Scored piston - What should I do?

burroak · Nov 16, 2010

I recently purchased an excellent condition Stihl 310 for $80. It's like new cosmetically, even 95% of the paint is still on the bar, however the piston is scored. With some effort I can get the saw to start, and once I do I can rev it up all the way, but when I let off the throttle, it will die. I've attached pics of the scored piston.

My question is, since it still has just enough compression to start and run, is there a "cheap" fix for it? I'm trying to decide what I should do with it - fix it and use it, or sell it as-is. What say you?

BTW, did I pay a fair price for it?

Duke Thieroff · Nov 16, 2010

Piston looks shot, for sure. Cylinder may be ok, clean it up and put a new piston and rings in it.

You'll do much better on it if you fix it up, for sure.

CT

blsnelling · Nov 16, 2010

You very easily may have ruined the cylinder when you started and revved it. Never start a saw that you know is sick. The only thing to do is to pull apart and see if it can be cleaned up. If so, stick a new piston in it. If not, buy a new MS390 topend from your dealer.

burroak · Nov 16, 2010

blsnelling said:
You very easily may have ruined the cylinder when you started and revved it. Never start a saw that you know is sick. The only thing to do is to pull apart and see if it can be cleaned up. If so, stick a new piston in it. If not, buy a new MS390 topend from your dealer.

That's what I was afraid of. I actually didn't know it was scored when I bought it.

Duke Thieroff · Nov 16, 2010

burroak said:
That's what I was afraid of. I actually didn't know it was scored when I bought it.

Always check them, if not before you buy it, after you buy it. Most definately check before you start it.

Always get a compression reading too.

CT

Trigger-Time · Nov 16, 2010

Never been in one, but why am I seeing what looks to be bottom
of piston skirt. Just below exhaust port on right side (left in picture)

TT

brages · Nov 16, 2010

If you don't like working on saws, just sell it again, you'll get your money back.

I don't think the piston looks horrible. If you want to save money, read a lot here and tear the saw down. Then clean up the cylinder with muriatic acid.

Then put in an aftermarket piston -- Northwood Saw has decent pistons for a better price than the ebay vendors. Do not use a Golf piston! Also, don't over-tighten the cylinder bolts... (!!!)

Then, it will either work like a champ -- or it will die again and you can re-do the job with a complete piston/cylinder kit. Maybe Bailey's will have their aftermarket 390 kit together by then...?

sefh3 · Nov 16, 2010

The 310 is similair to the 290. In my signature there is a thread on how to tear these down. I don't know of an aftermarket kit for these clamshell engine yet. Some of the sponsors here have pistons cheap. I would tear it down and put a new piston in it but before that read another post in my signature and build a vac/pressure tester to figure out if a seal went south causing the piston to seize.

audible fart · Nov 16, 2010

Would the problems with the OP's saw most likely be attributable to sawdust getting sucked through the carb?

CTYank · Nov 16, 2010

burroak said:
I recently purchased an excellent condition Stihl 310 for $80. It's like new cosmetically, even 95% of the paint is still on the bar, however the piston is scored. With some effort I can get the saw to start, and once I do I can rev it up all the way, but when I let off the throttle, it will die. I've attached pics of the scored piston.

My question is, since it still has just enough compression to start and run, is there a "cheap" fix for it? I'm trying to decide what I should do with it - fix it and use it, or sell it as-is. What say you?

If that's someone's idea of "excellent condition" I'd hate to see their "beater" or "fixer-upper." At least they did clean the exterior.

If you're going to fix it, I'd:
1. Second the suggestion about pressure/vacuum tester. (Crank seals are ?)
2. If keeping the cyl, do a major cleanup on the fins, besides cleaning the plating.

Why? Since the piston is cooked above the rings, the prime suspect is overheating. Possibly caused by crankcase vac leaks, or caked-up crap on fins. :blob2:

JustinM · Nov 16, 2010

blsnelling said:
You very easily may have ruined the cylinder when you started and revved it. Never start a saw that you know is sick. The only thing to do is to pull apart and see if it can be cleaned up. If so, stick a new piston in it. If not, buy a new MS390 topend from your dealer.

