We tell our kids "no means no". As adults we know the limits of rules

[harringtondav]

I'm prepped to get some hate mail on this one, but you can successfully violate the 'rules', sometime.
Merc manual states if the prop shaft runout exceeds .005", replace the shaft. Same instructions I'd give if I were selling prop shafts at 400% mark up.

I've successfully straightened two Merc Alpha II prop shafts, using one for three yrs now. Fixing my neighbor's '88 Cobra bent prop shaft, I just did the same. The bends normally occur in the high carbon, high chrome, non-heat treated section aft of the carrier pinion. Very tough steel. I always check the repair with Magnaflux Spot Check crack detecting penetrant, followed by their Spot Check developer.

This one bottomed out my .030" travel dial indicator. Visual said around .050" runout. Back to .0025". No cracks. No $330 new prop shaft for my neighbor. The press is a must for any DIYer.The pressure gauge add is a must for this job. The checking fixture was scrap wood, brass thumb tacks and an inexpensive dial indicator w/base.

 

[Grub54891]

Haters gonna hate....
I've straightened a few crankshafts using the same methods. Small engine stuff. Figured if I failed I wouldn't be out much. But hey it works!

 

[jakedaawg]

I do it a few times a season...good for you. Decide on your tolerance and stick to it. I aim for < 4 thou...

 

[harringtondav]

I do it a few times a season...good for you. Decide on your tolerance and stick to it. I aim for < 4 thou...

This OMC 3.0L Cobra prop shaft was more challenging than the beefier Merc Alpha shafts. The OMC shaft is necked down between the thrust washer taper and the splines. Seems it's designed to get bent. The Merc spline major diameter runs to the taper. I got my Merc shafts <.001", but when I tried to get this close with the Cobra shaft - with just another 100-200 psi, it over bent and I had to start from a new low spot. Tweaky freaky.

 

[gm280]

This thread reminds me of a machinist on the YouTube channels that runs a machine shop and straightens very long prop shafts. He uses heat and cold water and once you see him do his "magic", it is amazing how straight he gets those prop shafts. His name is Keith Fenner. Take a look at his prop shaft straightening process. You'll really like it.

 

[jakedaawg]

That's why I do less than 4 thou. Any better is just dumb luck for me on those. It's easy to put a twist in them also if your not careful...by twist I mean a slight corkscrew over a small distance.

 

[bigdee]

Good DIY! I used to work in a plant that made truck transmissions and we had to straighten many main shafts that were bent after coming from heat treatment...in fact most were! We broke quite a few too.

 

[harringtondav]

Good DIY! I used to work in a plant that made truck transmissions and we had to straighten many main shafts that were bent after coming from heat treatment...in fact most were! We broke quite a few too.

I worked as a factory engineer for Deere's Waterloo transmission factory. That is also where I learned the basics. Nearly all shafts were straightened after heat treat to restore spline, gear, and green bearing journal alignment so the journals would clean up in finish grind. Used to be 100% maganglow inspection after straightening. Now they have presses with crack detectors. And yup, there is a fairly good scrap rate due to cracks.