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Thursday, 12 April 2018

Repairing a Stihl 024 (or 026) with Huztl parts

An old 024 gets repaired with Huztl parts and a compression problem is resolved

A few weeks ago I found the badly battered and incomplete carcass of a fairly small, old chainsaw in the metal bin at Mt Nebo tip. Because it looked like a Stihl (it had an inboard clutch) and a professional model (it had a metal crankcase visible from underneath), I brought it home in the cargo bike to see if it was any use. I was busy for the rest of the day but by the end of the afternoon, our son Jasper had dismantled the saw and identified it as a Stihl 024: the smaller precursor to the 026 (later the MS 260, then superseded by the MS261) which is a very popular and durable small, professional grade saw. The 024 was produced from 1982-1994, 2.6HP from 41.6ccs. This saw could be 30 years old or more. 
Once it was in pieces, we could see the piston had a big score on the exhaust side - a common way for a chainsaw to fail, often from overheating caused by unmixed fuel or too lean a fuel-air mixture. 
The big score on the little piston, exhaust side
This is probably why the saw had been dismantled and discarded. There was a hole in the oil tank - probably from being thrown into the bin. Lots of parts were missing: carby, air filter, bar and chain, all the covers including clutch cover, rewind starter, cylinder cover, air filter cover. There was some fairly deep corrosion in the metal chassis around the clutch - a common problem when a saw isn’t quickly cleaned after cutting palm trees (and perhaps some others). 

Worth repairing

It would be easy to conclude that this saw wasn't worth fixing (someone clearly already had), but the cheap and easy supply of non-genuine parts makes solidly built saws like the professional Stihls very much worth repairing for frugalistas like me. 
My confidence in fixing this type of chainsaw has been greatly improved by watching Andy Reynolds' videos on chainsaw repair, and making a Huztl 036
We made a list of what was missing, and made an order of non-genuine parts from Huztl. The parts cost about $130 to fix the saw (but I also bought some spares - I like to have in stock what I expect to need). This seems very good to me - consider that the current comparable Stihl model, the MS241, costs AU$1350 new. 
Everything went together fine, but there were a few lessons for me on the way. 

Oil tank hole

I cleaned the oil tank carefully: first with solvent (mineral turps), then methylated spirits, then degreaser. I used some thick epoxy resin to fill the hole. This has worked fine. 
Epoxy glue sealing the hole in the oil tank

Air filter

I installed a new Huztl carby and air filter, which were both different from the originals (which were missing). This is the old-style small 024 air filter:

Here's the old type air filter, much smaller than the new ones
The old style filters aren't available from Huztl or any other non-genuine source I could find, so I had to change to the new style filter. 
The old filters fit onto the old-style carby, which looks like this from the back:
Old-type 024 carby, with tower-type fuel tank breather on left
The new style carby looks like this:
New-type carby for 024 or 026 with spout on top

They're actually pretty much the same, but the new carbies have that spout on the top, which is an extension of the diaphragm lid, that plugs into a hole in the air filter. 
If you're going to use a new-style air filter on the old type carby, you need to plug the hole which the carby spout goes into, otherwise you'll have a hole that will suck chips and dirt into the engine. 
Here's a new type air filter, with the carby spout hole plugged with roofing silicone
You'll also need to put on a new-style filter cover, because the old filter cover won't fit over the new-style filter. This is what an old-style 024 looks like with the filter and filter cover changed to new-style: 
This is an old-style  024 AVS with new-style filter and filter cover. A bit Formula 1? Some old 024s were made with only one bar nut, and the front stud un-threaded on the outside. I'm okay with this when I'm using such short bars.

Same saw from above

Something about the Huztl air filter and filter cover wasn’t right. As supplied, the filter cover would slip on fine, but would keep popping up while the saw was in use, seemingly because of pressure from my right hand. This wasn’t okay while working - having to keep pushing it back down. For some reason, the peg on the back of the air filter was too short to securely latch into the filter cover lock.  
I fixed this with a piece of steel nail and (one of my favourite fixing tricks) string and epoxy resin. I found a nail that fit snugly into the slot in the air filter peg, glued it in with epoxy, then wrapped the peg and nail in string with epoxy. A layer of resin over the string gave a nice smooth finish. This has worked perfectly for quite a few hours of work so far. For some reason, I haven't had this problem on other 024s. 
See the extended peg on the air filter, to give the filter cover a positive latch-on

Fuel tank breather

The original fuel tank breather (it lets air into the tank as fuel is drawn out) on this saw was the tower type which extends up alongside the air filter, but only works with the old type small air filters. To fit on the modern air filter I got from Huztl, I needed to pull off (it just levers upwards with a screwdriver) the original breather and improvise a breather from a piece of plastic tube with a screw in the end - like some old saws came with when new. This works fine, but will leak fuel if you leave your saw on its side with a full tank in the sun (like most breathers will leak too). 
Here's the old type breather - the vertical black rectangle in the foreground - with the old type air filter behind it. 
Here's the new type air filter with the improvised tank breather in low foreground: plastic tube with screw in top
You can buy a short breather (labelled a "tank vent") from Huztl very cheaply, it looks like this: 
The modern-type fuel tank breather, which fits on newer 026, 036, 044 etc, but doesn't fit on the old 024
But it won't fit in the tank spout of the old 024s with the small air filter and the tower-type breather. It's designed for the newer tank, like on this 026. That's why I needed to make a breather with tube and screw. 
Here's an 026 with a newer-type tank top, with a small vent that fits underneath the big new-style air filter

