How Does an Oilpump work? By Mike West Our own heart has many features that most of us are aware of. It carries food and lubrication to the various parts of our body and picks up the poisons and residue from work on the way back and sends it thru our kidneys for cleaning, then back to the first chore. Additional chores are to spread the warmth and cool the parts if too warm. It's a gear pump. One of the simplest and most reliable made by man. Past, present or future. First cousin to the Noble Screw it has all of his features of loyalty and sturdiness. Put it in and forget about it. This gear pump is the same design I used to use in Hydraulic Forklifts It will even pump Aspahalt and does in those machines that lay the stuff. You can pump air with the damn thing just not too well. It will of course also pump water. They just put in some brass gears and ship it. Can't think of the manufacturer right now. We're talking about the same pump for all three applications. Here's the way it works: there are two gears and one is driven from outside. In our case it's driven from the end of the camshaft. The two gears mesh and each has its own circular pocket in the housing they're in. There's a cavity on one side of the gear teeth to less then half the gear diameter of the gear. This closes down around the gear to just a gnats eye-lash where it reaches the center of the gear. There's another cavity on the other side for oil to be pushed out of the pump. The area in the center of the pump, where the two gears mesh is all closed in and tight. The sides of the gear, where the teeth ain't, are also within a gnats thing so the fluid can't escape down the sides. One gear is exposed to the incoming fluid and it picks it up with its teeth and then the fluid gets trapped there when the wall of the housing closes down and it is transferred to the other gear where it is rotating into the housing and is moved into the high pressure side of the pump. The high pressure or low pressure side only depends on which way you turn it. Works just as well in reverse. The only important thing to this pump working is the clearance between the teeth and the walls and the walls on the sides of the gears. The teeth in our small pump are steel same as the ones in my hydraulic system. The hydraulic system had a heavy cast iron housing and generated pressures of 5000 psi. All day long and sometimes 24 a day. The only difference is that housing. Ours is made of aluminum. That little pump would probably still go to a thousand psi before it blew a wall out. It be bad! :-) You wouldn't bother the camshaft to do that and probably not the gears that drive the cam. You may stall the engine tho. The weak points are the aluminum walls and the clearance between the walls and the gear. Flow rate: this is based on the size of the teeth and the rpm. A tooth can only hold so much fluid so that's pretty easy to see. From Tom Wilson’s book, "How to Rebuild a Volkswagen Engine" it shows seven different lengths of gears for those pumps. These different lengths equate to different flow rates for the different engines. For the type 1 there are four pumps according to Tom Wilson. The differences: Case oil passages thru '69 were 8mm. Starting in '70 the oil passages were opened to 10mm and the second oil relief valve was added. VW oil pumps and camshafts are matched parts! As I recall the difference here was the flat cam gear and the dished cam gear. Make sure you get the right one at rebuild. Oil pumps are named for the length of those gears. Cool!! The 40 hp thru '67 engines used a 17mm pump From 68 thru 69 the pump was 19mm. For '70 only thru August, a 24m pump was used. From August,'70 (71 model), a 26mm was introduced The 17 and 19mm pump go with the flat gear cam and can be inter- changed. Naturally the 19 has a 10 percent higher capacity so that would be my choice. On the other hand, to the best of my knowledge, you could use a later camshaft and go all the way up to the 26 mm pump. That is not a good idea unless you work out some way to open the oil passages in the earlier case. You'd have too much flow which would just become too much pressure. Drag your horsepower down etc. More to the point the earlier cases with the small oil passages and pick-up tube would starve a bigger pump for oil. Or blow it up because the oil couldn't get out the small outlet in the case fast enough. The 24m pump uses a 3 rivet dished gear and the 26mm pump uses a 4 rivet dished gear. Probably can use the 3 rivet for a 26mm pump but they put the extra rivet in there to drive the bigger pump. Could shear the gear off in extremes. Type 1-3 and type 4 pumps are inter-changeable per T. Wilson. The Bentley gives me 28psi operating press. using 30wt oil at 158F (70C) @ 2500 rpm, The oil light comes on at 2-6 psi. Unfortunately it doesn't say which pump or what the flow rates are. This little pump drags the hell out of your engine in the morning when the oil is cold. The main reason your idle is so low at that time. So we covered that pretty good let's get into a couple other aspects that need addressed. The fit of the pump case to the housing: The only seal around the inlet or outlet is the pressure from the case itself. If that pump doesn't fit hard the oil just runs out and back into the sump. The same on the inlet, you'll just be sucking air. Bob Hoover has a "Sermon" on this so check it out. Next is the sharp corners in the pump and the case passage which create some extreme pressure drops. Radius or chamfer them. Those gear/case clearances: it's in the book but I'll read it to you. Backlash: the slop between the gear tooth faces, .008" or less The end-play or side clearance: should be less than .004 without a gasket. I want .001" or less in there. The book shows no check on the clearance between the tooth end and the case. That's crap, I want the same .001"-.004 in there. Do not put a gasket on the cover plate where it mates with the gears if you don't have a good face, use a little permatex. Frankly if you don't have a good face how you going to get the .001" I told you to have? Between the case and the pump flange put what ever you want as thick as you want. Say a 1/16 gasket. The ruling factor on the above is where the holes in the pump line up with the holes in the case and the shaft end fitting into the cam gear. Adjust accordingly. You do this right and you will be justly rewarded. You do it wrong and you will still be justly rewarded, you just won't like it. This is the heart that pumps the lifeblood of your engine and it needs to be treated as such. "Serious as a heart attack" is not just a figure of speech. Now for the commercial: clear back at the beginning I spoke of how your blood carried away the poisons and crap and sent it thru your kidneys. Our little stock engine has no kidneys. We are expected to just dump it's blood out and put in new. It doesn't quite work that way. I don't care if you changed oil ten minutes ago, you do not have clean oil. On a big monster machine, I burnt up $12000 worth of hydraulic pumps and motors finding that out. It even had super bitchin' filters. They just weren't fine enough. President of the company running around yelling "flush the system, flush the system!" He didn't even know what that meant. So we need a kidney, guys, an oil filter. Full flow oil filter. If you look around, they make one that is a pump/filter combo these days and it's an easy fix. The other way, where you drill and tap the case and pipe it in is better in my opinion but get something in there. The bearings, being a lot softer than the pump parts are going to be the first to go. No disclaimers, Fram can send me money for every filter they sell if they want. :-) west