Tom's McCulloch BP-1

heimannm

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Since this has turned into a real project, I thought I'd go ahead and create a thread to document the whole story.

Background - Member EEkholt sent a message a while back offering his excess McCulloch parts if I would get his BP-1 running. It turned out I was going to be in his area a month or so back and we got together and I picked up the BP-1 in question along with a load of stuff.

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The saw as it arrived:

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Following some advice I've picked up along the way, I thought I'd pull the muffler and have a look. Turns out it was a good thing, some of the trash fell out of the muffler in the process but there was a bunch of stuff in there already as well.

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Mark
 

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heimannm

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Not sure what happened up above. Anyway, here is the saw coming apart...this one was really used. It had the OEM 1/2" pitch sprocket which indicates it was running the Auto Mac self sharpening chain which may explain all of the fine dust.

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There is a lot of stuff in the intake as well.

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I don't think the air vane governor would work very well.

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Really packed in everywhere.

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Mark
 

heimannm

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Getting down to the nitty gritty...

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That white hole is the "cap" that goes over the balance piston/cylinder.

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The balance end of the balance piston engine design.

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Top of the balance piston.

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Mark
 

heimannm

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Almost there, turns out the seals are the same size as the older 10 Series.

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On closer examination, I did find one of the core plugs missing. Another good reason to really take it down and give it a good going over.

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So finally, the crankshaft, rods and pistons, and the two opposing cylinders.

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The balance piston has no rings.

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Mark
 

heimannm

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The dismounted gearbox and the OEM style 1/2" rim sprocket.

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I noted elsewhere that the gearbox seemed to be locked up, turns out there was so much stuff packed around the clutch drum it could not turn.

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The back end of the handle with the points. I will try to get another photo showing the dogs that engage the flywheel to drive the shaft turning the cam to operate the points.

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Mark
 

heimannm

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The cover over the balance piston was badly corroded so I gave it a quick blast. The pitting was rather deep but did not go all the way through anywhere.

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A 1/2" welch plug is a perfect fit.

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A dab of JB weld over the socket just to make sure we have a good seal.

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I also did my best to coat the inside of the balance piston cover but I forgot to get a photo, perhaps tomorrow.

Mark
 

jacob j.

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Looking good Mark. Are you making your own carb diaphragms for these? I need to go through mine here soon.
 

heimannm

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JJ - I was able to pick up a few NOS fuel pump diaphragms along the way so I am covered there. I did send one of the metering diaphragms off to Joe Salva along with a fuel pump diaphragm, hopefully he will have some reproductions coming along some time. I found one NOS metering diaphragm in the parts bins in my attic today, I will have to try and get two limbered up and working since the BP-1 carburetor uses two metering diaphragms with an "actuator pin" between them.

First up, a couple of shots of the shaft and driver that runs the cam for the points.

This is the end that engages the flywheel

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And here is the other end with the cam

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I had the seals on hand as well as one last set of rings (59294) so I starter putting it back together today.

Here is the cover that goes over the balance piston cylinder with the coat of JB weld. I used my finger (nitrile glove) to insure every surface had a good coat on it to head off any future corrosion.

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And the obligatory shot of the crank shaft and pistons/rods. Note the interesting "windows" in the power piston.

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The cleaned up part laid out on the bench.

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Mark
 

heimannm

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The base with the cover for the balance piston, crankshaft, pistons, rods installed, power cylinder on.

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There is an extra counterweight behind the clutch, appears to have been a bit of an afterthought the way it is locked to the crankshaft with a ball rather than a more conventional key.

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There is a phenolic dust shield that installs on the operator end of the saw.

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Fuel tank on

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Have a look at the flywheel, you can see the two "pockets" that receive the drive yoke for the points shaft. One is smaller than the other so the shaft can only engage in the correct position to insure the points open at the correct spot in the rotation.

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Getting all of the parts fitted up.

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At this point I realized I had neglected to install the clamp that locks the gearbox to the engine, so I had to take it all back apart since the mounting screw is hidden under the fuel tank. Still and all it was a relaxing afternoon.

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Mark
 
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heimannm

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The saw is mostly back together now except for the carburetor.

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I have the carburetor disassembled and a few bits soaking in mineral spirits overnight to see if that will soften the diaphragms at all. I may have to try Tom's secret trick of steeping the diaphragms in some hot water with lemon juice.

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Check valve for the primer circuit.

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Mark
 

mcbob

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Mark what is the bore and stroke of both pistons and crank throws please ...................
 

awol

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That sure looks good, I really enjoy looking at those pics and the way these saws are put together. Is that balance piston made of aluminum?
 

heimannm

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Alan, really great to hear from you, I hope every thing is well with you and the family.

The balance piston is steel, no rings.

Power piston is 1.75" bore x 1.125 stroke, two thin steel rings.

Balance piston is 1.145" bore but I never thought to check the stroke.

The balance piston is 180 degrees opposed to the power piston so the vibration is substantially reduced as Bob pointed out. In addition, since the pistons are opposed the crankcase displacement is accentuated almost like having a supercharger. The engine is "piston ported" via the balance piston.

When the carburetor is right, the acceleration is instantaneous, idle to 12,000 RPM...

Mark
 

heimannm

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Now for today's update...let's dissect the carburetor.

The fuel pump diaphragm is somewhat conventional, mylar diaphragm sandwiched between two gaskets. The built in filter is sintered metal, and there is a primer pump mounted on top.

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The check valve for the primer is a spring loaded ball, there is a rubber seat in the section with the fuel filter; you can see the steel check ball peeking through the seat in the second photo.

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The needle and seat are pretty conventional as well, except perhaps for the fact that the metering lever is not captured by the fulcrum rod.

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The metering diaphragms are less conventional. There are two diaphragms with an "actuator pin" between them. One diaphragm as a steel plate attached that bears against the metering lever, the plastic pin goes through a separator and in turn bears against the regulator diaphragm. I have not done extensive research on the theory so I am not sure why there are two diaphragm in the metering portion. The real regulator diaphragm does not have the steel plate but I didn't have any more of those on hand so I had to use two of the metering diaphragms.

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Mark
 

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