What I learned about oil pumps this weekend

[Kong]

Let me ramble a bit for the guys who, like me, aren't ashamed to twist a wrench but haven't spent any time inside a chainsaw, not yet anyway. This is about the oil pump that is responsible for lubrication of the guidebar and chain.

One of my saws wasn't oiling. Its a Stihl MS361. I checked the oiling in the normal way; I removed the bar and chain and then started the saw. Running it at moderate speed for several seconds I watched to oil port to see if any oil came out. I did this when I first bought the saw (used) two weeks ago and it was oiling fine then, so I had a reference. When I checked it yesterday it wasn't oiling. I have to tell you that between the two tests the clutch bearing failed and was replaced - more on that in a moment.

So, there I was with a saw that wasn't oiling and me not knowing much about how saws oil.

First lets back up to the clutch bearing. This is important because on my (Stihl) saw the clutch drives the pump. Somehow or another I managed to lose the clip that holds the clutch and rim sprocket together and on the crankshaft end. The saw kept on cutting and my son didn't recognize the odd sound it was making as anything unusual. The clutch wiggled around and ate the bearing and also touched the oil pump and wore a hold in its case causing it to malfunction. Of course I didn't know that at first.

I repaired the clutch and put the saw back together but it wasn't oiling. That's when the learning experience began. Apart it came. I thought I'd simply explain what is down there, how it works and what to do if yours fails. Its really very simple once you see it all apart.

OK, here is how it works. The drum part of the clutch has a slot in its edge. The oil pump is driven by a heavy wire that engages that slot. So when the clutch spins the oil pump drive, which is under the clutch, spins too. So that's the first thing, make sure that the little tab of wire is engaged in the clutch.

The pump itself is under the clutch. To remove the clutch devise a way to stop the engine from turning and then using a 19mm (3/4") wrench remove it by turning clockwise, its is a left-handed-thread. With it removed the oil pump is exposed. The drive wire attaches to a nylon ring that surrounds the crankshaft, lift it off. Under it the pump body is held in place by two screws. Remove them and the pump will lift up and off.

The first part I inspected was the drive gear. It is a spiral nylon ring gear with what looks like large screw threads. The threads on mine were undamaged and so the drive gear is good. The pump body exposes the pinion which is driven by the nylon drive gear. It is metal. Putting my finger on the metal gear I discovered it spun easily and smoothly, it too appeared to be good. It was at this time that I noticed the worn grove in the pump body on mine that exposed the end of the shaft on which the pinion gear rides. I did not know it at the time but that exposed part was right on top of the actual pumping mechinism.

Looking at the pump body I saw that the adjustment screw could be removed by taking out a pin,which I did. I then saw that the method by which flow rate is controled is by moving the initial position of the pinion gear, which moves ever so slightly in or out of the pump body, out increasing flow and in decreasing it. I then pulled the brass bushing out of the end of the pump and removed the pinion gear along with the saft/piston, washers (2) and spring that rode on it. It was then possible to see how the pump works.

Lubricant is held in the tank section of the saw and enters the pump through a hose that has a filter on the tank end. The end of the pre-formed hose acts as a bushing as the hose passes through the saw body and seals the inlet to the oil pump. When the pump is bolted in place its conical inlet is pressed into the rubber hose. So with the pump removed now is the time to clean the hose and filter and inspect if for cracks. On the suggestion received on another message board I cleaned the line with compressed air. Pfffssssttttt, all cleaned out.

Once the oil enters the pump it moves in a straight line fron inlet to the outlet with the pump's piston causing the flow from the center. The end of the piston has a cutout cavity about half the size of a drop of water. It is that spinning cavity pulsating in and out just a few thousandths of an inch, that pumps the oil. In effect it is a centrifugal pump with a power assist with the little jiggle provided by the eccentric on the end of the drive gear. The gear is held under tension by a single spring with a very small thrust washer on each end.

- - to be continued - -

 

[Kong]

Looking at the oil flow cavity it was clear that both the inlet and exit holes were considerably larger than the internal oil passage, the top of which could be seen through the exit hole. The internal passage is the limiting factor so there would be no gain in flow to be had by enlarging either the inlet or exit port; to do either would be a waste of time. I only mention this because I've seen it suggested in posts that I found when searching for informaiton on this subject. The passage is a blind hole on the end where enlargement would be beneficial, so basically there's nothing you can do here unless you plan to drill all the way through the housing and then find a way to plug the end of the hole you just drilled.

Flow could be increased, however, by enlarging the size of the cutout on the end of the piston. A very small amout of metal removed to enlarge this dimple would go a long way to increase oil flow so one should be careful if they intend to take the grinder or a file to it, that that's the only way I can see to increase flow with a stock pump. Early Chevrolet small blocks oiled the same way and I used to modify them by enlargin the oiling grove on the last cam journal the same way - it made a huge difference in oil flow so if you try this do not remove much metal.

Putting the thing back together is easy enough and there are no suprises. Just make sure it spins freely when you're done. Oh, I counted the number of turns it took for the drive gear to spin the pinion gear. The ratio is 9:1, so this ~14,000 RPM saw is turning the oil pump about 1,500 RPM and along with the pulsating of the piston caused by the eccentric end there is the centrifugal force and if you can increase the miniscule amount of oil that force moves then you have accomplished your task.

On mine the cavity in which the piston pulsates was cut into so what oil was sucked out of the tank exited the pump body, not through the outlet leading to the guidebar; lots of oil was coming out of the saw, just not in the right place.

So that's about it. It comes apart easily and goes back together just as easy, no tricks to it, No special tools needed other than a small set of Torx sockets. Air was helpful, I had a three quarter inch wrench. Oh, a new replacement pump for this saw would cost about $70, I paid $30 for a good used one delivered - ebay.

All you see is all I know, but I'll answer any question I can.

 

[dante2]

Slip some rope through the spark plug hole to stop the piston from reaching the top. Works great.

 

[Kong]

I have a more barbaric method, but I won't post it for fear someone may do as I say. It involves the ugly combination of the words fins and wrenches and I'll say no more.

 

[LLigetfa]

Slip some rope through the spark plug hole to stop the piston from reaching the top. Works great.

When you do this make sure the piston is high enough up on the stroke so the rope doesnt get caught in a port.