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Wednesday, 30 August 2017

Shortening a chainsaw bar

Shortening an 18” 3/8”pitch Huztl “Holzfforma” brand bar to 14”

Short bar on Huztl 036

Why shorten a chainsaw bar?

I like my chainsaws to have bars as short as practical. This has many advantages: a lighter unit to lift, gives the motor an easy life, less bar and chain to lubricate, more power for the teeth in the cut, quick sharpening, etc. (I go on more about the benefits of short bars in my electric chainsaw entry).
Given all this, I wanted a suitable bar for my newly assembled Huztl 036, intended for thinning and limbing small trees. In my experience, the 036 is suitable for no more than a 16” bar. It can comfortably pull 16” of 3/8” chain through lighter hardwood, but at 18” it is bogging down. When the Stihl 036 was relatively new (in the early 1990s), Stihl’s recommended bar lengths were 37cm (15”) and 40cm (16”) and I think they were right. The current Stihl catalogue shows the current version of this saw, the MS362, fitted with a 20" bar as standard. That's a toddler wearing daddy's boots...
So for my new saw, I wanted to try for 14” – a radically short bar for these days of bar length anxiety. Walking into a Stihl dealer was financially frightening, I couldn’t find a suitable bar online, and the shortest 3/8" pitch bar from Huztl was 18”. So I thought I’d try something I’ve been daydreaming about for some time: cutting a bar to a shorter length. I had a cheap 18" Holzfforma bar from Huztl, and was willing to risk it for science.

How I went about it

First step was to work out what length of chain to use, then to make the bar to fit. I looked at a 60 link chain on a 16” bar and rim sprocket:

I like Stihl’s practice of making bar lengths to fit chains with even numbers of teeth. This means that the alternating pattern of teeth continues around the whole loop. Given this, the next shorter length of chain was 4 links shorter, so I made a chain this length and tried it in place
The 56 link chain trying the bar for size
I used a genuine Stihl bar to mark out the pattern of slot and holes on the Holzfforma bar.

Using the Stihl bar to mark the slot and holes
Then I punched and drilled the bar.
Drilling the holes

cutting the slot with angle grinder with cutting disc, using a steel bar as guide
I did have an interesting challenge in this process. When centre punching one of the bar tensioner holes, there was a small bang, and I found myself with a splinter of steel surprisingly deep in my pinky finger, a punch with a flattened point and no mark left on the bar. By chance the tensioner holes aligned with spot welds from laminating the bar layers, which had left hardened patches of steel (spot welding brings spots of steel to melting temperature, and when the electrical resistance heat is suddenly stopped, the heated spots are quenched by the cold steel around, which will cause hardening in carbon steels). I first tried to deal with this by moving the hole a little, but ended up having to temper the steel with heat to make it drillable. Thus the tensioner holes are a little misaligned.
The bottom hole is right on the hard spot weld - see the failed punch mark

Cutting the slot between the holes
Filing the slot after grinding

Chamfering the hole edges

Chamfering the hole ends

Cutting the bar to length - there's no turning back now!

cutting off the corners

Grinding a curve onto the bar

Smoothing the bar shape on a linisher

Grinding the bar slot into the bar base

Chiselling a wafer of middle laminate so it can be ground away

Tempering the bar to enable drilling

Drilling the bar after tempering

Drilling the oil holes using the old bar base as template
The new bar fitted to the motor


After all this, the bar fit on the chainsaw. I had to make some modifications to the newly ground slot at the bar base, to remove wafers of centre laminate I had missed when grinding the slot - the chain links were catching on a remnant.
The bar looks a little wider than normal at the base, but I can live with that
The saw cuts fine with the new shortened bar. It's currently only had a few minutes to test, but bar problems would normally show quickly: pinching in the groove, failing oil flow, tensioning problems etc..
The job took about an hour and was greatly extended by caution and newness. The most difficult part was grinding the chain link groove into the newly shaped bar base - it was very hard to see what I was doing.
I recall 25 or more years ago when I was doing a lot of chainsaw milling, my chainsaw shop had a machine to regrind the slot in hard nose bars. This had a grinding wheel the right thickness, held the right distance from a table, to reliably grind bar grooves. That would be very useful for this job.
If I shorten a bar again, these would be some tips to help:
  • Temper the steel at the bar base before drilling any holes
  • It would be really good to have a proper bar groove grinding setup
With these things in place, shortening bars would be relatively simple - which is good, because there are so many over-length bars around....

Update on the bar in action

So far, the bar and saw work very well. I really like the shortness and lightness.
After 3 hours of metered running hours (I've fitted an hour meter to this saw to see how long it goes), I removed and inspected the bar. In use - mostly thinning very small cypress trees in a thicket - it worked perfectly. On inspection I did find some burring of the bar rail edges at the base, on the top side, in the section where the chain meets the bar groove after going around the sprocket. 
see the burred bar edge above the tensioning hole, in the heated section
It looks like the steel I softened here with the gas torch is deforming under chain impact. 
Chainsaw bar rails are heat treated to make them harder, you can see in this photo below: 
This bar lost a patch of paint, showing the heat treatment colours along the rails, bottom side
The blueish strip along the bottom side of the bar shows that there has been some heat treatment - probably induction hardening - to make the groove rails harder and more durable. 
I'm not expecting this to be a problem, as there will be some work hardening of the steel by the chain hitting it, and there is surplus steel in this section. However it is worth keeping an eye on.