Sprocket replacement time!(?)

[Overlooker]

I have a pretty good feel for when it's time to replace a saw chain, but I struggle a bit more deciding when it's time to replace a sprocket. I recently purchased two saws for parts or repair, an ms390 and an ms391. The 390 was in several dozen pieces in a box, the 391 was "running" but in need of a P&C. In trying to decide what to do with the 391, I checked the B/C/S; the chain is about done for, the bar is mostly blue from running hot, but check out this sprocket:

On the left is a brand new 009 sprocket for comparison. In the middle, the worn sprocket I found in the box of 390 remains. But look at the one on the right:

How did that saw run at all??! Now with this specimen, I'm not struggling with whether or not it needs to be replaced!
So, if I decide to rebuild this 391, my plan is to use Meteor P&C and fit it with a 25 inch B&C. I know Stihl recommends max bar length of 20 inches, but I have plenty of 20 inch users. Would anyone else think it wise to do this for an occasional use saw? O

 

[Wood Doctor]

I would say throw away the one on the far right and perhaps use the one in the center only as a backup. Worn out sprockets can damage chains, generate excess heat, etc.

 

[pioneerguy600]

Run em til the wheels fall off, typical non pro users.

 

[Huskybill]

Get a rim and drum throw the sprockets away. A bar and chain combo. The saw will cut better, fast with less load on it. I change every saw over to rm and drum. With a sprocket nose bar it’s rollerized.

 

[J D]

Agreed, if you're replacing the drum anyway get a rim drive... Saw will need all the help it can get if you want it to pull a 25" B/C.
A MM & running skip or semi-skip chain will help it too.
I'd also be concerned the oiler won't keep up... Some Stihl's have a pin you can punch in to get a little more out of em

 

[Overlooker]

The saw will cut better, fast with less load on it.

So, if I'm putting an extra long B&C on this ms391, I should certainly use a sprocket nosed bar for less friction. I'm also thinking that I should avoid more power-demanding chains like .404. Perhaps 3/8 semi skip? This saw has an adjustable oiler; will it keep enough oil on a 25 inch setup if I keep it set for max?
FYI: I pulled the jug today and found very minimal roughness in the cylinder. I'm planning to use muriatic acid & super fine sandpaper to salvage it. I've ordered a Meteor piston/rings/wrist pin set.
I found there was no cylinder gasket, just a red sealing substance. Is that to be expected? I I understand this saw had a previous cylinder issue that the dealer repaired.
And, NO! I can't throw that sprocket (on the right) away; it's artwork, don't you think? O

 

[Overlooker]

Agreed, if you're replacing the drum anyway get a rim drive... Saw will need all the help it can get if you want it to pull a 25" B/C.
A MM & running skip or semi-skip chain will help it too.
I'd also be concerned the oiler won't keep up... Some Stihl's have a pin you can punch in to get a little more out of em

Thanks, JD. I was composing my last post and didn't see your suggestions until I had already hit "post reply". O

 

[Overlooker]

A MM & running skip or semi-skip chain will help it too.

What's MM? O

 

[pioneerguy600]

MM is a muffler mod, make the exit hole larger.

 

[J D]

So, if I'm putting an extra long B&C on this ms391, I should certainly use a sprocket nosed bar for less friction.

Yes, sprocket tip bar & rim drive setup on the clutch drum ideally.

I'm also thinking that I should avoid more power-demanding chains like .404.

Most definitely.

This saw has an adjustable oiler; will it keep enough oil on a 25 inch setup if I keep it set for max?

That oiler is designed to oil a ~20" bar set to max... Your mileage may vary.

FYI: I pulled the jug today and found very minimal roughness in the cylinder. I'm planning to use muriatic acid & super fine sandpaper to salvage it.

Take a metal rod/ old screwdriver shaft or something similar, cut a slit in the end, insert a long strip of sand paper in the slot, fix it in your battery drill & you have a makeshift buffing wheel. Can do the same thing with a strip of scotch pad to polish it after.

I found there was no cylinder gasket, just a red sealing substance. Is that to be expected?

It's had the gasket "deleted", that is fine assuming it still has enough squish. I'd put it back together without a gasket, be sure to use a proper 2 stroke sealant like dirko/threebond (RTV/Silicone isn't petrol rated & will deteriorate over time).
Measure squish & pressure/vacuum test once back together.
Clean carb out & put a kit in it if it has stiff diaphragms/damaged gaskets/leaky metering valve.
Run fresh non ethanol fuel with at least 40:1 mix of quality oil & tune slightly rich for the first few hours of running.

 

[Overlooker]

Take a metal rod/ old screwdriver shaft or something similar, cut a slit in the end, insert a long strip of sand paper in the slot,

How wide should the slot be? 1/2 inch? 1 inch? Thanks for all the good tips. Does drum/rim sprocket really have significantly less friction than a star sprocket? I'm surprised. I was planning on a star sprocket, as they are cheaper in the short run. But I want to reduce friction wherever I can. O

 

[Gord404]

How wide should the slot be? 1/2 inch? 1 inch? Thanks for all the good tips. Does drum/rim sprocket really have significantly less friction than a star sprocket? I'm surprised. I was planning on a star sprocket, as they are cheaper in the short run. But I want to reduce friction wherever I can. O

They maybe cheaper but in the long run they are not and also dictate what kind of chain you use and rather than change the whole drum every time when you have excessive wear a sprocket is a cheaper option, don't really know about more power thing, someone else can explain that.

 

[cscltd]

It's had the gasket "deleted", that is fine assuming it still has enough squish. I'd put it back together without a gasket, be sure to use a proper 2 stroke sealant like dirko/threebond (RTV/Silicone isn't petrol rated & will deteriorate over time).
Measure squish & pressure/vacuum test once back together.
Clean carb out & put a kit in it if it has stiff diaphragms/damaged gaskets/leaky metering valve.
Run fresh non ethanol fuel with at least 40:1 mix of quality oil & tune slightly rich for the first few hours of running.

ms390 and ms391 have no base gasket to delete- they are gasket less crankcase/cylinder mounting, only the sealant that you recommend

 

[HarleyT]

Here in Ky., we just fix them with a stick welder!!

 

[HarleyT]

Here in Ky., we just fix them with a stick welder!!

I am THAT good!!!

 

[pioneerguy600]

I am THAT good!!!

I have fixed clamshel saws with a 24 ton excavator, a 30 ton is even gooder.

 

[HarleyT]

I have fixed clamshel saws with a 24 ton excavator, a 30 ton is even gooder.

Do you repair chainsaw carbs while wearing leather gloves, like the techs in the Stihl videos?

 

[pioneerguy600]

Do you repair chainsaw carbs while wearing leather gloves, like the techs in the Stihl videos?

Wit myn welding gloves on over those black mechanix kind to prevent cancer.

 

[pioneerguy600]

Oh, an happy birthday, Harley.

 

[J D]

How wide should the slot be? 1/2 inch? 1 inch?

I use one of these...

Followed by something like this...
On something like this...

You can also do this...