McCulloch PM850 rebuild

[heimannm]

This saw was rescued from the warehouse in Los Angeles about the time McCulloch was moving to Arizona. As you can see it has not had a lot of run time, and was evidently built from whatever parts were available as the flywheel cover was not intended for use on the PM850 (missing a couple of holes for mounting screws at the back and the bottom. It must have been some kind of developmental project as the top of the cylinder is stamped "XX" rather than "A", "B", or "C" (or nothing).





I'll have to get a photo of the top of the cylinder another day.

The saw came to me as it quit in the middle of a job. First thing to check is compression...seems a bit low.



The spark plug certainly looks like it had been running quite lean.



Let's pull the muffler off and see how it looks.




Houston - we have a problem.

Mark

 

[heimannm]

Might as well tear it all the way down and see the extent of the damage. I noticed a couple of things as it was coming apart, first off the cap for the manual oil pump was cracked and popped off when the fuel tank came off. I couldn't have run that way very long as there was not a lot oil spilled and crud collected around the manual pump.



The rear anti-vibe mount will also have to be replaced if this saw will be put back to work.



Many times, particularly of the saw has not been previously molested you can remove the 4 hex head 8-32 screws that help hold the crankcase halves together. In this case the saw had been molested and one of the screws on top had been rounded enough that the 1/4" socket could not get a grip. Not to worry, a 6mm socket is smaller by just enough that you can force it on with a few light hammer taps, a hand held impact wrench come in very handy for an occasion like this since you apply some down force with the hammer blow in addition to twisting the fastener.





With the last fastener out, the crankcase pops open and the piston, connecting rod, and crankshaft come out. The piston looks even worse when you can see it all.





Hopefully the cylinder will be salvageable.

Mark

 

[heimannm]

Sadly, the chrome has been chipped off in a few spots and the scratches are through the plating as well.







I did my best to clean it up with a quick ball hone and some emery cloth on a mandrel to remove the aluminum transfer but the damage was too extensive to chance using it again with the plating gone in a few spots. Now what? Let's see what I can find in the attic...

Mark

 

[heimannm]

Well, for some good news I found this...



It even has one of the hard to find "A" pistons!







Well this saw may just live again, more to follow as time allows.

Mark

 

[Jasonrkba]

That will be pretty awesome once complete.

 

[Matt93eg]

Wow, beautiful saw. Hope it will be a runner again. Looks like it will.

 

[SEAM]

I like your attic

These are very nice saws - actually very small, almost "tiny" for their engine size. They sold a lot of PM850's in Japan it seems.

 

[mogulmasher]

Awesome build thread. Nice to have NOS parts lying around in the attic. Should be a runner again before to long.

 

[PogoInTheWoods]

OK. I'm in. LOL

 

[heimannm]

Here is the "XX" stamp on top of the original cylinder.



I should point out that the piston had the "B" stamp.



The owner of the saw (let's call him Mike to keep it simple) also noted this saw had been the test bed for the development of an RPM limited coil as evidenced by the extra hole in the back of the shroud. Mike father was an Engineer work at McCulloch, primarily in the area of electronics including the electric start saws, the fast charger for the batteries in the electric start saws, and the 170 welder as well as electronic ignitions and a myriad of other things. I have copies of at least half a dozen patents awarded to him and assigned to McCulloch.



Since this is going to be a complete rebuild, I went ahead and replaced both seals. For the flywheel side the easiest way to remove the seal is to drill a small hold and use a coarse threaded drywall screw in a slide hammer.



For the PTO side, it was a simple matter to press out the old seal using the bearing, then put everything back in place again. To simplify the task I have made a few tools since acquiring my small Grizzly lathe. While I had good luck in the past just using some sockets as tools, I discovered that trying to install a new bearing without a mandrel in the bearing to keep the needles properly aligned can allow the cup to partially collapse causing the needles to cock and the bearing to run off the shaft.

Below are the fixture to hold the bearing insert, the I.D. has a step to hold the insert and a through hole large enough to allow the seal and bearing to pass through. The driver with the smaller O.D. will pass through to allow the bearing to be pressed out, the one with the larger O.D. is used to properly set the position of the bearing and the seal in the insert.





