Your Commando Doesn't Need To Wet Sump

Wet sumping is an issue many riders have found with their classic Nortons. Before I was aware of the extent of my Commando's wet sumping problem it would cough and run rough for about 30 seconds or more after it was started if it hadn't run in a couple of days. After that it was fine for the remainder of the day. Now, if it hasn't wet sumped or if I drain the sump before starting, it starts right up and idles smoothly.

Here are four simple steps that can be followed to eliminate or minimize wet sumping. When all are used together, the problem should be solved, meaning more time can be spent riding your bike rather than emptying the sump and refilling the oil tank.

1} According to The Norvil Motorcycle Company you should use the correct Monograde oil (SAE 50 in summer and SAE 40 in winter). However, not everyone agrees that monograde oils are the way to go. Nick Hopkins at Andover Norton International writes, "We have had some interesting results with different oils. There are those who swear by the old-fashioned mono-grade oils but quite why I have never really understood and our tests confirm that these oils don't work very well, at least in a Commando engine. A great deal of frothing occurs and when this happens the oil pressure drops quite dramatically because the pump is passing so much air. We got the best readings using a regular 10W-40 motorcycle oil which contains anti-froth additives amongst others." Fred Eaton, our friend and owner of Old Britts told me he uses 20W-50 Kendall GT-1 in all his commands.

2} You need to leave your pistons on compression after shutting the engine down for the day. To do this, slowly press down on your kickstart about an inch or so until it reaches the top of the effective swing. This has the effect of raising your pistons and the big end journals to the top of the barrel so oil has further to go before it can drain out.

3} MKIII Commandos have a build in anti-wet sumping valve in their timing covers. This prevents the seeping of oil down onto the bottom of the crankcases. On Dominators & Commandos (pre 131257) you can have an anti-drain valve fitted inside the timing cover, (AFTER the oil pump) if you send it to The Norvil Motorcycle Company, (reference their workshop service number WB12). Their cost is 90 pounds. On Commandos with engine number 131257 and onward, Norvil has a new timing cover which can be fitted to solve the problem (part number 066161A). This is NOT the dangerous type of anti-drain valve, which is fitted by some people into the oil feed line above the crankcases. Anti-drain valves fitted into oil feeds, always cause oil starvation to the big ends for a brief time when starting the bike. If you use an on/off tap instead and forget to turn it on, you will destroy your engine in short order.

4} If the above three steps have failed to solve your problem, you probably have a faulty oil pump. Oil can flow backwards through the oil pump from the feed side to the scavenge side of the pump if it is in need of attention. Most oil pumps can be repaired and restored quite simply. An additional step is to have O-rings fitted to your oil pump shafts to stop oil draining from the feed to the scavenge side.

If you are unaware if you have a wet sumping problem I highly encourage you to monitor the oil level in your Norton's oil tank immediately after shutting it down and before starting it up. If after a few days of sitting there is a significant difference then drain your sump and measure what comes out. You might be surprised or as in my case SHOCKED! Anything over about a cup is due to wet sumping and can cause problems. With my Commando, 1.5 inches on the dipstick equals 1 quart of oil.  Over an extended period of time it is possible for the oil to drain down to the oil tank's feed line.  If that happens when the engine starts there will be no oil pressure until the scavenge side of the oil pump moves oil back into the tank.

I have found that my MKIII Commando likes the oil level around the "L" mark on the dip stick.  If it is much higher than that it seems to blow the excess out the breather.  I made a calibrated ruler that I use to determine exactly how much oil has drained from the tank into the sump.  It is a pdf document and can be downloaded by clicking HERE.

Although the MKIII has a built in wet sumping value it doesn’t work. Before I installed Old Britts wet sumping valve  my Commando would wet sump about a pint and a half.

To make draining the crankcase easier I use Old Britts sump drain plug with drain bolt.   The stock steal drain bolt can easily strip the aluminum threads in the crankcase.


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Revised: January 05, 2018.