<VV> VirtualVairs Digest, Vol 108, Issue 57

[djtcz at comcast.net]

Hi James, 

Would you be willing to loan me one of those cracked pistons for a little modeling and analysis? 

Some tuning aberration likely exists that should be identified and corrected. 
But Corvair pistons with slotted oil ring grooves have a pretty bad rep around here in high boost engines. 

Slotted oil ring grooves do a nice job decoupling the piston skirt from the dome and allow running tighter skirt clearances than designs with drilled oil holes and thus quieter running. 
As a result slotted oil ring grooves were pretty common in factory forged pistons in the early days to permit street use. But in real severe service that lovely advanced dark gray TRW aluminum alloy was no match for the stress concentration that a crude slotted oil return causes. 

When Jim Hall's Chaparrals were using aluminum block 327 engines they were using factory HP pistons and experiencing some piston cracking. In typical, practical, Jim Hall fashion they simply modified the otherwise acceptable piston with a clever detail right out of standard machine design text books. According to a couple of sources Hall later said something like "The pistons are stock but for a stress-relieving hole drilled at each end of the slot in the oil-ring groove to stop cracks." 
http://www.sportscars.tv/Newfiles/Chaparral2.html 

Hot rodders and race car builders lavish all kinds of attention on all kinds of //other// parts. 
It seems like incorporating "Jim Hall" piston modifications in Corvair pistons would be appropriate and actually carry some bragging rights all by itself. 

(Geometry can trump exquisite material every time) 

Dan T - 01886 


----- Original Message -----


-------------------------------------------- 
On Wed, 1/22/14, James Cuneo <jamescuneo at hotmail.com> wrote: 


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...................... 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