My chainsaw snapped the rod & the manufacturer only sells a replacement
as a rod/crank assy for $150, that is more than I want to pay as the
crank is still like new.
Can I just use any chunk of misc Al from the scrap bucket or is a
certain alloy required? The rod uses needle bearings on both ends if
Are you going to dissassemble the crank? I had a Stihl that needed a rod, I
pressed the crank apart and put in a home-made rod with needle bearings and
pressed and welded the crank back together. It was a BITCH! I spent more
time on it than it was worth.
For that price you won't be able to make your own at all.
Un-press the crank
Mill a new conrod
Make new bearing bushes (hardened and ground)
Press crank and align
Do you have the tools? A drill press and a hammer is not enough!
Yes, I have a Bridgeport. The needle bearings come as a unit, the shell
is included so I just need to bore the proper size hole & press them in.
Also, since when does a true HSM'er care about time or cost?!? :)
I also made a 20 ton arbor press recently. I looked at the HF ones but
shipping was a killer so I cloned it.
BTW I got all the steel from the structural steel joint up the street
for $60. I can post pix or details if anyone is interested.
Ah, that type. Then it's much less work.
So go on and *sorry*!
Normally Al conrods are drop forged, so you should go with the toughest
alumin(i)um you can find.
After machining the conrod and pressing in the shells, check for alignment!
But a bar though each of the bores and see that they are parallel. You
often need to bend or twist the rod straight.
Aligning the crank is best done with a copper mallet.
"Beefier" as in heavier? At those rpms, I'll bet life will be
shortened and vibration would become a lot worse. Guys I know control
the weight of such parts down to milligrams.
Please join my team in the fight against cancer.
Unless one knows PRECISELY what is going on (this requires what amounts to a
masters degree in small engine technology) any change in the design/mass of
a piston rod in a high-speed / high performance engine such as a chainsaw is
bound to end in disaster!
There is no way a bodge will work in this aplication as the chances of
getting it right are vanishingly minute.
Get a scrap chainsaw with a good piston rod/crank assembly.
Throw in a new ring set for good measure and go back to cutting.
Anything else is not worth the effort - the 'fun factor' not withstanding.
Probably would be best to match the mass of the original rod, otherwise
the balance will be off and you may have vibration problems or worse.
The overall rod mass, and the mass at each end, should be the same as
the original design. Hopefully you can press the crank pin in and out
without damage, while maintaining the press fit....
Ok, thanks guys!
I did not think of the difference in mass. All the pieces are still in
the engine so I guess I can fish them out & weigh them as accurately as
The old rod has an I-beam cross section & was just going to make the new
one from flat bar stock but I guess I could use a ball mill to
approximate that shape in the new rod to get the weight matched to the old.
Do you really think one cannot be made? (just trying to learn, not be a
I can now see the weight causing vibration being a problem, but not how
an approximation in it's design would affect operation.
No, you can make it and it will work.
It's not that big a problem. Each engine (talking about singles) has
counterweights on the crank-shaft. It is a simple decision by the engineer
(and not by pure math) to select the size of the counterweight. Some
engines have 50%, some 80% balance of masses. This simply determines in
which direction the engine will shake more. Along the cylinder axis or
traversal to it. But it will *always* shake.
Translatoric and rotating masses is the keyword.
For example in bikes, the engine is balanced to shake back and forth and not
up and down.
Balancing rods or pistons to milligrams is snake-oil. Just one drop of oil
and it will be "out of balance and will fall apart in no time".
Just to add to the [problem] mix, even if the material is the
same, the method of manufacture can have a big effect on not only
the gross strength but also the fatigue resistance. With a
normal forging, the grain lines are oriented to maximise
strength, and with a die casting, much of the strength is in the
"skin." A high strength aluminum alloy such as ZA-27 may also
have been used.
Some products are designed to have a finite service life. It
would be good to ask about how long does your particular chainsaw
typically last. If the con-rod went about this time, it may well
be that another part is about to go.
The reason that an "H" section is used, is to move as much of the
material away from the axis of the rod to get as much bending
strength as possible in the plane of the bearings with minimum
weight. There is nothing magic about the simicircular grooves,
just that these are the easiest to put in a casting or forging.
It is entirely possible to simply cross-drill the web as the only
thing the web does is hold the heavy sections apart, much like
the plaster does the paper in sheet-rock.
Of course you can make a connecting rod. Some one had to make
the first one. The only thing is that you may have to make
several to get one you like, just like the designer/engineer did.
One other thing -- as this is a two stroke engine, crankcase
volume will have a big effect on efficiency. An old two stroke
hop up trick [any Vespa, Lambretta or Villers "Goldstar" people
in here?] is to install packing pieces in the crankcase to
increase the pumping efficiency and power output.
You might try posting this to the motorcycle/scooter newsgroups.
Good luck and let us know how things work out. I sense a saga
Can anyone suggest a source of titanium to make the rod out of?
Anyone for carbon fiber reinforced aluminum?
Unk'a George (George McDuffee
A man may be a tough, concentrated, successful money-maker and
never contribute to his country anything more than a horrible example.
A manager may be tough and practical, squeezing out,
while the going is good, the last ounce of profit and dividend,
and may leave behind him an exhausted industry and
a legacy of industrial hatred.
A tough manager may never look outside his own factory walls or
be conscious of his partnership in a wider world.
I often wonder what strange cud such men sit chewing
when their working days are over, and
the accumulating riches of the mind have eluded them.
Robert Menzies (1894-1978), Australian Liberal politician, prime minister.