McCulloch sp125 Teardown Rebuild

[HerbalSage]

Inherited a bunch of chainsaws from Grandpa that havent been run in lord knows how long. My first focus is getting this 125cc saw into operation. It looks like a hose is snipped and he modified the air intake. What can I do to insure this saw runs good? The dogs on this thing are insane. Very proud to have it passed down to me!

 

[Bob Hedgecutter]

Nice old muscle saw- mine is pretty much bog standard.
I guess you have to figure out how to connect the fuel line and impulse onto the carb modification- then its a matter of the usual perishable items like the intake manifold rubber, seals and things like that.
Chip guards on the starter can get pretty brittle over time- I managed to find a guy that 3D prints them and replaced what was left of mine.

 

[HerbalSage]

Nice old muscle saw- mine is pretty much bog standard.
I guess you have to figure out how to connect the fuel line and impulse onto the carb modification- then its a matter of the usual perishable items like the intake manifold rubber, seals and things like that.
Chip guards on the starter can get pretty brittle over time- I managed to find a guy that 3D prints them and replaced what was left of mine

I found the pdf file for the manual, going to take it apart and see what's up. Think I can get the hoses easy. Thanks for indicating the modded carb, I have no idea what's stock. Seeing that intake I knew right away he must have run this saw pretty good. Do you think I could run it with the intake mod? Hope when my mom tears down some boxes he had the top to it will show up.

I'll make a post eventually showing off my saw collection.

I'm keeping this one and giving the 3120xp Husqy to my brother. Got a couple other Macs with the haul he even had the mini Mac.

 

[Bob Hedgecutter]

No ID tag any more- but it was likely a saw that had the non adjustable carb on it- they did that for a while because guys were getting them, screwing them up for max rpm, eliminating the governor and burning the saws up- so taking away to option of fiddling and the problem was somewhat solved.
But then people put alternative carbs on them- I took the original off of mine and fitted the carb off an old Stihl 051 to have an adjustable carb.
I am guessing this was your Grandpa's way around it and better air filtering to boot.

 

[HerbalSage]

After some time I have decided to dig into the saw. Did a complete teardown, faced the dreaded 10/32 spline bolts for the piston rod, hammered out the bearing, slipped the piston out.

I knew this saw had low compression just playing with the pull cord and my friend confirming. Looking through the muffler and intake I could see scoring. But oh man I think I know why my fuel lines looked snipped rather than rotted off.

Upon inspection, it seems the rings have melted to the intake side of the piston. 3/5 of the rings feel free to move but ultimately this is what caused the low compression I could notice without a gauge.

Thankfully the cylinder wall looks immaculate, such a suspense waiting to see how it looked trying to figure out how to get the crank out etc.

https://www.ebay.com/itm/385390248189 matches the same part #. Can anyone vouch for Lil red barn? Seems like the way to go.

 

[Bob Hedgecutter]

Personally- I would not touch Lil Red Barn stuff.

Id say something far worse than a lean condition is going on there- looks like something metal has gouged the side out of the piston- like a bearing having let go.
You shuda kept the 3120.

 

[HerbalSage]

Just an update. Bought the piston on ebay so I can return promptly if incorrect. I might get Caber rings if the rings from them are no good. HY, Meteor does not make this piston. If anyone knows a honey hole for a piston let me know.

The manual says there are 2 sets of roller pin bearings. Set of 24 for the crankshaft, set of 22 for the wrist pin. Upon counting all bearing pins are accounted for so it makes sense why the cylinder wall doesn't have a catastrophic failure as result of whatever in the world it chewed up. My suspicion is someone was running to lean and overtime it made some chunk, or something dropped in there.

When the piston comes I'll do a side by side comparison to at least have an example for the forum.

Definitely aluminum transfer in the cylinder, need to address that before the piston comes.

Work bench is like a stack of cards right now, its well organized but makes my head hurt.

P.S: My brother is probably never going to use the saw 3120xp. In Highland Park Mi we are the "city of trees." many are invasive or old and need to be cut down. My grand plan is to have my 1994-2004 520 Lopi Liberty heating my house from now on. Lots of norway maple to cut down and my brother understands its best for me to keep it. He's probably keeping the 371xp once I tune it.

 

[HerbalSage]

So, that LRB piston came.

Measured both pistons and wrist pins with verneer calipers. Pins are spot on the same at 14mm, Pistons are differential by a half a millimeter, most likely due to the obvious damage. However the new one measures as advertised 57.83MM.

I have not tried the fitment of the rings in the cylinder. I have to clean up all the bearings and prepare for surgery sometime soon.

I have some pics to show direct comparison of them if anyone else is interested.

I know LRB has their rep and I'm not trying to shill out for them. But so far this piston looks like its worth a million bucks, to me.. XD

***Changed forum name to Mcculloch Rebuild so others who google it can find this.