Author Topic: Bore small rod end on crankshaft?  (Read 264 times)

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Offline Cut4fun .

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Bore small rod end on crankshaft?
« on: May 16, 2011, 09:48:19 am »
Was wondering if any of you have any experience with taking say 10mm small rod end and boring it to say 12mm or taking a 12mm to 13mm etc?

What type of machining would be involved?
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Offline Al Smith

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Re: Bore small rod end on crankshaft?
« Reply #1 on: May 16, 2011, 11:48:59 am »
I have not but can tell you how an automotive type rod is made . The rod and crank pin ends are microfinished using a very sophisticated lapping machine .The finish is in microns of smoothness .

You could possible do it using a machine called a jig grinder which has an indexable chuck which can be offset so that it cuts the inside periphery of the part .The part could then be lapped using a lapping arbor in  a lathe with compound .

This is exactly the same way they do Harley fork and blade rods for restorations on older machines .It's kind of complex to do it correctly .

I've got indexable micro laps that use diamond paper in sizes from 1" to about 1/4" but I've never used them .

Offline Old Iron Logging

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Re: Bore small rod end on crankshaft?
« Reply #2 on: May 16, 2011, 05:17:35 pm »
Have a friend that faced a similar situation. He solved it by cutting the con rod and welded a 12mm small rod end on in place of the original 10mm one.

Offline Al Smith

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Re: Bore small rod end on crankshaft?
« Reply #3 on: May 16, 2011, 07:31:14 pm »
There was a guy that claimed he bored one out with a milling machine still attached to the crankshaft . I have no idea how that could be done with any precision .Nor can I figure out how it could be welded with any precision .

Now I've welded crankshafts back together and they held but it was on an air compresser turning 1000 rpms with babbit bearings not a 12,000 rpm saw engine .

Offline Old Iron Logging

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Re: Bore small rod end on crankshaft?
« Reply #4 on: May 16, 2011, 07:38:07 pm »
Saw the welded rod. He ran it for 5 races before it failed. Stihl 026 base. Big Poulan topend. Not far behind my TW 260.

Offline Al Smith

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Re: Bore small rod end on crankshaft?
« Reply #5 on: May 16, 2011, 08:24:21 pm »
What failed, the rod ?

Offline Al Smith

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Re: Bore small rod end on crankshaft?
« Reply #6 on: May 19, 2011, 10:00:55 am »
I've got a theory ,never tried it but it might work .You'd have to be darned good with a lathe but it could be done .

A milimeter is 39 thou .In the case of 10 to 12 mil  that would be approx 78 thou diff .So if you made a blind end bushing that pressed in the piston with a hole in it about the size of the hole in the pin so it could get oil it might work .You'd have to either cut the pin a tad shorter or find a pin a tad shorter to allow a flange like thing with enough meat to hold the circlip .Now I have no idea if an aluminum bushing would hold with a circlip or not but the flange would  hold the pin in place .

In the case of 12 to 13 mill that would only be around 18 thou wall thickness for the bushing .Might work,might not .

If I had my Monarch 10" EE running and I was in the right frame of mind I think I could cut some out .Trying to run that tight of tolerences is a beech though . To work properly the fit would have to be tight else the thing would fly apart like a dollar watch .

 

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