Another tool I made is a tapered sleeve that allows me to slide the bearing and seal in the insert over the step in the shaft without risking the lips of the seal hanging up and possibly dislodging the garter spring in the seal.



Another tool that is nice to have is the driver for the wrist pin. I have done this many times using a socket but this tool has a pilot that keep the driver properly centered over the wrist pin making the whole operation a bit easier. When you are changing pistons, make sure to get the piston installed correctly with the wider skirt up towards the intake side since the intake port is wider than the exhaust port. If you get it wrong the skirts won't cover the intake port and you have to take it apart again and assemble the parts correctly.



Mark

 

[Jethro 2t sniffer]

Nice to see these tools Mark and very interesting about the man working there.

Is there anything else different about the cylinder? Have you ever seen or heard of the xx before? Just wondering about porting or something they must have had a huge stack of differently ported cylinders somewhere at some stage

 

[heimannm]

I don't know anything further regarding the cylinder. I am sure there would have been a lot of very interesting stuff in the warehouse on Century Boulevard.

The crankshaft in the 10 Series saws is fixed in position by a ring on the O.D. of the flywheel side bearing.



On the 70 and 82 cc saws, the PTO side needle bearing (BH108) is mounted in a "bearing insert" rather than just held by the bore in the crankcase. On certain early models the bearing insert was held in place by a similar ring to the flywheel side, on later models there is a small dowel pin in the crankcase and a hole in the insert to locate the insert. Notice that the locating hole is off center so make sure you have the bearing and seal properly oriented when you press them into the bearing insert.





Also, always remember that the piston rings are pinned, if you try to install the piston and don't have the ring gaps properly aligned something will be damaged.



Before I put one of these saws together, I like to clean up the 12-24 screws as best I can including running a die over the threads to insure all of the old sealer is removed.



Mark

 

[heimannm]

The 12-24 x 1-3/4 "sealing screws" are equipped with rubber washers to insure a good seal at the head. I have not found a good replacement so I use a #008 O-ring instead when I reassemble a 10 Series saw.



Due to the problems with the hex head 8-32 fasteners used to hold the crankcase I substituted socket head cap screws.



Poge alerted me to the benefits of Dirko HT some while ago and searching e-Bay you can find any number of sellers in (Eastern) Europe that can provide a 70 cc tube for less than $15,00, or you can pay a lot more if you would prefer to find it more locally. Dirko is an RTV (Room Temperature Vulcanizing) compound that cures by exposure to humidity in the air and I find is a lot easier to clean up that Threebond 1184 and similar products. I put a light coat on the first half of the crankcase including the bearing bores. Make sure you have the bearing insert properly positioned and oriented so the dowel pin will align with the hole in the insert when you install the oil tank half of the crankcase.



A light coat on the second half as well. I have also been successful just coating the bearing bores but it's not like you use so much that it is cost prohibitive to coat both halves. Make sure the dowel pin engages the insert when you put the two halves together.



I'll back up for moment and note that you will find one of the holes for the internal 12-24 fasteners breaks out in the bearing bore.



For that reason, I also put a bit of sealer (Dirko in this case) under the head of the fastener and on the threads as well. When you take one apart you always find quite a bit of sealer on the body of the screw as well as the threads so I guess this is a good practice.



Mark

 

[heimannm]

To simplify installing the 8-32 fasteners, leave the manual oil pump off when you are putting one together. Not an absolute requirement but does make the job a little quicker and easier.



Leave the automatic oil pump out as well so it doesn't get in the way when putting the 12-24 screws inside the oil tank.

I have put together a kit of gaskets and plates to facilitate sealing the intake and exhaust ports. Note that on the rigid mount saws the screws in the intake side of the block are 12-24, but on the anti-vibe saws the screws are 10-24. The 10-24 will thread into the 12-24 tapped holes easily enough, but will damage the internal threads if you apply too much torque. You can also see in this photo that I have pulled the impulse barb from the block, more on that story in a moment.



The exhaust cover plate for the 82 cc saws is an original McCulloch part, not sure where it came from though.



I also made up some blank gaskets to help insure a good seal.



In spite of my best efforts, I had a leak in the crankcase joint revealed by a pressure test with some soapy water. At this point I also noticed the impulse barb fitting was leaking too.



