Sunday, December 20, 2009

Poulan 1420 Electric Chain Saw Overhaul

The big boss where I work is an ace firewood scrounger. He's used the subject machine for several years in his garage for cutting up lengths of hardwood logs that he comes across. The saw started making some grotesque noises awhile ago. He bought himself a new one and brought me the casualty to look into.

I took it apart and found a curious failure. The motor bearing at the business end of the motor (the end with the drive gear on it) was still in good condition, but the bearing at the commutator end had failed catastrophically -- to the point where it had spilled its balls. One would expect it to have gone the other way 'round. The business end bearing is the one with all the stress on it; the commutator end bearing is pretty much just along for the ride. It may be that the commutator end bearing is more vulnerable to airborne particulate contamination; the business end bearing is more sheltered

Following is a detailed teardown and overhaul procedure.


[I don't write teardown procedures as linear exposition. I find that approach unhelpful and eye-glazing. What follows is a list of each item to be removed or dealt with in order, along with any and all information pertinent to each item. I normally use 'bullets' as sub-item leads, but I can't do that the way I'd like to here; Blogger's text entry apparatus is not a full-blown word processor by any stretch of the imagination. Instead I'll use the character '>'.]

>'A/F' = 'Across Flats'; i.e the span of a hex nut.
>'w/' = 'with'.
>All screws are No. 2 Phillips recess.

1) Chain Drive Side Cover
>One No. 10 x 1 3/4" black pan head threading screw at the lower front. Note that there's a rolled steel spacer associated with this screw. It will be free to fall out when the cover is removed. The spacer is 1" long, 1/4" I.D., 3/8" O.D.
>Seven No. 10 x 3/4" black pan head threading screws.)

2) Chain Bar Clamp Plate and Bar/Chain
>Two 5/16" - 18, 1/2" A/F hex nuts w/integral washers.

3) Chain Tension Adjuster
>Lift it out. It's an 8-32 x 1 3/8" round head screw with a 1/2" square washer under its head, and a travelling, female-threaded steel 'finger' that engages the chain bar. The screw's end has been purposely 'distressed' so the travelling 'finger' can't be removed.

4) Gearbox Cover
>One No. 10 x 3/4" black pan head threading screw at the cover's rear, and then it just lifts off..

5) Switch
>The interlock button is a little snapped-in plunger. Pry it out, then the switch can come away from the frame.

6) Bar Oil Reservoir
>Lift it out. There's no fastener.

7) Large-Gear/Chain-Sprocket
>One 5/16" - 18, 1/2" A/F hex nut w/integral washer.
>One 5/16" fender washer, 1 3/16" O.D.
>The item just slips off its spindle. It has a combination sleeve bearing and needle roller bearing in its bore. The sleeve bearing portion supports the large gear at the inboard end. The needle roller bearing supports the chain sprocket at the outboard end. The gear is some sort of plastic material.

8) Motor Assembly
>Five No. 10 x 3/4" black pan head threading screws.
>The assembly is pulled straight out.
>The motor is a Johnson Motor No. U-9832.

And there we are. We have the motor out. Now the real fun can start.

Needless to say, every item listed above was filthy with greasy sawdust and needs cleaning. Now, prior to dismantling the motor, is a good time to do that. All the cleaned stuff can go in a bin and be set aside out of harm's way. I'll have my bench clear for the work on the motor.

Motor Teardown

A word of caution: Armature and field windings are easily damaged. Exercise great care when dismantling and handling motor components. A split-second's carelessness can cost you the motor.

Another word of caution: Before taking something like this apart, it's a good idea to mark components' relationships to one another. An electric engraving tool is most helpful here. Also, a centre punch can be used to produce permanent alignment/orientation marks. Take care to make your markings unambiguous. Axial dimensions should be measured and recorded; things like shaft protrusion lengths can turn out to have a bit of variability at reassembly, and it's good to have a record of the original, factory dimension.

1) Switch/Linecord
>The two motor leads have to be disconnected to free this item from the motor. The switch's wiring terminals are the 'push-in' type. By poking the shank of a 1/16" twist drill in alongside a wire, you can deflect the terminals' springy contact and release the wire with no harm done to the contact.

