Rod Straightness 101

This test for rod straightness always seems rather mundane.  
If your motor was running ok, why check it? 
Here's why:

Especially, if you are building an engine from a variety of parts - this is a very important step in the process.  S&S Cycle and Jims both offer a long "precision pin" which you can insert in your bushing, and run down to the cylinder base.  You can use a feeler gauge (I use a thin piece of carbon-type paper) and determine if your connecting rods are slightly bent.  The pin should hold your paper between the pin and the base on each side - you should feel a drag on the paper . . . On my engine, the front rod was perfect.  The rear rod was off (+.016) which is quite a bit !  I bolted my lower in the stand, C-clamped it to the bench, and using the special tool the boys made years ago . . . proceeded to bend my rod back for straightness.  Do a little at a time.  First, I got to +.006, then +.004, then .000 (perfect).  However, the rod may "relax" a bit back to a positive reading . . . so you may want to go beyond (-.001 or so . . ) and it will relax back to a zero reading.  It sounds crazy - but, this is the way you do it.  S&S has full instructions how to make a tool.  Now you have the best chance for less drag, better ring seating, better compression, increased power and longer engine life. 


WhitelinePsycho said...

Unreal Noot, thanks for the lesson, love the solid gudgeon pin thingy.

Wes said...

That's the part of the process I hate!! How do you feel about the tools that bend the rod from the wrist pin location?