Clutch drum and oil pump

The old clutch drum was worn, and the new clutch drum needed modifying to fit my old 024. 
The older models of 024 and 026 used a crude but effective oil pump arrangement which is now superceded. More modern saws only pump oil into the bar when the chain is running (actually when the clutch drum is turning), but this old 024 has a simpler setup that pumps oil whenever the motor is running. A short steel worm drive fixed to the crankshaft turns a gear on the pump unit. This is wasteful of bar oil if you leave the saw idling for long periods - which isn’t a good idea anyway, as idling causes extra wear of the clutch drum needle roller bearing. I bought the parts to convert it to the modern system, but decided not to use them yet: the old system works fine, I’m using free old fryer oil, I don’t yet know how to get the steel worm drive off the crankshaft, so the cheapest good-enough option is to leave it all alone. 
This is the oil pump on the old type continuous oiler, behind the clutch
024 and 026 saws with the old continuous oilers use a clutch drum that is just deep enough to fit over the centrifugal clutch. The modern ones have a deeper drum with a rim that extends past the clutch (on the motor side), where there is a little notch that engages with the wire quill that drives the pump. When the clutch drum starts to turn and drive the chain, it also drives the pump by pushing the wire around. 
The clutch drum I bought was of the modern wide type (I don’t think Huztl sells the old narrow type). This is too deep for the old style saws: the drum edge rubs on the oil pump. I wore a bit of a groove into my oil pump (without causing any problems) before I realised I had the wrong clutch drum width. 
You can see the groove in the oil pump from the over-width clutch drum, centre of photo, showing some brass
Fixing the clutch drum was simple enough, using the bush engineering technique of angle grinder and lathing in place. I simply reversed the clutch drum on the crankshaft where it could spin freely, and held an angle grinder up to it to grind and spin it at the same time. I ground until the oil pump quill notch was gone, chamfered off the sharp edges, then reversed and installed the clutch drum as normal. Here's a little video how:

Cylinder squish and compression

I had noticed that the old cylinder was probably useable: there was a bit of a score on the exhaust side, but only below the port. However I installed a new Huztl piston and cylinder. The saw ran and cut, but I felt like something wasn’t right. The saw seemed to be weak under power, had a lag on throttling, and most of all: pulling the starter cord was too easy. The motor would spin over too smooth and fast, without the noticeable lumpiness of the compression strokes. It seemed like the compression was low. 
Looking at the Stihl specs for the 024, I could see that the 024 had both a shorter stroke and smaller bore than the 026, by 2mm each way. 
But I saw that Huztl sold one crankshaft that was supposed to fit both 024 and 026. That caused me to wonder if the Huztl cylinder - which would presumably fit the Huztl crankshaft - was made for the longer stroke. If so it would probably be 1mm too tall for the 024, which would give it low compression. 
I tried a "squish test" on the Huztl cylinder. I poked a piece of solder in thru the spark plug hole, right to the back of the cylinder, turned the crank over one revolution, then measured the thickness of the squashed end of the solder. I got 1.4mm (I needed to find some thick solder), which seemed big compared to what I could find online. 
Then I swapped cylinders, re-installing the original Stihl cylinder (after cleaning up inside pretty carefully). On these saws, a cylinder swap is surprisingly easy, taking only a few minutes. 
On the Stihl cylinder, the squish test measured 0.9mm, which was 0.5mm shorter than the Huztl (some online sources proposed that about 0.5mm squish was good). This was consistent with my measurements of the 2 cylinders: the Huztl measured 61.9mm from base to squish ring (flat area around perimeter of head), and the Stihl measured 61.4mm. 
Measuring the depth of the cylinder with vernier caliper. Huztl cylinder is the dark one on left, Stihl is the light coloured one on right
Looking into the 2 cylinders, you can see the "squish ring": the bright circle of flat machined metal around the cylinder crown, right at the bottom (which is really the top)
When I started the saw with the Stihl cylinder, I found a different saw. It was lumpy to pull over, and when it started, the idle speed was so high I had to back off the idle screw several turns (it had been almost all the way in). Cutting wood, it had heaps more power. It was now a really sprightly little saw. 
I’m hoping the old Stihl cylinder goes well for a long time. I feel the score below the port shouldn’t cause harm - the piston and rings need to go past all the ports, which are much bigger than a little score. If/when I need to replace the cylinder, if I’m still using the old crankshaft, I can restore good compression by shaving a little off the Huztl cylinder base: this can be done on a lathe (first machining a spigot to fit inside and hold the cylinder), but I imagine it could also be carefully done with a file. 

Chain and bar

Liking short bars (and son Jasper having a very satisfactory 13” bar on his 026), I made a very light 13” bar from a Holzfforma 16” bar from Huztl (I used a .325" pitch, 1.6mm or .063" gauge, small format bar for Stihl 025/MS250) ; this cost approx AU$10 when a genuine Stihl is about $75. My post on shortening bars can be found here. I bought some long .325” chains from Huztl, and cut and shut them to 56 links for the short bar. I made 3 chains to rotate with the sprocket: all to be replaced together when the chains are worn out. 

The working saw

With its short bar, good compression, and compact, light motor, my 024 has become my favourite saw for thinning and pruning in the family forest at Stanthorpe, where it has done a lot of work cutting small White Cypress trees (which badly overcrowd the forest there). Once warmed up, it starts first pull over and over all day (I do a lot of stop/start). The continuous oiler certainly keeps the chain oily - if I was paying for oil from the shop I’d consider swapping over to the clutch drum-driven oil pump. It is very economical with fuel: a tank full will thin for well over an hour. I greatly value the saw's lightness when lifting it up and down to prune small branches off the trees, and bending over to cut them off near the ground.  
In conclusion, for light work, the old 024 is a great little saw. Robust, almost indefinitely repairable, light. Worth fixing!

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