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Pressing a Crankshaft

motomedik

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Did a crank rebuild for an engine job (KTM300) at work. Most times we just sell a complete assy, but in this case the custumer had the oem kit and the complete crank is $665 from KTM. Thought it might be fun to show the process, and I think this could apply to the saws if it was something that needed reworking or the builder wanted to change stroke, or even adapt a different piston (pin diameter).

So getting started, I use a square to scribe reference marks for reassembly. Also measure web width to achieve on reassembly. This i on the bearing surface.

Then we begin to press the two halves apart....


moto_0056.jpg moto_0057.jpg

And we have the parts that make up the crank

moto_0058.jpg
 

motomedik

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Once the two halves are separated from the pin, rod, bearing, and thrust washers, I cleaned the halves thoroughly and also polish the surface the thrust washers ride against. The reassembling begins with the installation of the pin in one half. I take great care and time pressing it so it is perfectly straight. You don't just want to slam it home crooked. I start it in the hole, tap it square and go a little more, checking it several times. Then the washers, rod, bearing etc go on with a coat of moly paste.

moto_0059.jpg moto_0060.jpg


Now the fun part... Start the other half, check for true, tap, tap, tap, press, tap, tap, measure etc.


moto_0061.jpg moto_0062.jpg moto_0063.jpg
 

motomedik

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In the pics the line I scribed is not visible. I scribe a line parallel with the length of the crank across the webs opposite the pin. This allows me to use a straight edge as i press to keep things relatively aligned. Every so often I pull the assembly from the press and measure the distance across the webs at different points around the circumference so I know it's straight. The next step is finishing the crank final assembled width. I've had to pressure the assy and then tap the bed of the press with a hammer to get that final bit. And sometimes press it back and forth a bit to get it just right and centered as well as the correct thrust clearance for the rod.

moto_0064.jpg

Then to finish it up we true it. This is my makeshift home made truing stand, clamps to the bench. This crank came out virtually perfect from the press, got lucky, sometimes it can take quit a bit of 'bopping' with the brass mallet to be trued.


moto_0073.jpg
 

Gink

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Real cool thread, I'd like to see more of the truing stand. Do you just measure the run out from side to side with a dial indicator?
 

mweba

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Some of the best info I've seen in a while! Great job. Been making an attack plan for a rod swap for a while now. 385 rod on a 372 crank that is.

Thanks
 

motomedik

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Thanks for the rep guys, thought the process might interest some of you guys.

The hard part I found with pressing a saw crank is the webs are not full circle, so when you disassemble, it is hard to support around the pin. Need to make a fixture that will support the crank to get started- then I suspect it would be a fairly easy job.


The truing part I ran short of time, had to bag it up and head to work for the day. I will try to show some pics and explain that part of the process soon. Basically I try to support the crank on the stand on the bearing surfaces, and then set the indicators on the ends of the crankshaft. Sometimes you have to get creative to get a good reading and then i mark the spot where it's "high" and remove it from the stand, whack it with the brass hammer and repeat. I've had ones that I hit them harder each of three times and got nuttin' and then hit it out the other way. At times it is best to walk away and do something else for a while. I find when I go back to it I have a new aptitude and will finish it with just a few good taps. The "fun" ones are when the customer REMOVED the crank with a hammer, and you finally realize the dang thing is bent!
 

nmurph

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Very good thread. This is not something you read about on chainsaw forums.

What is the "pin?"
 

motomedik

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Very good thread. This is not something you read about on chainsaw forums.

What is the "pin?"

The crank pin that the rod rotates around. And no, nobody messes with the saw cranks, but think of the possibilities if you could change the stroke!
 

motomedik

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Some of the best info I've seen in a while! Great job. Been making an attack plan for a rod swap for a while now. 385 rod on a 372 crank that is.

Thanks

One thing that I'd like to do is try to make a stroker 272, just to see if I can and see what characteristics the longer stroke imparts. Most of the motorcycle engines use a longer stroke for the given displacement. So the piston sweeps farther and creates more base pressure? There must be a reason the saw manufacturers choose the dimensions they do, probably rev a bit slower.
 

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