Post date: May 6, 2012 8:41:44 AM
Lifted from the Made in England email list
The gear pump in a Commando would be the simplest form of pump ever. Just around and around, even the plunger pumps in Matchy singles is more complex, combining rotary motion with reciprocating motion. An efficient gear pump must have very fine tolerances. Very fine tolerances between the gear side faces and the pump case end plates. As well there must be a very fine tolerance between the gear teeth and the case past which the teeth pass. Shaft sealing must be spot on. In bigger pumps the shaft can have lip seals and the endplates can also have seals, sealing up against the gear side faces. The tolerance between the radial end tips of the gear teeth and the case must be as fine as possible. The next best pump after a gear pump is a vane pump whereby instead of teeth the rotor has vanes that actually contact the housing as they rotate. They even get forced out by oil pressure and centrifugal force.
No such luck with gears. You got wot you got, thats it. You can get a brand newy, but being so small, it's only as good as whoever made it. Bottom line here is, the hot thin oil after a good ride will slip easily past most Norton gear oil pump gear sets and dribble off into the sump. The oil will go the way it should, and that is through the pump to the crank and out through the big ends. There are gaps big enough so that over the period of a week or two you will lose the contents of the oil tank into the sump. Both my Commandos would do it over a period longer than two weeks, the Model 18 does it almost over night.
There are taps and ball valves and all sorts of goodies to help the inept.
I've got one on the Model 18, wouldn't dream of it on the Commando. The Commando needs oil pressure because it has plain bearing big ends, the Model 18 has rollers all the way and needs bugger all oil pressure. I've forgotten to turn the tap on, on the Model 18, nipped it up and got away with it. Just nipped the piston to bore. Oil starvation on the Commando would mean instant disaster and a major engine rebuild, as the plain bearing big ends would sh*t themselves in a big way. They definitely need oil, it's their nature. Balls and rollers can get away with it.
What happens if all the oil drains into the sump and, so what if it does...............
With a sump chockers with oil it's hard to imagine the engine will seize. So you wouldn't worry too much there. Sumps need to be a specific size. In a wet sump engine like most cars, the sumps are huge and the crank doesn't spin in the oil, it spins above the oil. In a dry sump engine such as most Brit bikes, there normally is no oil in the sump except half a cup ful of transient stuff on it's way back to the tank. So, in a dry sump engine there is really only room for the crank and not much anything else, including oil and including air. If you imagine your pistons going up and down, from the underside, they will be displacing quite a bit of air. Almost as much as the nominated capacity of the engine. It's like an air pump in there, pushing and sucking as the pistons go up and down. As the pistons approach BDC all the air is shoved out of the way, as the pistons return up to TDC, the air is sucked back in. This is why we have crankcase breathers on our engines, sucking and blowing, in and out. If the sump is big enough, should I say, if the volume of air contained by the sump is big enough, it will compress to a certain extent and absorb the space occupied by the pistons on their upward and downward journeys. The air will be getting compressed. The bigger the sump, or the better and bigger the crankcase breathing, the less adverse affect this suck and blow will have on the engine.
If you have a smaller sump, or reduce the air space, then there is less space for the suck and blow to happen. Think about allowing the contents of your oil tank to drain into the normally dry sump. Oil will not compress very easily. The sump, now partially full of oil has lost probably half it's capacity to absorb the suck and blow from the pistons as they go up and down. The blow side of things as the pistons come downwards will compress the limited airspace and will want somewhere to push the air and oil out of it's way!!!! It will look for the weakest point, especially if the oil in the sump is also covering the breather outlet / inlet. Like the breather in the very bottom of the crankcases on a Combat engine!!!! Why did they move it back up to a higher point in the back of the timing chest a la 850's????????? I wonder!!!!!
In most cases the weakest point will be the crankcase seal behind the drive sprocket. The crankcase compression will easily push that out of the way, dumping the oil from the sump into the primary. First thing you will notice is a slipping clutch as everything in there gets over-lubricated. Next you'll probably wonder why your bike suddenly needs oil, so naturally, you top it up, only to have it drain into the primary again. Bike will also probably blow smoke and try to run like a dog as well. Major dramas eventuate. Other weak spot is to blow the camshaft oil seal out, then try for the sealing surfaces such as through the vertical split in the cases or blow out a cylinder base gasket.
