Shot Peening

[flyingriki]

A neighbor is rebuilding his Continental. The rod caps came back from the shop with some minor grinding for balancing. They look to have been shot peened originally.

Question: Does the grinding create a need for re-peeing those areas to relieve them of the possibility of cracking?

It seems like a good question since the freshly exposed metal will have different properties than the untouched metal according to this definition:

"Peening a surface spreads it plastically, causing changes in the mechanical properties of the surface. Shot peening is often called for in aircraft repairs to relieve tensile stresses built up in the grinding process and replace them with beneficial compressive stresses. Depending on the part geometry, part material, shot material, shot quality, shot intensity, shot coverage, shot peening can increase fatigue life from 0?1000%."

[terrye]

I think you just answered your own question. IMHO if a rod has been shot peened originally, then it needs to be shot peened after grinding. However, you need to make sure it was shot peened in the first place. Some forging dies have a mottled surface that looks a bit like shot peening.

I would check with some reputable engine shops and also with Continental.

[terrykohler]

Originally Posted by terrye

I think you just answered your own question. IMHO if a rod has been shot peened originally, then it needs to be shot peened after grinding. However, you need to make sure it was shot peened in the first place. Some forging dies have a mottled surface that looks a bit like shot peening.

I would check with some reputable engine shops and also with Continental.

Check with the manufacturer! You can't be certain it was shot peened by looking at it. In fact, forming of ferrous parts above ambient temperature results in the formation of scale-this the result of oxidation. Virtually all of these are "cleaned" by using hardened steel shot to remove the scale. This is NOT shot peening and does not result in any beneficial stressing of the surface. It does however, give the surface a rough appearance.
Terry, CFI
RV-9A N323TP