2) Pinion Gear
>There's a small external snap-ring at the end of the shaft, suggesting that the gear should just slip off the shaft once the snap-ring is removed. From what I've seen of most readily available snap-ring pliers, pliers with small enough tips to fit a snap-ring this size are not so easy to come by. I solved this problem for myself years ago by filing the tips of a pair of snap-ring pliers to fit small rings. You may have to do the same. It's a painstaking bit of work, but quite doable.
>With the snap-ring off, the gear proved to be stuck on the shaft. There wasn't space to get a puller on it, and prying with a screwdriver didn't work. And what there is to pry against is the plastic end-frame of the motor, which limits how forcefully one dares to pry. As I was rummaging through my drawer full of pullers, I came across a vehicle door trim remover that I'd bought long ago and all but forgotten that I had. I tried it on the gear, and it turned out to be the perfect pry bar for the job. It provided sufficient leverage while doing no harm to the plastic end-frame.
>One can expect to run across this sort of snag on old machinery. All it takes to make a slip fit an obstinate fit is a tiny bit of rust or sticky fouling on a shaft or in a bore.

3) Cooling Fan
>Another snag. The cooling fan is a plastic disc with twelve straight, centrifugal blades on it. Its means of attachment to the shaft wasn't obvious. It didn't appear to be threaded, though I tried twisting it in both directions -- to no avail. That left pulling it off, but I could see no way of applying a puller that wouldn't damage the part. With a 1/16" twist drill, I drilled the plastic hub directly alongside the shaft at nine places around the shaft to relieve the hub's grip. That worked. I was able to twist and pull the fan off. 'Turns out the fan's bore was pressed onto a finely splined shaft -- a very secure fit indeed. There's still enough left of the fan's bore that the fan should fit back on properly, and at final reassembly I'll fill all the drill holes with epoxy.
>If I had it to do again, I'd try drilling two 11/64" holes through the fan 180 degrees apart so as not to imbalance it at all. I'd get the holes as near to the shaft as possible, where the disc is thicker. Then I'd have a means of installing a couple of 8-32 screws with nuts and rigging a puller.

4) Brushes
>The brush holders and springs were well thought out in this motor. The springs are torsion springs that are easily backed off and moved aside for brush removal. There was a snag, though, in that one of the brushes had overheated and was stuck in its channel. I did some harm to the face of the brush as I pried at it to free it up. Nothing fatal, but not good. I should have sprayed it with WD-40 first; that might have made it easier to nudge the brush loose.

4) Commutator End Frame
>Two No.8 x 2 3/8", 1/4" A/F hex washerhead screws.
>Since the bearing had no balls in it, the end frame came away easily. With the two long screws out, the motor's field winding frame and its other end frame were no longer attached, and I wanted them to be held together until I could get the armature out. I put the screws back in with two 7/8" long, 1/4"-20 coupling nuts under their heads for spacers, to take up the screw length formerly taken up by the commutator end frame.

5) Outer Bearing Race
>This part was a light interference fit in the plastic end frame. I was able to punch it out a little at a time by engaging a small chisel with the groove in the bearing race from the outer side of the end frame.

6) Armature
>Easily pressed out of the remaining plastic end frame on a hydraulic press. There was a wavy washer on the bearing's outer face, presumably to provide a bit of axial preload to the assembly.

7) Business End Bearing
>There was a substantial e-clip on the shaft serving as the bearing's axial position reference/stop. With the e-clip pried off, it was easy to get a small, two-jawed puller on it and pull the bearing off. Prying off the e-clip had to be done carefully so as not to harm the windings.

8) Inner Race of the Commutator End Bearing
>This was tricky because the part was too small to afford any purchase to a puller's jaws. As with the other bearing, there was an e-clip to remove. With the e-clip out of the way, I had just enough room to cut through the race with a small cut-off wheel in the Dremel hand grinder. I did a bit of harm to the shaft's surface, but not enough to have any ill effect on installation of a new bearing.

9) Business End Frame
>'Removed the two screws that I had put back in with spacers in item 4 above.
>I blew off all the greasy sawdust that I could and degreased it in the parts washer. For a follow-up on parts like this, I spray them down liberally with Fantastic and rinse them with scalding hot water while wearing nitrile gloves. Blind screw holes must be blown thoroughly dry before reassembly. Pipe cleaners are helpful as well for drying out blind screw holes.

10) Armature
>'Needs a thorough blow off with compressed air.
>I chucked it in the lathe and burnished the commutator with 600 grit silicon carbide paper, followed by 1200 grit. A jeweller's screwdriver works nicely for scraping the gaps between the segments. Just prior to reinstalling the commutator end frame, I'll wipe it off with lacquer thinner to make certain that it's free of any oily contaminants.


Both bearings are the same; they're NSK No. 6900ZZ -- 10mm bore x 22mm O.D. x 6mm width, shielded both sides. I mentioned to the guy at BDI Canada, the bearing distributor, that a sealed version might be in order as a replacement, and he said that sealed bearings are avoided in hot-running applications; the seals will tend to quickly go to ruin. A serious wood cutting session in July would no doubt make for a very warm motor, so I went with a shielded replacement. The BDI fellow tried to find me a less expensive bearing, but it turned out that NSK is the sole source for this particular bearing, so NSK it had to be; $15.80 plus sales taxes. A new saw is about $70.00. Small wonder that the landfills keep filling up, eh?

