PP AH-58 almost made it to the finish line

[Terry Bentley]

Well I almost made it to the finish line. OK, only metaphorically speaking. Not on the track, but more like in the pits. But not really there either. Actually on the work bench this time.

Another one of my project chainsaws was begging me for a little bit of attention. It has been feeling neglected for way too long. After tripping over yet another box full of parts too many times over the last ten years, this one project finally made it to the work bench.

I like to mess around with these old dinosaurs and actually put them to use making firewood. I recently installed a pot belly stove in my cabin, so this saw has a purpose. My preferred choices are David Bradley and McCulloch gear reduction chainsaws. Mainly around the time karts made their debut.

Todays project is a David Bradley 5G21, using the Power Products AH-58 engine. This translates to 5 horsepower, gear reduction with 21 inch bar. It was picked up more than 10 years ago and been sitting around collecting way too much dust. The powerhead of this chainsaw was also used on karts. At least a slight variant of this engine was used.

I tore this saw down to find out why it died. Did not take long to figure out the problem. The engine still had some compression, and showed very slight scoring on the exhaust side piston. After removing the intake manifold and reed plate, there it was.


broken reed plate


reed parts

You can see the broken reed petal. Also how small the openings on plate are.

These engines are mostly solid. But the reed plates are a very weak link. So far, half of my David Bradley projects were dead from a single broken reed. Others were from weather damage or just sitting up too long. Rarely ever do I find one melted down or with broken rod.

Being gear driven makes this engine left hand drive. Most of these old David Bradley chainsaws are powered by the AH-47, 58 and 81 engines. Attempting to use one of these engines for a kart is not the best choice on earlier models. Not unless you want to make some seriously needed improvements first.

After the first generation of engines was adapted for karts, they showed their weaknesses immediately. Mostly made of magnesium and ba1ance issues plagued them severely. With the lack of ba1ance, the engines would shake themselves to pieces.

Vibration at higher rpms than originally designed, caused some of the bolts and studs to work loose and strip out. The cylinders were removable and the ears would break off sending them flying into oblivion. Think rocket launcher or punkin chunkin here.

It didnt take very long before most of the problems were resolved. The new models were cast from aluminum and heavily reinforced compared to the older ones. Originally a three bolt for engine mount, now has four mounting bosses.

Another major improvement was material removed from the sides of crankshaft rod journal for better ba1ance. Also larger diameter of shaft on flywheel side so it did not shear off. These improvements substantially increased the reliability of this model engine.

But on my little project, these improvements would not keep the reed assembly from failing. Once a single reed petal breaks off, the engine is out of service. If engine was running at full speed when it let go, that broken petal would make its way to the intake ports and lodge between piston and cylinder wall.

Fortunately that did not happen on this engine. But after tearing it down, I also noticed the piston rings were also worn out. Overall the rest of this saw was in good shape. The piston rings are only made from cast iron, and not chrome plated. So they dont last all that long anyway.

I decided to do an upgrade and see if this reed plate could be improved. One thing I always thought is the stock reeds are way too small for most of these saw engines. The rest of this engine is not too bad overall to make some decent power.


reed plate slotted

So I took the stock reed plate and cut out the center section leaving two ribs. Originally it had six very small openings that barely worked out to be much more usable area than venturi of carburetor. If reed opening is too restrictive, the carburetor cant draw enough to utilize the venturi to pull in adequate air and fuel mixed to produce much power.


carbon fiber reed

I used a single piece of carbon fiber and cut it to fit over the larger opening in reed plate. It now runs full length instead of split in half. I also have the reed petal opening so the rotation of crankshaft does not work against it.


side view reed plate

In the second pic above of reed plate disassembled, you can see the two long slots on each side of bolt holes to retain the reed stop. This presented a problem that was easily resolved.  What needed to be done is machining down the thickness of reed plate so the one piece reed would seal. I removed about 0.045" of thickness from reed plate until the two pockets were eliminated. Then sanded on a slab of granite to make sure was flat.


clearance for reed screws

With the reed plate thinner now those screws stuck out. Instead of cutting them shorter, I drilled some pockets for them to clear in the manifold. You can see the holes that were punched in gasket that match the holes in manifold. But not so clear in this pic. maybe I should take a better pic of this.


rings

The piston rings were noticeably thin on the opposite side of gap. You can see the difference between the old rings and a new one here. After assembly the engine had decent compression. But other problems plagued this project.

There was a ton of carbon buildup behind rings, so took some effort scraping it out using what was left of an old ring.

As I was about to bolt on the intake manifold, noticed the threads were stripped out in the crankcase for one of the studs where it fits. This is a common problem if engine has been torn apart a few times. Magnesium doesnt like being tightened up too many times. Most people usually over tighten these small 1/4-20 screws anyway.

If using a torque wrench to spec these, you would swear they were not tight enough. Thinking back now, I remember this intake manifold stud was missing when engine was originally torn down. I should have checked it over more closely. But the rest of the saw showed little signs of being worked on before.

