PM700 scored cylinder

[FixIt20]

Good afternoon,
I have basically no experience with small gas engine rebuilding/repair, let alone chainsaw rebuilding/repair. I have PM700 that wouldn't run. First of all it didn't have spark, so I got a new coil ($50) for it and that fixed that problem. For some reason, I didn't make sure it would fire before I tore it apart; I guess I just assumed that I could take it apart, clean it up, get a carb rebuild kit and tune up kit for it and put it back together and have a good saw. (Never assume things) Turns out that the piston and cylinder are scored and I for sure need a new piston and I'm afraid I need to get a new cylinder as well. I'll post some pics of the cylinder and the piston for y'all to see. The saw has had a lot of use by the looks of the drive sprocket, and will need a fair bit of money to completely fix up. I tried to clean up some of the transfer with a green scotch brite pad these pics of the cylinder are before and after pics.

I would appreciate some experienced advice as to if this cylinder will cause too much lack of compression and/or ruin a new piston or not, and if its worth putting 500+ into the saw to fix it up.

Thanks in advance

 

[Maintenance supervisor]

It looks like a foriegn object or a bearing came loose in there and removed some of the chrome plating as well as gouged the wall.
I dont think that cylnder is salvageable.

 

[heimannm]

That one is finished.

$500 to repair? Only if it hold great sentimental value. Otherwise, keep it for parts and find another project saw.

Mark

 

[FixIt20]

Thanks for your responses.
FYI, in case you're curious, I put it back together and compression with that piston and cylinder is mid to upper 120s and it fires on starting fluid.

 

[heimannm]

Don't use starting fluid unless it also has some lubricant in it. Your 2 stroke engine needs the oil in the fuel.

120 should be enough to run, let us know how long it continues to work.

Mark

 

[FixIt20]

Makes sense about the starting fluid. I didn't think of that at the time, but definitely a potential problem. It only ran a couple of seconds on starting fluid so hopefully no further harm was done.

 

[muddy42]

Thanks for your responses.
FYI, in case you're curious, I put it back together and compression with that piston and cylinder is mid to upper 120s and it fires on starting fluid.

Interesting. Sorry what did you end up doing - did you fit a new piston and new rings with the old (cleaned up but still gouged) cylinder?

 

[FixIt20]

It is currently put back together with all the old parts including old gaskets. I ordered some gaskets and a few other parts, and am planning to use the old piston and cylinder. I know it might seem crazy to put that old piston back in that cylinder, but it also doesn't seem the best idea to spend money on a new piston without getting a new cylinder as well. And I don't feel like putting that kind of money in it at this point either. Maybe it will be easier to put more money into it later, after I fix it up a little and use it for a bit. At this point I am planning to try to lessen the severity of the scoring in the piston (maybe knock down the rough edges with a scotch brite pad) and see what happens.
I don't use a chainsaw very much, (just to cut up firewood for campfires and such) so hopefully it works for a little while at least.

 

[muddy42]

Firstly, just look very hard for whatever caused that scoring - bits of loose metal in the muffler or crankcase. As has been said, it looks like the damage was caused by metal parts flying around rather than heat. You MIGHT get away with re-using the cylinder (after cleaning it up) based on the fact it appears the damage is in the lower part rather than the combustion part. Even then, generally a new piston and rings are recommended to ensure a tight seal.

 

[jakethesnake]

I’d run it. Wouldn’t sell it to someone but I’d run it

 

[FixIt20]

When I took it apart, I couldn't find anything that looked like a cause for the scoring. Those pics of the cylinder are on the intake side, and the exhaust side of the cylinder looks pretty good--not new, but nothing that can be felt with a fingernail.

 

[FixIt20]

And one the reasons why I didn't run it on fuel is because about half of the fuel line was in a melted pile in the fuel tank.

 

[muddy42]

I am no expert, but my understanding is that damage to the exhaust side are caused by excess heat and lean fuel/air mix. This could be caused by straight fuel, air leaks in the fuel line, carb, intake boot, seals or one of those gaskets you are replacing. Pressure/vacuum test to confirm. Damage on the intake side is from something getting in there, dirt or metal breaking off and rattling around between the cylinder and piston - a piece of the ring, a bearing or the bits of metal that hold the rings on. This is what it looks like from your photos. That is why I am skeptical about reusing the piston. By the way it is acceptable to save the cylinder and simply replace the piston and rings. You don't need to replace both and critically the piston is cheaper.

 

[FixIt20]

I'm just afraid that the cylinder will ruin the new piston.

 

[Maintenance supervisor]

I've got a pm800 with a tiny chip missing from under the exhaust port, a 82cc homelite that had the piston pin come loose and grooved 2 slots in the cylinder, and a poulan 5200 that was obviously on fire at one poont and has a severely poc marked cylinder. They all run fantastic!
The only thing that I would have suggested is you should have swapped the bottom ring and the top before putting it together, the bottom ring usually has much less wear and will seal better.

 

[FixIt20]

So you're saying to let the piston go for now?

That is a good idea about the piston rings. I was planning to take the saw back apart when I get gaskets so I will switch those around when I have it apart.

 

[Maintenance supervisor]

So you're saying to let the piston go for now?

That is a good idea about the piston rings. I was planning to take the saw back apart when I get gaskets so I will switch those around when I have it apart.

Well honestly I don't think I'd use that piston, you can get away with a cylinder but the piston is where the action is, and yours should be replaced.
Part of the problem is parts availability for the 70cc McCulloch saws. LBR makes a replacement piston but it is absolutely know for its failure in keeping the pin in place. You may get lucky with it if you reuse the original piston pin bearings , but keep in mind that they are a press fit and should be fully supported when pressing them in or out.

 

[jakethesnake]

Gently sand down any high spots on the piston.


Run the saw. You’re not going to lose anything worth mentioning power wise. It’s silly to replace the piston with lower quality and have less than you have now. If you can get new rings that would be super cool. I wouldn’t risk breaking them to swap them from top to bottom.

A scratch in a piston isn’t going to change compression or the way the saw functions.

I’m just saying it’ll work. If parts were easy I’d change it out. If it was mine just to run I’d leave it alone

 

[FixIt20]

I can get NOS piston pin bearings are available ($35) and LRB piston ($40)
NOS piston rings are also available ($40).
All from ebay

 

[Maintenance supervisor]

I don't think the same rings fit the LRB and oem.
If you have the appropriate size 1/4" socket to fit inside your original piston bearings you can press them out and reuse them.
A vice can be used but I bought a little arbor press from harbor freight.