I recently received an email from an Australian blast cleaning equipment provider. He said he had a customer that wanted to peen connecting rods for a V8 engine. He said the guy was a "back yard" mechanic and wanted advice on how he should proceed. Without expecting the guy to invest a lot into the project I responded and thought I'd post my response with context here:

It sounds like your customer is what I'd call a "shadetree mechanic". Shot peening certain automotive parts is becoming popular with weekend autosport enthusiasts. I've found that unless they're working for a big-time team, most people play fairly loose with the rules of shot peening. The fact is shot peening, even if done poorly, will yield benefits. I can give some basic tips that might help them out, but make sure to let them know this is like comparing using a coloring book to trying to recreate the Mona Lisa. My advice is meant to just help them keep within the lines. One gearhead to another type advice.

The media they are using should be as large as they can get without being too big to fit properly in the smallest radii if their parts. The general rule his a 1:4 ratio when comparing the smallest part radii to the average media radii. They can use cast steel shot but should make sure that the size is relatively consistent and broken shot with damaging sharp edges should be kept to a minimum. If the media is too abrasive it will break the #1 rule of shot peening: no material is removed from the part when blast peening. In peening we only want to dent the surface of the part.

Next they need to be sure they are blasting the part hard enough to make dents. The higher the air pressure, the bigger the dents. The bigger the dents, the more fatigue resistant the part will be. However, they can peen the part too hard. If the surface looks too beat-up (under magnification) they would want to dial down the air pressure. In real world peening this is controlled by measuring the "intensity" of the blast stream. If they want to get a bit more serious about this they can buy an Almen gage and learn how to use saturation curves to determine the intensity they're creating. Then they could refer to the old AMS-S-13165, Table VI for target intensities to try.

Then, they need to make sure they peened the part long enough. This is as simple as examining the part surface to make sure they didn't leave any un-dented area. They may only want to peen a specific area of the connecting rod. In example an area where they've seen fatigue cracks develop or where the part breaks. This is up to them to decide. I would also advise against peening the bearing surfaces as peening will "roughen up" the surface. They can use a couple layers of masking tape to protect those are
as.


Dave Barkley
EI SPT Director, Peening Preceptor & Product Engineer