So, it all had to come apart again. I pulled the barb and coated it with some Indian Head gasket shellac and pressed it back into place. I have had success with Indian Head in this exact situation before so I was pretty sure it would be a good fix.

I also decided to flatten the mating surfaces of the crankcase halves just to be sure I would get a good seal on the next assembly attempt. I have a piece of marble that was one part of a partition in a bathroom (in my old Junior High School) that is nice and flat. I use a little spray on adhesive to stick a piece of silicon carbide sandpaper to the marble and make some figure eights.



Both halves are now flat and smooth.



Mark

 

[PogoInTheWoods]

Possibly a pinch or deformity of the 'o' ring at that fastener being the leak source?

 

[heimannm]

There will be a bit of sealer around the heads of the internal screws when it is all together. Give it plenty of time to cure and don't spend a lot of time trying to remove the excess. When you take one apart for the first time you will see the factory did the same thing. In order to pressure/vacuum test the crankcase it must be sealed which means the automatic oil pump must be installed to cover the impulse port that drive it.

Gasket in place.



Since I am going to use the spring clip on the automatic pump to insure a good seal, it recycled a couple of the brass inlets in the pump body that will prevent the steel clip from digging into the plastic pump body.





The pump installed, but the clip had not been "wedged" into position.



I use a couple of large screwdrivers to press the spring clip into position and wedge the back side of the pump against the gasket.



Again we'll back up for a moment, since this was a NOS block it did not have the oil pickup tube installed. The process involves forcing the flange of the pickup tube through the hole in the top of the oil tank. This jobs requires a bit of patience, determination, and good old fashioned concerted effort. Warming the flange first may help but I avoid lubricants since you will likely have to pull and push on the flange to get it to slip through and any lubrication can complicate the process.



If anyone has any good advice on fitting the screen into the end of the oil pickup tube please let me know. I have several NOS oil pickup tubes with the screens loose and have not come up with a good way to fit the screen into the bell on the tube.



Mark

 

[heimannm]

I don't know what caused the leak, but on the second go 'round it sealed up nicely Poge.

With the crankcase sealed (pressure and vacuum - check) I began the rest of the reassembly.

The oil pickup tube in position, the flange sets in a slight counterbore in the top of the oil tank.



Gasket and check valve (the small aluminum disk on later saws, phenolic on earlier models) in place.



As I pointed out near the beginning, the cap on the manual oil pump was cracked so I needed to install a new one. These fit pretty snuggly and I have had them crack during installation so I have gotten into the habit of warming them up (hot water works nicely) and carefully pressing the cap on the manual pump body.



With the manual pump in place, I like to test the system to make sure it works before going in further.



Looks like we're good.



Mark

 

[mogulmasher]

This is a great build thread Mark. Thanks for the detailed explanations and pics.

 

[heimannm]

Reassembly continues, installing the upper shroud. Get all of the screws started before you tighten any completely.

10-24 x 3/4" with internal tooth lock washer



Insulator, then rear anti-vibe mount (replaced with a new one) and the 12-24 screw and keeper. Note how the shroud includes a notch for the keeper to fit in.





There are two holes in the front of the shroud, one for an 8-32 screw to hold the cover in place, and the other for one of the screws holding the flywheel cover in place. Please note I have shown the mounting screw in the wrong hole to help you remember not to do it this way....



Mark

 

[heimannm]

The new anti-vibe mount is much stiffer than the old one, so to simplify installation I like to knock the roll pin out that holds the choke button in position. This makes it much easier to slip the tab up into position. The first side goes in O.K. but it takes a little effort to get the second side in place.



I made a small punch from a 6d finish nail to knock the roll pin out, and a tool to hold the roll pin and start it back in again. This is a little easier than pounding on your finger & thumb to get it starter and easier to hold in position than using needle nose pliers or tweezers.





The lower end of the boot is held in place by the upper shroud, the upper end is held in place with a clamp that attaches with two 8-32 screws from the top side through the air box. If you line up the easy one on the PTO side and start to tighten up the screw you can usually get the second screw to go in easily enough.





This shows the correct position for the fillister head screw on the front of the shroud. The flywheel cover screw goes in the open hole.



Mark