<VV> Cracked Pistons

[James Cuneo]

Guys,I need some help from you engine gurus out there.I have cracked 3 pistons in the #5 cylinder in the past four years. 2nd time I thought it was a coincidence that it was the same cylinder, third time, now I wonder what is happening in #5 that sets it apart from the others.What I have:2G rochester carb modified.F/B turbo on 140 heads. Clarks Hi Tech pistons D cut to 8.25 crCrank, flywheel, Pressure plate, rods, pistons balanced.TB20 otto cam329 turbo dist.Wide band o2 sensor, cruise afr 14.5, boost afr 12.0SafegardWater injectionSecond time this happened was just last august so I recently installed water injection. Was doing a tuning run last Sunday when after a 5 second run at 10psi I felt the piston give way. Safegard indicated no knock. I had done a lot of homework on water injection and was sure this would be the cure to this problem, it was not. Spark plugs all appear dark brown, #5 a little lighter brown in color. Piston failure mode is a crack forms in land between #2 ring and #3 oil control ring, then travels to oil return slot in #3 groove.Questions: 1. what is unique to #5 cylinder running hot, lean? I can imagine the hot air off the turbo being distributed there. 2. Is there a better, more appropriate piston to be using?I have decided to replace all 6 pistons with new. It appears there are only 2 choices of forged pistons easily available, Clarks brand and Sealed Power, which one?I appreciate all and any advice.Thanks, Jim Cuneo39 year corvair owner, Journeyman auto mechanic, FAA Airframe and Powerplant mechanic, Farm/ Industrial equipment mechanic.