What to do..........accept the fact that you ride an old pommy bike that not only has a stuffed oil pump, but it was probably like that from new and either go buy a new one and experience the same thing again or get on with your life and adapt.
Hints & Tips
Before going for a ride, amongst all the other things you check......check the oil level. If oil level is above half, your pretty well OK. If oil level is half way down the dip stick, start the bike carefully and don't rev it, just set on a slightly fast idle to warm up and gently pump all the oil back from sump to tank. Be gentle with it and don't give it heaps of revs, you don't want to blow that seal!!. If the oil level was further than 1/2 or off the dipstick all together. Get your clean oil drain pan and drain the oil from the sump. Ignore the fact that there was three litres in the sump. Just go make yourself a coffee and sit and chat to the trouble and strife for a while. Go back to the bike, tip the drained oil back into the oil tank, screw the sump plug in and go for a ride.
Come to terms with the fact you are riding a bike with an oil pump and lubrication system that was designed circa 1930, get over it and don't expect to be riding around on something with the oil pump performance of a Jap bike from 1962, it's just not like that. Develope a little system for yourself. If checking and draining the oil was the very first thing you did when the idea for going for a ride entered your brain, then by the time you scratched around and donned relevant bike clobber it would only take a moment to tip the oil from the bottom, back in the top, screw in the screw and take off. You only need to drain most of it out, not every last livin drop!!! This will only take 5 mins.
Resume Lotus position close eyes hold fingers in a funny circle thingy and repeat ........... hhhhuuuuuuuuummmmmm ..........hhhhhhhhhhhuuummmmmmmmmmmmmmmmmmmmmmmmmmmmm I love my Norton, I love my Norton, I love my Norton.
As far as I know, the Mk3 850's are the only twins to have a check valve. Interestingly I have a single (ES2) timing cover from 1961 which has the design check valve in it as the 1976 Mk3 850. All the rest rely on close tolerances to "hinder" bypass leakage.
The aftermarket in line check valves are fitted to the pump suction hose between the oil tank and the pump. They have a ball and a spring inside then, the spring holding the ball on a seat against the flow of oil from the tank. This idea relies on the spring being very weak, as vrtually all it resists is the head pressure of the oil from the tank to the valve. This is the pressure created by gravity acting on the weight of oil between the difference in vertical height between the check valve and the highest level of oil in the tank. As soon as the pump starts to work a partial vacuum is created in the hose between pump and check valve. The weight of oil in the line above the check (head) is then supplemented by atmospheric pressure that balances the partial vacuum in the line below the check and forces the spring ball off it'd seat back against the spring and allows oil flow. It really doesn't take much in the way of crud etc to stall the opening of this ball check and either not allow oil past or restrict the flow enough, especially at high revs, to damage the motor. As someone else said, they do seemingly have a good record, although when one fails, kiss your engine goodbye!!!
The Mk3 Commando check and the check in the timing cover of the 1961 ES2 I have, is positioned in the timing cover on the pressure side of the pump, in the gallery that leads from the pump pressure outlet to the pressure relief valve and banjo bolt to the top end, and of course to the crank.
When the motor is stationary, the head of oil leans on this ball, which can have a much stronger spring. The oil dribbles down it's usual path through the hose to the pump around the gears and out into the gallery in the timing case. Now, instead of continuing on to the big ends where it leaks into the sump, it is stopped in it's tracks by the ball check in the gallery.
Being on the pressure side, and as everyone knows, a Commando can achieve big oil pressures on startup, the full pressure force of the pump pushes the ball check out of the way. Even if there was some crud behind the ball, the average oil pressure of, say, 40 psi, is much much greater than the pressure exerted on the inline ball check by the head weight of the oil alone.
The gremlin in the woodpile here is the condition of the shaft sealing in the pump, because the Mk3 style check is after the pump and this leaves the integrity of the pump itself in question.
Some people use little shut off valves in the suction line, as I have on my Model 18 and rig up all sorts of inventions to remind them to turn the damn thing on before they start the engine.
In the end.........choose your own poison.
Fri Sep 7, 2007 3:15 pm