Piston Rod

I believe different alloys can be used but all are T6. Childs & Alberts uses 7075-T6.

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.

A lot of dealers/repair shops have a good supply of parts machines and will sell used parts for about 50% of new price. Gerry :-)} London, Canada

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!

Nick

I beg to differ on that old chap !

I don't know him, but I kinda doubt the "chap" judging from his question. If I was wrong, then my honest apologies!

Nick

Ok, thanks, I don't think I have any of that series. Mostly have 6061, do you think that will be ok or should I go the the scrapyard?

MikeB

I thought about that but was thinking of making it slightly beefier so I don't have to go thru this again!

MikeB

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.

MikeB

Of course, we're interested. You don't ask on this site, you brag...

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.

Nick

"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.

Pete Stanaitis

--------------------

MikeB wrote:

We are all ears if you would care to elucidate.

Wes S

Exactly! 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 I can.

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 wise-ass)

I can now see the weight causing vibration being a problem, but not how an approximation in it's design would affect operation.

Thanks, MikeB

MikeB

Because weight isn't as important as weight distribution, and that's harder to clone. Not impossible of course, but hard.

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".

Nick

================== 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 unfolding here.

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.