Since the saw was now mostly assembled back together, I attempted to heli-coil the stripped out hole. On these magnesium crankcases, there is very little material around the threaded hole. Even less when drilled out and tapped for a heli-coil.

The hole is also not so deep.  Even using a bottom tap, the threads will taper down slightly. No problem here until attempting to screw in the stud. It started in and immediately got tight. Another partial turn of the stud and I heard the dreaded noise of the crankcase cracking.

Even tho the heli-coil is held in well enough to risk, I didnt want to take the chance. That small chunk usually breaks off and stud will stay in place. Right up until it fails sometime later on. I didnt want to spend all this time and knowing it would eventually break out. Welding this spot does not hold up so well either, since magnesium usually ends up porous and weak.

Since the damage was done and no way of repairing this, I walked away. The next day I decided to swap out the crankcases. This was a good time to replace the crankshaft bearings while it was torn down this far. Something I noticed on most of these engines, they have no end play on crankshaft. The last two of these I have torn apart, the case halves had a very slight air gap when bolts were removed. But not really measurable even with the thinnest feeler gauge.

It took almost no pressure to mate the surfaces together, but still not good in my mind. I changed the crankcase and the new one was the same way. I guess when up to operating temp it is not a big problem. For a racing engine, I would have to machine the bearing pockets slightly deeper for at least a minimum bit of movement.

I spent some time to check out other David Bradley projects. Then noticed two of the AH-47 crankcases needed the bearings replaced. There was no end play at all and bearings were very noisy.


closeup

So here is the stud boss that is very weak. Some other magnesium cases I have laying around have been repaired and threaded to 5/16. The Heli-coil was a failure, so options are limited. On newer aluminum cases, not so much of a problem.


cases

The crankcase I swapped, is a newer one with the four bolt mounting. But it is still magnesium. All of the kart engines with this newer style crankcase are aluminum.

Going to all of this trouble I had to make it look pretty. Fortunately the old paint is easily removed. Adding a few ounces of lacquer thinner to kerosene or naptha, while washing the parts, old paint comes off fairly easily. For tough spots, straight lacquer thinner did its job.

So after doing a lot more work than planned, but that is usually the case, I finally made it to the end. Everything checks out and carburetor was disassembled, welch plug removed and all orifices were cleaned out and sized accordingly.

Now the saw is close to as new without being on display in the lawn and garden department at your local Sears store back in 1959.

The next step is filling gas tank with some real gas mixed with 4 ounces per gallon of blue oil. If there are no unforeseen problems, this saw may become a productive part of society once again.

[Terry Bentley]

The carburetor on these saws are nearly identical to ones on the kart engine. The particular model carburetor on this saw was HL-65A. I did not like the wear it showed, so swapped it out to an HL-92A. If these carburetors did not have a model number stamped on the flange, you could not tell them apart.

The models HL-35, 45, 64, 65, 66, 73 and 92 are almost identical. These carburetors are common on David Bradley chainsaws. What few differences there are would not change how they work on any of the chainsaw models they were equipped on.

The HL-66 is actually the correct model for AH-58 and AH-82 kart engines. Except for the throttle shaft being slightly different, only one orifice size have been changed in the fuel circuit compared to the chainsaw carburetor model HL-65. Of these carburetors listed one model may have a larger air bleed to the low speed circuit, while another one may have slightly larger high speed needle seat.

The difference in these two carburetors for my saw were in both air bleeds. One was slightly smaller, while the other was larger. The newer HL-92 had low speed air bleed only 0.001" smaller, but the high speed air bleed was 0.009" larger.

The size difference in high speed air bleed size would be very significant on the low speed circuit, but not so much on the high circuit. But even that is not so much of a deal considering some of the carburetors are not even drilled with the low speed air bleed.

The HL-88 used on Westbends, and HL-93, used on Macs do not utilize the air bleed orifice. Either of these carburetors could benefit with having it added.


Tillotson HL-65

Wow it looks just like any other Tillotson you have seen. This carburetor looks almost identical to the very first model in 1953, the HL-1A.

Tearing the carburetor apart, I needed to remove the welch plug. Super simple but super easy to screw the carburetor up too.


Welch plug 1

You drill a little 1/8 inch hole in center of welch plug. But only drill far enough to barely penetrate the thickness of that plug. BTW it is very thin, so be careful. If drill bit catches as it penetrates, will pull the tip into the body and go right thru. Then you have junk.


Welch plug 2

With a small pointy object, like this pick you see here, just pry it out. Almost impossible to screw things up doing this. But there was this one guy.....


low speed circuit

You see those three very tiny holes down in the low speed cavity. The left on is low speed jet, middle is the transition jet, and that last one at the edge on right is the air bleed.


