<VV> Cracked Pistons

[Smitty]

Smitty Says;  James I don't know about being a guru but I do have some 
thoughts on the subject.  I think it is generally accepted that # 5 and # 2 
are the cylinders that suffer most from heat and pressure damage.  I believe 
the consensus is that the warm air vent steals cooling air from # 5 and the 
oil cooler steals air from # 2.  I wouldn't argue that either way.  The 
piston land cracking sounds more like pressure than heat damage.  I have 
experienced some piston failures and always contributed broken lands to be 
pressure failures.  It is my opinion that the Hi Tec pistons may be stronger 
than regular forged but they are also more brittle.  (harder).  A forged 
piston can yield a bit without breaking.  I have had forged pistons warp 
over the wristpins to the point they trapped the rings in their groves. 
Runaway boost does some weird stuff even with a safeguard.  I have not found 
any indication of heat damage on any of my failed pistons.  As long as the 
engine is not detonating the Safeguard does nothing to prevent combustion 
pressures.

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.