Topic: con rod polishing

[bikehenge]

Hi all,
Polishing con rods to reduce "stress risers" that could lead to cracks is not uncommon in automobile engine rebuilds.  Apparently the typical process involves a die grinder and 40 grit sandpaper.  Has anyone done this for SOHC4 rods? 

My greatest fear is that the work could distort the rods.  Any "conventional wisdom" concerns related to doing this to SOHC4 rods? 

thanks,
Rob

[mlinder]

I don't think polishing will do very much unless the surface is hardened by shotpeening prior. The shotpeening is far more beneficial than the polishing part.

[MRieck]

I don't think polishing will do very much unless the surface is hardened by shotpeening prior. The shotpeening is far more beneficial than the polishing part.

Mark....I actually polish the beams then shot peen them. It works well. I agree that the shot peening is the most important part.

[Zaipai]

Dumb question coming.. what is shotpeening?

[Don R]

It's kinda like sandblasting but with bigger media. Metal balls I believe.

[BLUE71TURBO]

Exactly. Some what like shot gun shot.

[mystic_1]

Wikipedia says:  http://en.wikipedia.org/wiki/Peening

mystic_1

[bikehenge]

Hi Mike,
Does this process cause any distortion that needs to be corrected by additional machining?
Also, when you say "beams" do you mean that you do not work the entire rod?

Thanks,
Rob

I don't think polishing will do very much unless the surface is hardened by shotpeening prior. The shotpeening is far more beneficial than the polishing part.

Mark....I actually polish the beams then shot peen them. It works well. I agree that the shot peening is the most important part.

[mlinder]

If Mike says polish first, you should prolly polish first.

[Zaipai]

Ah..I see.. What did we ever do with out wikipedia? Thanks for the info..

[Nikkisixx]

Hi Mike,
Does this process cause any distortion that needs to be corrected by additional machining?
Also, when you say "beams" do you mean that you do not work the entire rod?

Thanks,
Rob

Hey Rob,
I'm not Mike but I did sleep at a Holiday Inn Express... 

Smoothing the rod removes a few grams and allows the rod to shed oil quicker, which reduces reciprocating and rotating mass. It also reduces small inclusions that can lead to failure.  When polishing, it makes sense to smooth out most of the rod, but focus is placed on the beam since this is where most failures seem to occur.  Polishing does leave small scratches that are then further reduced by shot peening, so this is done after polishing. When polishing one should stay away from machined surfaces (the flats on the side of the rod, flats for the nuts, the journals, the hole for the pin) and  if polishing is done to extremes (a bench grinder on the thin part at the top by the pin for example) it could weaken the rod.  As a matter of practice, rods should be balanced and reconditioned (the big end made round to size) as part of the process. 
 

[bikehenge]

Thanks all,
that is helpful information

Rob

[bucky katt]

the cycle exchange used to sell cb750f2 rods, shot peened, polished with some really large arp studs in them. ken swore theyre good for a street bike that doesnt live at 11,000.