Fuel circuit

You see how these are drilled from the angles. That extra one you see is the fuel pickoff. It feeds fuel to both low and high speed circuits. Fuel flows down into another drilled passage connecting both circuits together. Newer model carburetors have separate pickoffs that better control amount of fuel to each.


high speed air bleed

Here you see how the high speed air bleed is drilled. The actual orifice size is abound 0.028". But it is recessed in a pocket drilled out larger to 0.060". This larger pocket helps direct air into the smaller hole instead of just passing over.

The fuel flowing thru high speed jet drilled straight down in venturi is controlled by size of high speed needle seat. The size of it is not so critical, but usually around 0.062" on many of the models I have checked

Not all of the carburetors utilize the air bleed for low or high speed ircuit. What the air bleed does is mix incoming air with the fuel being drawn in from engine. This actually leans down the mixture a lot. It also better atomizes the mixture too.

The high speed jet on these early carburetors is just a straight drilled hole. Later models utilized a brass check valve. In the late 70's John Hartman contracted Tillotson to build the HL-307 eliminating the check valve going back to a straight drilled hole.

He had already done the same thing earlier with the alcohol carburetor models HL-293A and B. The model "A" had a check valve, model "B" was straight drilled. So which one was faster on the track?

So I cleaned out all of these small orifices being very careful not to enlarge them. Some of these orifices are only 0.001" different from a similar model. I have logged hundreds of these carburetor sizes and from the first model to the latest with similar dimensions, all fall within a very narrow range.

The big differences are in how the fuel circuit is routed. A few of them look like an engineers pipe dream of complexity or just a nightmare on acid. Others are so simple, you wonder what those other guys were even thinking. Or maybe that was the problem, they were not thinking.

[Terry Bentley]

I guess you have some interest in this thread if clicking again to check out this new post. I had to demote a couple of donor chainsaws to parts only for this one project becoming a reality. That is always hard to do. Sacrificing one for the greater good. How can someone choose which one will live and which one has to be sacrificed for the other? Oh the humanity!

If I were following this thread with interest, my curiosity would be wanting to find out what this project looked like when started and what the final results are. Because anybody can steal pics off the internet and make up some BS.

Unfortunately for some of these old projects, there are always a few of them that dont make the cut. In this case there were several. Most of the internal engine parts are salvageable, but the chainsaw parts may not be worth keeping. The gas tank has holes burned thru. Magnesium doesnt like alcohol or water so much. Long term contact eats away until there is nothing but white powder left. Not the kind some people like to snort up their nose either.

Surprisingly the gear reduction components survive the neglect and abuse. The big gear is made of phenolic and shows no signs of wear on any of these saws.  I am sure there are plenty of these that failed over the years. But not any of the ones on my project list.

The earlier models powered by an AH-47 have straight cut gears. The later model 47 with more hp, the 58 and 81 powered saws have angle cut gears.



This is used on the Ah-47 model saws. The gears are 18/59 working out to a 3.27 reduction.



The higher output models use these angle cut gears which are 17/39 at 2.17 reduction.



I sacrificed two other saws to put this one together. There may still be enough pieces between them to complete another build, minus a few missing pieces.

Now for the piece de resistance.

[louie figone]

Very nice Terry, Power Products AH 58 and 61's are one of my favorites.

[Terry Bentley]

Very nice Terry, Power Products AH 58 and 61's are one of my favorites.

Thanks, I enjoy the heck out of resurrecting those old saws. Especially when they actually function. Finding the 1/2 pitch chain is not happening anymore. I find out so many people tossed out saw parts when building up kart engines. I am down to machining rivets and piecing shorts together. Not very successful with the sections stretched differently.

[Jim Donovan]

Very nice Terry, Power Products AH 58 and 61's are one of my favorites.

Thanks, I enjoy the heck out of resurrecting those old saws. Especially when they actually function. Finding the 1/2 pitch chain is not happening anymore. I find out so many people tossed out saw parts when building up kart engines. I am down to machining rivets and piecing shorts together. Not very successful with the sections stretched differently.

I have the chain and the sprockets in the 1/2" pitch.  #420 and and #41 are the same chain but #420 is pre-stretched.  #40 is heavier and has a roller that is .312 diameter versus .306 on the #41 chain.  The 1/2" pitch you want to stay away from is .428 because the roller is .333 and it will not go into the valley of either a #41 sprocket or a #40 sprocket.

[Terry Bentley]

Great info there Jim. Hard to find much specifics about chain without a lot of digging. Well it used to be that way before the internet and cell phone tech. And dont leave out amazon. I can type on an Underwood, peck on a laptop or yell at my cell phone in frustration. Then it answers me back with the info I was looking for.

But the 1/2 chain I need is for this old saw. Not to be confused with "This Old House"

Here is the big 1/2" stuff compared to full size 3/8" chain. Same pitch as most vintage karts also known as #35.

(You see how I just tied this in to kart related so not off topic? This way I can show off some of my old dinosaur chainsaws)