McCulloch SP125C piston

[Gregford]

Hi all,

I've just bought an SP125C from an old logger. When I took the muffler off to inspect the bore, I could see that the piston is a bit scored and damaged. The rings look okay, as does the side of the bore that I can see (the side I don't need to inspect, of course). I'm hoping the bore will be okay as the saw hasn't done a lot of work..

Does anyone have pistons for these? I'm in New Zealand and any source of parts for these big Macs has long since dried up over here.

One other question - the crankcase appears to have a lot of what looks like chain oil in it. Did these saws have oilers which could let chain oil seep into the crankcase if they malfunction?

Looking forward to firing this baby up! It'll keep my Canadien 271s company.
:chainsawguy:

Greg

 

[Al Smith]

One other question - the crankcase appears to have a lot of what looks like chain oil in it. Did these saws have oilers which could let chain oil seep into the crankcase if they malfunction?



Greg

The oiler is an externally mounted impulse driven piston pump,located forward of the engine,on top of the oil tank.There is a #10-24 screw coming out of the top which is the adjustment screw for the amount of oil per pump cycle.It basically limits the amount of piston stroke.

Next to this screw is the ball check valve assembley,consisting of a spring and a small steel ball.If it's leaking,that most likely is the cause.

An age old trick in dealing with Mac oilers is to run a tank full of diesel fuel through them.90 percent of the time this fixes the problem.

Unless the piston is really chewed up badly I wouldn't worry about it.These pistons pop up on e-bay every so often.

If you do change the piston you must remember the rod bolts are a special splined head #10-32 .That wrench is rather rare .

After thought:To be on the safe side,run that old puppy on 32 to 1 mix.

 

[Gregford]

Unless the piston is really chewed up badly I wouldn't worry about it.These pistons pop up on e-bay every so often.

If you do change the piston you must remember the rod bolts are a special splined head #10-32 .That wrench is rather rare .

After thought:To be on the safe side,run that old puppy on 32 to 1 mix.

Thanks Al, I'll try the diesel oil trick and see if it fixes the oiler.

I'm hoping to use the saw for cutting wood, as all my saws get worked. This is why I was asking about another piston. I didn't want to risk ruining the bore if there is damage I can't see.

I run all my saws on 25:1 mix, so it should be quite happy!

One more question. Does the cylinder bolt to the crankcase or is it one complete casting? I can't see any bolts because I haven't taken it apart.

Thanks again

Greg

 

[thomas72]

The cylinder and the crankcase are one casting. But the saw is still simple to work on, and I was able to reassemble mine in about 45 minutes; maybe a little less. As for the piston I would put new rings in it and take a file and smooth out the bad areas until the piston fits without rubbing the cylinder wall. I take a permanent marker and mark the piston skirt where I think it is rubbing and I reinsert the piston in the cylinder and remove it. Then I will file the piston where the ink got rubbed off.

 

[Gregford]

Found out why the piston is scored

Took the fuel tank and carb off the saw and found that the little rubber manifold between the carb and the reed valve block has a split in it. It would have run lean until she conked out. The old owner couldn't get it started again afterwards.

Is there a simple way to fix these manifolds? I thought of duct tape and silicon sealant but I wondered if anyone else out there knows of a better way, or even something else entirely different which would do the job. Vacuum cleaner hose?

This problem is exascerbated by the anti-vibe mounts, which have separated. This puts strain on the rubber mainfold. Unless I fix the mounts, I can see the same trouble happening again and again. Is it possible to re-cement them and if so, what glue would I use?

All info appreciated! Thanks in advance,

Greg

 

[Gregford]

The cylinder and the crankcase are one casting. But the saw is still simple to work on, and I was able to reassemble mine in about 45 minutes; maybe a little less. As for the piston I would put new rings in it and take a file and smooth out the bad areas until the piston fits without rubbing the cylinder wall. I take a permanent marker and mark the piston skirt where I think it is rubbing and I reinsert the piston in the cylinder and remove it. Then I will file the piston where the ink got rubbed off.

Thanks! I'll try that.

 

[Gregford]

Fixed the piston

For those that are interested,

I cleaned the piston up on super-fine wet and dry, cleaned out the ring grooves and sanded the odd point where the ring grooves were too narrow due to the damage. It looks pretty good now!

The rings were fine so they'll go back in. Next task is to hone the cylinder and I'm hoping that the damage won't have gone through the chrome.

There's a place in town which can re-vulcanise engine mounts so I'll ask them if they can put new rubber on the anti-vibe mounts for me. The old ones are decomposing thanks to fuel/oil getting on them.

The rubber intake manifold between the carb and the reed valve block is still proving to be a puzzler. It has a hole in it. Has anyone replaced these with something else? I'm hoping to find a more or less permanent repair rather than fix the old one with silicon sealant.


All advice appreciated!

Greg

 

[Al Smith]

How in the world did you get the rod bolts loose ? You must have the splined wrench.

I wouldn't do any more to that cylinder than maybe go over it with a Scotch brite pad.If that thing has started to delaminate you are faced with either having it rechromed or sleeving it with an iron liner.

 

[Gregford]

You may not like my methods....

How in the world did you get the rod bolts loose ? You must have the splined wrench.

I wouldn't do any more to that cylinder than maybe go over it with a Scotch brite pad.If that thing has started to delaminate you are faced with either having it rechromed or sleeving it with an iron liner.

