Welding recommendations for this part ...

[Gary Fowler]

We dont think about metal compressing and stretching but that is what it does just like a piece of rubber. Anything that loads up any metal form to the point of yield in compression or tension will at some point in its life fail due to fatigue. In this instance, it looks to me like the partial penetration fillel weld failed in the heat affected zone which is normally the weakest point in a non heat treated weld. One can see from the last photo that the weld was not fully penetrated as only about half the rod diameter was welded. This was actually a good thing in this case as it allowed for rewelding. If the rod had fatigued then it would be total replacement perhaps even including the piston housing if it had gotten crosswise and scarred it up internally.