size of the oil-ways in the pipes and cases and obviously leakage in the system.  On a Dommie motor the Commando pump will increase the flow-rate somewhat but remember that the scavenge gears are bigger than the feed gears so the return flow should stop a build up of oil so long as the oil circulation is getting it back to the sump effectively.  The drain hole at the back of the head and down through the barrel can allow a build up of oil in the head gallery and cause some smoking.  If so these holes can be enlarged a little with care with high spots polished away.  It is usual to drill the oil-ways and fit Commando unions for racing and performance motors to get the benefit of the increased flow-rate afforded by the better pump.  Give it a go and enjoy your Dommie. Cheers, Howard

The oil pump on the twin is the same as there no such thing as a Commando oil pump, it changed little in the life of the twin. It is how it is driven which determines flow rate, pressure amongst other things is determined by the PRV as described above. The ideal pressure is hard to achieve using the shims and a compromise will need to be made, this has been determined in some recent testing. 

Mods are OK, but remember, it is a minefield just because you are seeing pressure, does not mean you are getting flow to all the correct places, bare this in mind if you make changes. Big ends will run for about 40 miles with poor / no supply before it all gets messy, as some those that do not change the oil pump feed washer everytime the timing cover is off have found out, when it has dropped unseen into the oil catch container. 

Ashley is quite correct - there is no such thing as a Commando oil pump.  The Commando pump is virtually the same as the bigger capacity SS pump that was fitted to the heavyweight twins after 1962.  There are elongated slots replacing two of the 3/16 inch holes in the earlier pump and consequently have a much bigger flow rate than the earlier pump.  The SS pump had an S in the front casting.  There pumps were fitted to the 88SS, 650SS and Atlas.  They then went on the Commando and Mercury - but no longer wore the S on the outside. These pumps were not fitted to the 7,88,77 nor 99 so it is a different oil pump to that which most Dominators have fitted.  Good Luck.

I think this is what I said in my first post.  IE  the current new pump WILL supply more oil than the 88/99 pump , Not because it has a six start gear set ,but because it  is different inside. 

Yes, I'm sorry to repeat you Robert.  Paul's question though was really about the effect on the oil pressure and scavenging with the later pump fitted.  I think that's all been agreed now.  Bye all,  Howard