Tuesday, January 26, 2010

Farmboy Gear dimension problem?

I received this e-mail from Curtis Strong:

"My name is Curtis Strong. I purchased the plans and started biulding my Farm Boy engine almost a year ago. I just finished it resently and it looks and runs very well. I did run into one problem on sheet 3 the rear frame. The dimension of 1.026 from the center of the crankshaft to the center of the camshaft may be too short.Although this is the first time I have made gears, I did folow the plans and did end up with very nice looking gears but when I tried to assemble them to the frame they were to tight to assemble. I took a long hard look at the plans and did some figuring which led me to believe that the plans actualy put the gears at apox. .003 interference. after doing the math using trig I figured the dimension on sheet 3 should be aprox 1.034. The fix was fairly simple. I just enlarged the cam shaft bore in the frame and changed the center of the larger bore to 1.034 then installed a press fit alluminum bushing. This dimension has produced a clearance that has resulted in the proper gear engagment. If there is somthing I may have over looked please let me know.
Thank you, Curtis."

If anyone can shed some light here, it would be appreciated.




Hi Curtis, I built serial #3 (Finished it March 19, 2009) and went by the plans and everything came out just fine. I did the math also and by my calculations the distance between the crankshaft C/L and the C/L of the camshaft hole is 1.1203 " and doing the pitch diameters the distance between the centers of the gears the distance is 1.1220" . That's a difference of .0017". When I run the numbers with 1.034" I come up with the gears being .0056" too wide? I think the numbers on the drawings are well within tolerance.

Jim G

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