Pro Mac 850 Rebuild

[Brian Thacker]

Doing a rebuild on a McCulloch Pro Mac 850.
Started out pretty dirty. Compression at 150psi.
Performed a leak down test and found blow by threw the rings.
Performed pressure test on the crank case and found the PTO side crank case seal leaking. During tare down it had string wrapped around the PTO crank case seal. See pic)
After a lot of cleaning got it apart.

 

[Brian Thacker]

A lot of dirt inside the engine.
The starter side of the crank I don't have apart yet but the PTO side has an issue. It has a grove in it right where the seal contacts the crank. It must have gotten moisture in there and formed some rust. Not sure what I am going to do at this point. I polished the crank as best as I could with crocaus cloth but the rust spot is still enough you can hang your fingernail on it. Maybe .006in. deep. It looks worse than the picture shows. The next picture is the string that was wrapped around the PTO shaft and into the seal. The seal is trashed. Luckily have seals for both sides. Checked the bore for runout, it has about .002in. runout and about .001in out of round. Hard to tell in the picture, the face of the gauge was too bright. The ring end gap was just out of spec. Willbe working on it maybe tomorrow and will add more to this thread. Got to check the crank to make sure it is true, cleanup all gasket surfaces, check ring gap on the new rings and start putting it back together. Should be a good saw when done.

 

[jeff taswelder]

Im in

 

[rwoods]

Brian, let me know if I need to pick up a new crank. Ron

 

[Brian Thacker]

Well done a little more work on the 850 today.
I cleaned and rebuilt the carb. Ran it threw the ultrasonic cleaner just using water and just a small amount of white vinegar. Replaced diaphragms and gaskets. Checked pop off pressure and now it's assembled and willbe ready when I get ready to assemble the saw.
Got the crank seal off the starter side of the crank. The bearing must have gotten some moisture in it because it is all black with some corrosion in it. Thank a new crank is in order. Did not perform a runout test on it because we will more than likely replace it. The pulse tube was really bad.

 

[Brian Thacker]

Also cleaned the chain oil cap and put a new O ring on it just to discover that the threads were all messed up. Sometimes we overlook the obvious. But I did clean the oil pump and it seams like it is working, time will tell after assembling the saw. If not it's easy to get to. I also cleaned the manual oiler and new gasket for it. Blew air threw the pickup tube and the passage from the pump to the chain oiler port on the chain side of the body of the saw. All seamed good and clean.

 

[Roll Tide]

I picked up a pm850 not long ago from a former member here and I fell in love with it. Very strong running, and smooth saw.

 

[Brian Thacker]

Have a PM800 and it is very smooth. Cuts great.

 

[blsnelling]

A blow by test on a 2-stroke? That's a new one to me.

 

[Brian Thacker]

OK, been a while sense I have been able to post any updates on the PM850 rebuild. After finding the crank had a bad area on it have been waiting on a new to this saw crank and time to dedicate to this project.
Polished a couple of minor rough spots on the crank, replaced the starter side bearing and installed new crank seals. Got all surfaces cleaned and prepped, ready to assemble the saw. Chased all cylinder threads and bolts. Cleaned the ring groves and installed new rings. Hope I will get time to work on it tomorrow.

 

[Brian Thacker]

More

 

[Sty57]

How much of a difference is there between a PM 850 to a SP80?

I picked up a SP80 not to long ago and it seems to run good. But I haven't had much time to really check it over.

 

[rwoods]

How much of a difference is there between a PM 850 to a SP80?

I picked up a SP80 not to long ago and it seems to run good. But I haven't had much time to really check it over.

Mark H. can fill in, but the main differences are electronic ignition vs points (lope less crank at least some 850s) and two shoe clutch vs three shoe clutch.

Ron

PS to Brian, keep up the good work.

 

[Brian Thacker]

Made a little progress on the PM850 today. Got the cylinder and clam shell together. Performed a vacuum and pressure test after I got it together. Only leak I could find was a small leak around the compression release. I will have to pull it and clean it. Fot the fuel line replaced along with the fuel cap gasket. Got the intake adaptor installed and the rubber connector from intake to carb. Had a rough spot on the exhaust port. Got it all clean and smooth. Checked crank for trueness.

 

[Brian Thacker]

More

 

[Roll Tide]

Looking good !

 

[Brian Thacker]

Got a little more done this evening. Got oil pump installed, Top AV mounts installed, carb connector and pulse tube installed and muffler. Coil mounted, Flywheel in place and coil distance set from flywheel.

 

[heimannm]

As noted, the PM850 is electronic ignition and the SP80 and early SP81 saws are points. The later SP81E came originally with a two piece electronic ignition but the later one piece will also work and is more reliable.







Many will tell you that the PM850 is the best of the 82 cc McCulloch saws, but I have an SP81 with a fresh piston and cylinder that would hang with most.

JoeyMT33 as a PM850 that will break starter covers if you do not use the compression release.

The PM800, 805, and Super 850 as well as the DE80 and PM8200 all have the oversize mufflers with the large screens. Very quiet saws by comparison but seem to be lacking some of the attitude of the SP80/81/81E and PM850 versions. That may simply be an impression left by the lack of decibels.

The SP80 came with a three shoe clutch but the two shoe version from any 10 or 600 Series saw will work just as well.





Mark

 

[cgraham1]

I picked up a pm850 not long ago from a former member here and I fell in love with it. Very strong running, and smooth saw.

I think I know the saw you speak of...

 

[Roll Tide]

I think I know the saw you speak of...