Hi Al,

I tried all round town to get the wrenches and was almost threatened with Grevious Bodily Harm by ex-McCulloch agents. Had more warmth from the Stihl guys.

:censored: So I used side-cutters - stuck the jaws in the little grooves in the rod-bolts and gave them a twist. They had no choice but to come off. I've since bought a pair of Allen bolts the same size which I'll use instead.

Yes, the cylinder will be more of a worry. I'll run a scotch-brite pad over it tonight and see what happens. There is a bit of alloy from the piston which has transferred itself to the bore and I'm not sure how much of that I can remove with a Scotchbrite pad.

The rubber anti-vibe mounts are shot and the local people who sometimes revulcanise them said the rubber is ruined and they can't make new ones without a mold. So it looks like I'll have to bend up some 1-inch flat steel and make her a SP125CV (V for vibrationful). This'll mean that I'll be able to repair the old cracked manifold with silicon sealant and duct tape without worrying that it'll crack again because with no anti-vibe, the manifold won't be flexing.

At least the saw will be going and as I'm used to old saws with no anti-vibe, it'll just be another one!

Greg

 

[thomas72]

I had the same problem with the intake boot. Someone ran racing fuel in one of the saws and destroyed every rubber seal in the saw. I was able to make a 084 intake boot off a Stihl work. I just trimmed the boot and used seal all on the tank side, and was still able to put the spring assembly back on the Stihl replacement.

 

[Al Smith]

Not to be a bearer of bad news but a regular #10-32 socket head bolt will hit the stuffer plate.A low head might work.

I turned the heads down on a socket head,using a threaded piece of round stock to hold the bolt,when chucking in my lathe.This was before I had a splinned wrench to remove and install them.When machining the bolt,I left as much shoulder as the original spliined bolts.

 

[thomas72]

Oh, Matco sales that socket, if you are in the need of one.

 

[Gregford]

I had the same problem with the intake boot. Someone ran racing fuel in one of the saws and destroyed every rubber seal in the saw. I was able to make a 084 intake boot off a Stihl work. I just trimmed the boot and used seal all on the tank side, and was still able to put the spring assembly back on the Stihl replacement.

Excellent! Just what I was hoping for! I'll order one tomorrow.:hmm3grin2orange:

 

[Gregford]

Not to be a bearer of bad news but a regular #10-32 socket head bolt will hit the stuffer plate.A low head might work.

I turned the heads down on a socket head,using a threaded piece of round stock to hold the bolt,when chucking in my lathe.This was before I had a splinned wrench to remove and install them.When machining the bolt,I left as much shoulder as the original spliined bolts.

Nothing's that easy, eh? Okay, I'll have to do what you did. But not to worry - if life was easy then there wouldn't be much fun figuring out how to solve these sorts of problems.

Thanks for the info. Glad you told me now!

Greg

 

[Gregford]

The saw's going!

I got the SP125C going today. I had to make up new engine mounts - unfortunately they're not rubber ones so now it doesn't have any anti-vibe but I can live with that. By putting solid engine mounts on it, it also fixes the problem with the split rubber intake boot. I just had to clean it in white spirits and seal it up with silicon sealant, seeing as it won't have to concertina any more. The bore was damaged thanks to the split in the intake boot leaning out the mixture. I had to clean up the piston quite a bit and give the bore a very gentle hone, but now it's running well with heaps of compression in spite of the scored bore. What a lovely sound it makes - just like a 125cc dirt-bike.

Now I'll have to pick up the bar and chain from the previous owner and see how it cuts wood!

One puzzler - I can only find one mixture screw on the carburettor - is this normal for this model?

Thanks everyone for your help,

Greg

 

[Al Smith]

Some of the 125's ,105's and 10 series had a fixed main jet carb.The reason Mac did this was an attempt to lesson warentee problems resulting from the owners adjusting the carbs too lean.Some run o.k. most don't.

The fix is to find a Tillotson HS or whatever model Walbro it was,with adjustable mains.You have to drill a little hole in the side of the air box cover to be able to adjust the carb and plug it with a rubber stopper or something.

You need the large venturi model on either of thses carbs,I think it is 3/4".Some where in my pile of junk I had about a half dozen but for the life of me I can't find them.

On that bar,about any large mount McCulloch bar will fit,such as 250,1 series,super 44 etc.Most were .063 gauge but a few older ones are .058 so you have to kind of watch it because .404 by .058 is getting rather hard to find.

 

[Gregford]

Some of the 125's ,105's and 10 series had a fixed main jet carb.The reason Mac did this was an attempt to lesson warentee problems resulting from the owners adjusting the carbs too lean.Some run o.k. most don't.

On that bar,about any large mount McCulloch bar will fit,such as 250,1 series,super 44 etc.Most were .063 gauge but a few older ones are .058 so you have to kind of watch it because .404 by .058 is getting rather hard to find.

Thanks Al,

Mine seems to run okay with the existing carb., which is great. The last owner still has a 42" bar and chain so I'm just waiting for him to dig it out from wherever it's hiding. I also have a big 42" roller-nose Mall bar which fits but I'd have to get some .404 chain made up.

I don't really need such long bars on my saws so I put a bid in on a McCulloch 1-43 with a 24" bar but the owner stopped the auction so I'll have to look for another old McCulloch.

Thanks for all the help!

Greg