+1

Hopefully the only thing you did is maybe transfer some more crud to the cylinder that can still be rubbed off. Good luck!

blsnelling · Nov 16, 2010

AM 290 topends are available, but not 310 or 390. I just saved a 390 cylinder myself and put a new Episan piston in it.

JLB-MT · Nov 16, 2010

Yesterday I looked at a 660. It had a greyish coating on the piston, very minor scoring. The rings had rust on them. The coating could be wiped off, however it looked like it was flaking in areas. Never saw this before. The saw had not run in awhile. Compression seemed ok but did not run it on a gauge. Not sure what to think of that one.

audible fart · Nov 16, 2010

audible fart said:
Would the problems with the OP's saw most likely be attributable to sawdust getting sucked through the carb?

Anybody know? As an MS390 owner I'm curious about what causes this.

burroak · Nov 16, 2010

sefh3 said:
The 310 is similair to the 290. In my signature there is a thread on how to tear these down. I don't know of an aftermarket kit for these clamshell engine yet. Some of the sponsors here have pistons cheap. I would tear it down and put a new piston in it but before that read another post in my signature and build a vac/pressure tester to figure out if a seal went south causing the piston to seize.

Thanks for the help, sefh3, and everyone else! This site is a huge help. I'll take it to a stihl dealer later this week and see what they think about it. I've never worked on a scored engine before, so I don't know how to tell if simply replacing the piston/rings will fix it, or if it needs an all new upper.

brages · Nov 16, 2010

audible fart said:
Anybody know? As an MS390 owner I'm curious about what causes this.

Looks like scoring from getting the piston too hot to me. Too lean or straight gas.

JustinM · Nov 18, 2010

burroak said:
Thanks for the help, sefh3, and everyone else! This site is a huge help. I'll take it to a stihl dealer later this week and see what they think about it. I've never worked on a scored engine before, so I don't know how to tell if simply replacing the piston/rings will fix it, or if it needs an all new upper.

IMHO if you're not willing to do the work, i would just sell it on ebay (make sure you disclose everything you know about it). But thats just me. There's nothing wrong with this option - some guys just wanna cut wood, not work on saws too!

If you take it to the dealer I think you're going to find the cost prohibitive.

blsnelling · Nov 18, 2010

audible fart said:
Would the problems with the OP's saw most likely be attributable to sawdust getting sucked through the carb?

No, that would not be from a bad air filter. That would show on the intake skirt over time. This kind of damage comes from a saw running too lean or with no oil in the fuel.

JLB-MT said:
Yesterday I looked at a 660. It had a greyish coating on the piston, very minor scoring. The rings had rust on them. The coating could be wiped off, however it looked like it was flaking in areas. Never saw this before. The saw had not run in awhile. Compression seemed ok but did not run it on a gauge. Not sure what to think of that one.

I would seriously recommend you not start it. Tear it down and find out what's wrong and fix it. You may save yourself the cost of a new cylinder.

brages · Nov 18, 2010

blsnelling said:
AM 290 topends are available, but not 310 or 390.

There's a 310 aftermarket top end on ebay right now: http://cgi.ebay.com/STIHL-MS310-PIS...979?pt=LH_DefaultDomain_0&hash=item20b456e0bb

47mm bore, according to the ad. I haven't seen any pistons yet, though.

blsnelling · Nov 18, 2010

brages said:
There's a 310 aftermarket top end on ebay right now: http://cgi.ebay.com/STIHL-MS310-PIS...979?pt=LH_DefaultDomain_0&hash=item20b456e0bb

47mm bore, according to the ad. I haven't seen any pistons yet, though.

Excellent! 47mm is indeed for a MS310. We're getting closer to a AM 390 topend

Scored piston - What should I do?

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