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395xp oil pump drive gear failure

Nate Harp

Nate Harp

New Member
May 17, 2019
Casper WY
Hi all, I haven't posted here before but I have done a lot of reading.

My 395xp quit oiling during a milling session.
Upon inspection I discovered a stripped plastic gear for the oil pump that is driven by the clutch drum.

This may have originated due to use of veggie oil and improper storage, I replaced the whole oil pump assembly, and also got a new clutch assembly, with bearing.
At first I thought it was fixed, lots of oil on the bar, proceeded to tune the carb.
A day or so later I re-visited the carb tuning with a handy engine RPM app on the smart phone. No issues.
At one point, I did notice the chain turning at idle and idle adjust and low jet would not solve that.
Another start later and that issue went away? The oil probably stopped then to.
I did blow out the oil holes to the bar and tank when I had the pump out.
The only washer is between the crank seal and the plastic gear.
The wear pattern on the failed parts looks like the center of the threaded part of the plastic gear meshes with the piston pinion of the oil pump, looks like they line up correctly.

Decided to work over a couple new ripping chains, and I discovered that the chain would not turn freely, the crank shaft was turning with the clutch drum due to an interference fit of the new plastic pump drive gear.
I don't think I ordered the wrong part, I bought a set of 4, likely not OEM.

Some research here and around the web. I sanded the bore of another plastic gear so that it was a slip fit on the crank shaft.
Ran the saw on a couple slabbing cuts, and noticed no oil again. Opened it up and had another stripped gear.

1. stripped gear - original had worked, unsure why it failed maybe oil blockage.
2. stripped gear - new one was a tight fit on the crank shaft.
3 stripped gear - Modified a new one to slip easily on crank shaft.

I have two more new gears and don't want to repeat this insanity.
I haven't checked for play in the crank shaft itself yet, will do so today. The seal looks good and I haven't noticed any play when working with the oil pump and clutch. I haven't had the other side apart.

Any advice is appreciated. I plan to check closer that all the parts are right.

a couple quick questions?
1. Is the clutch drum supposed to have end play, how much is normal? It can move side to side a little bit, The clutch comes up tight to the bearing but I can still move the drum in and out, Similar to how the sprocket can move side to side on the drum splines. This creates some bell ringing.
If this is not correct, I can see how the plastic gear might be able to move out and strip.

- I can see how if the bearing was too long, that this might be the case, I will compare later today with original clutch parts and bearing.
- maybe I need a another washer in there?
- the normal rotational direction should work to suck the plastic gear in toward the case.
- I put some grease in there on the gears.

2. I read somewhere that someone else sanded out the bore of the plastic gear like I proceeded to do. Is this normal, did I get a bad batch of gears?
- doing this did allow the chain to move freely without turning over the crank.

- are there any other modifications to the gear or parts that will help it along.

3. Is there a proper break in procedure for a new oil pump or pump gear.

Hopefully a simple issue, a little long winded but maybe that will help narrow it down.
Nate Harp

Nate Harp

New Member
May 17, 2019
Casper WY
Thanks for the reply.
I have been installing the gears after the pump piece is already bolted down.
Carefully screwing down with the clutch drum by hand, using the same to get them out.

Opening up the bore of the plastic gear so it doesn't stick to the shaft is a must, the first new gear didn't get that treatment.
I may have done a couple things wrong with that first one. I recall that when I put it together the first time the adjuster screw was out a little too far and it interfered with the saw body, after I noticed that the pump bolts needed another touch, I am not sure if I had the gear in a this point or not.

I tried the second gear without thoroughly cleaning out the new pump. Although I did sand out the bore so it was free of the shaft. The debris in the pump likely cased it to stick and cause the second new gear to fail.

Hopefully the third time is a charm.
I took apart the oil pump and cleaned it out. Got everything together and feeling good by hand.
The plastic gear drive ears also needed a tiny shave to fit into the drum slots.
Idled it with the clutch back on and got to see oil flowing.
I have the bar and chain back on and ready to see how it goes, I will start out with the adjustment turned in and low. I think I can get the idle speed adjusted up to where it will start clutching, let it run there for a while and let the gear find its own real good before giving it impulses up to 12000 rpm in a fraction of a second.
Or is it better to give it the speed and risk shearing the threads.
I think its going to work this time.

Upon close inspection and play with the oil pump, it may be best practice to avoid a bind to make the click increment adjustments while turning the pump.

The original oil pump piston had black hardened crap on the beveled face; wrong bar oil got too hot or too old or both. There is a clean circle near the center where it had been set and notch over to the side a little. The pump adjustment screw didn't like a different location on that face, binding the piston rotation, and stripping the plastic gear.