The one good original bearing spun very freely, which is indicative of a grease-starved bearing. It's possible to pry off a shield on a bearing with relatively little damage to it by shoving the sharp point of a scriber between the shield and the inner race. I did that and, sure enough, the bearing's interior looked like a grease-free zone. I let it sit in the parts washer's solvent for a week, then thoroughly rinsed it and blew it dry. (By the way, it's considered a poor practice to set bearings to spinning with compressed air; you can cause them to spin without lubricant at outrageous speeds by doing that -- not good.)

For a bearing grease, I'm partial to Canadian Tire's "Motomaster" brand "Wheel Bearing & Chassis Lubricant". (My truck's front wheel bearings have been running in it for a couple of years now since I replaced the rotors and bearings.) I'm probably guilty of overfilling the bearing with grease, but I figure that if excess grease seeps out when the thing warms up, no harm can come of it since this bearing will be at the end opposite to the commutator. I reseated the shield by going around its outer edge with Channellocks, gently squeezing its lip back in place. It might be possible to press or punch it back in place, but you'd need an adapter the full diameter of the bearing, and you'd need to get everything set up for a dead square start -- easier said than done.

Motor Reassembly Notes

A 10mm, six-point, deep 1/4" square drive socket served as a pressing tool for pressing the bearings back on the shaft by their inner races, but a bit of modification was called for. As with most socket wrenches, the hex opening of this one was quite severely chamfered inside to make it easy to slip the wrench over a nut. I chucked it in the lathe and with it spinning, applied a small grinding wheel in the Dremel tool to it to grind its end down to where most of the chamfer was gone. That gave me a pressing tool that presented flatly to the face of the bearing's inner race.

I found it easier to get the bearings started squarely by hand with the pressing tool and a plastic-faced mallet, than to start the bearing directly on the hydraulic press. The hydraulic press is a wonderful tool, but some items can be awkward to set up on it, and this was one of them.

The remainder of the work was easy and straightforward. The bearings' outer races are a hand press fit in the plastic end frames.

I drilled the fan in two places, as I'd mentioned earlier that I should have done in the first place. I'll be able to apply a puller now should I ever have to remove it again. I used my woodworking vise as a press for putting the fan back on. It went on squarely and looks like it will hold fine, in spite of the nasty way I went about getting it off. I'll still shove as much epoxy as I can into all the holes I drilled, though.

Motor Bench Test

I clamped the motor securely in the woodworking vise for a trial run. (Don't even think about doing this sort of thing unless you have an absolutely secure way of holding a motor that in no way involves your hands. Eye protection is mandatory. Universal motors have considerable starting torque; the torque reaction at start-up can be startling. Then they run at a no-load speed in excess of 20,000 rpm. They're not an item to be toyed with. See my anecdote "A Cautionary Tale".)

It runs fine, though the commutator sparking looks a bit excessive to me. It's going to have to do; the likelihood of me finding replacement brushes for this motor is slim to nil. Time to reassemble the machine.

Saw Reassembly Notes

Pinion Gear

The pinion gear can go back on the motor's shaft either end first, but it's best in reassembly to get gears back together in the same mesh they've always had. Though the pinion's teeth exhibited no wear to speak of, it was evident which side of the teeth had been in contact with the large gear's teeth, making it easy to determine how to put the gear back on the shaft.


I broke the little snap-ring for the pinion gear while trying to correct a bit of distortion in it with pliers; sometimes it's best to leave well enough alone. I have spares on hand, so that caused me no real grief. (Princess Auto puts up assortments of common mechanic's fasteners in compartmented boxes. They're well worth having in the shop.)


I packed the large-gear/chain-sprocket's needle roller bearing with wheel bearing grease, and smeared the sleeve bearing and spindle with it as well. Before slipping the item onto it's spindle, I filled its gear teeth with grease. After installing the gear and giving it a few rotations, I took away all the squeezed out grease. For an arrangement like this one, that's as good as it gets for gear lubrication.


All of the casing screws are of the 'threading' variety; i.e they form their own female threads in the plastic when they're first installed at the factory. The type used here do that by displacing and compressing plastic; no plastic material is cut away as with some types of threading screws. It makes for a very sound and strong female thread. At reassembly, it's important to get the screws started correctly so they run back into the original thread and don't start forming another thread; that tends to lead directly to a stripped thread. A way to ensure engagement with the original thread is to present the screw lightly to its hole, then turn it counterclockwise until you feel it drop into the female thread's starting point. The screw should then run in easily.

