High Quality Chainsaw Bars Husqvarna Toys

Poulan 3400,3700,3800, 4000 stroked crankshaft project. Part 2

dieselfitter

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I stroked the crankshaft in my 3800 by means of an offset crank pin. I added .094"increasing the stock stoke of 1.319" to 1.413". I posted a thread on this forum("part one").
I thought it would be fun to increase the stroke as much as possible within this saw's crankcase. To do this, I decided to make a 5 piece crankshaft. I split the original crank and cut the counter weights off of the shafts.
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I cut two square plates of annealed 4140 and ground them parallel. I tack welded them to prevent any movement while boring the center hole and crank pin hole. I bored the crank pin hole(@ .810" offset(throw) from center.
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Here is a pic showing the difference in crank throw.
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I cut the corners off the square plates with the bandsaw, pressed in the shafts and ground them round on the surface grinder using a spin fixture. Before I could stuff the crank, I needed to do some work to the case. I trued up the cylinder mounting surface.
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dieselfitter

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Boring the case halves created a problem. I bored through the case into the screw pockets. I filled the case with some pour-able epoxy.
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I cut the scallops out of the halves on the bandsaw.
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The piston skirt is hitting the counterweights.
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I chamfered to outside edge of the halves in the lathe. This gave clearance for the piston skirt at BDC.
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dieselfitter

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I cleaned up and trued the scallop cuts in the mill.
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I balanced the crank by removing additional metal on the rod side. I may need to do further balancing but it is pretty close now. The crank pin is just a slip fit for mock up at this time. Once the final pin is ground and the three pieces pressed together, I will have a T.I.R.(total indicated run out) to report.
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To figure out what weight to balance it at, I dug out my precision scale and weighed the piston and stuff.
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It is surprising how sensitive it is. I think a piece of hair will register.
 

dieselfitter

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I happened to be painting the transfer case axle assembly on my wheel loader to it was a good time to paint the case halves.
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Time to fill the original cylinder mounting holes. I used 1/4" aluminum rod, threaded and cut a relief to twist it off. Lock tite green will keep it in place.
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dieselfitter

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I milled the aluminum stubs flush, drilled and tapped the new holes in the proper location.


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Now a spacer block to mount the 2100 cylinder. I'm starting with 1" and will mill it down to the proper height.
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dieselfitter

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it's cool to see someone else doing a crank like this. what kind of run out did you get? check it between centers?
Originally I was setting it on V blocks and getting .0002" runout.
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Joe/Red97 showed me a better way.
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I have less than .003" run out. I have checked a few other crankshafts for comparison. They are checking out at .002".
 
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