An interesting feature of the screws used here is that their threads are a double helix; you can see it if you examine the end of a screw closely -- there are two helix starts. (Hence, 'double-start' screw.) I can't really prove this, but I suspect that such screws are prone to have very slight asymmetry between the two helices. The result of that is that a given screw may run back in more easily from one of its starts than from the other; something to be mindful of if you find that a screw is offering more resistance than feels right.

Wiring Lay

Wiring lay can be an aggravating thing to re-establish in portable power tools; there's often very little space for it, and even stranded conductors have enough springiness to make a nuisance of themselves as you try to get them to stay put while you close things up without pinching any wires. This machine was actually pretty good in that regard; plenty of space, and a well thought out wiring lay.

Whenever you're closing up a wiring cavity, and you start to feel a 'squishiness' as you tighten a screw down on what should be a rigid interface, stop. You're pinching a wire. Open it up again and correct the problem.

Chain Tension

Poulan's instruction manual is a bit less than enlightening on this. After explaining the action of the chain tensioner and taking one through to an initial point of tension, one is told to, "Continue turning the adjusting screw until the tension is correct." [!?] One is not told what the criterion for 'correctness' is. I'll go with 'no slack; no tightness'.

Bar Oil

I looked into this and learned that ordinary lubricating oil is not recommended. Chainsaw bar oil is formulated to be sticky so it doesn't get flung off the chain. I went to Canadian Tire for some and it turns out that the stuff is also formulated for cold weather or warm; what they had on the shelf this time of year was marked "Fall/Winter". At least the price is within reason -- $3.49 for a Yankee quart (946 ml).

And Here It Is

I began this post before I'd acquired a camera, so I don't have a 'before' picture; 'Before' would have looked about the same, only grimy.

- - -


'Oiled the chain with a little oiler I made from a ladies' hair colouring goo bottle, filled the oil reservoir and tried it on a 2x4. It runs and cuts.

The bar oiler appears to have ambitions to be an automatic oiler, judging by the puddle it's leaving me when I set the saw down. I'll have to look into that. Wouldn't hurt to sharpen the chain, too, although it's not in too bad shape.


The Bar Oiler

The oil reservoir is a flexible plastic 'bottle'. To dispense oil, you press down on its filler cap, which squeezes the thing and forces oil out of its dispensing orifice. It does seep oil out of that orifice when it's sitting idle. There's no check valve of any kind associated with it, so it's not clear to me how it could ever not seep oil. Perhaps when new, the orifice only opens when there's some pressure applied behind it, but age and use have made it leaky.

Poulan's website is quite impressive for providing service parts, but I won't pursue getting a new oil reservoir -- a bit too much of business and commerce for my liking. If I hang the saw by the hole in the nose of its bar when it's not in use, the oil will be down away from the orifice and won't be able to leak out. Whenever the saw is actually in use, the leakage really won't be a bother; problem solved.

The Chain

On closer inspection, I can see that this chain has been resharpened many times, and not uniformly -- some teeth are shorter from front to back than others. If I had more confidence in my motor overhaul, I'd get a new one. As things are, I'll give this one a touch-up sharpening and live with it. I'm actually unlikely to have much use for the saw.

The chain is made by Oregon, which outfit appears to pretty much dominate the industry. As with all things, when you start looking into the subject of saw chain you discover that there's much to learn, not that I really need to know all that much of it to deal with this chain.

This chain appears to be a fairly light duty, very low kickback design. Typically, every other link of a saw chain has a cutter tooth. On this one, every third link has a cutter tooth, the two intervening links are toothless 'bumper' links. This is probably not a chain that a working professional would have any use for -- it's no doubt been specifically designed to be as safe as possible for occasional use by homeowners.

Saw chains are sharpened mounted on the bar. Presumably, the filings fall away and don't pose a great wear hazard to the chain. Here's a photo of a good setup for sharpening:

There's quite an array of sharpening gear available out there, but at bottom all that's really needed is a round file of the correct diameter for the cutting teeth (5/32" in this case). Adherence to the original chisel angle of the teeth, and consistency of tooth length are what one wants to strive for. That's probably easier said than done; the odd tooth may need more filing than others, and so end up getting shortened more. (By 'shortened' I'm referring to front-to-back length of the tooth, not height. Teeth are never filed on their top surface that I'm aware of.)

It's helpful to dab one link with Wite-Out to indicate a start/end point on the chain. I'm inclined to file from a tooth's chisel edge inward, so as not to turn a 'wire edge' on the tooth.

Anyway, this chain is too far gone to be worth fussing over. I've given it a touch-up filing. I'll see what use I find for the machine.

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