<VV> Cracked Pistons

kevin nash wrokit at hotmail.com
Wed Jan 22 16:17:11 EST 2014


> 
> Message: 8
> Date: Wed, 22 Jan 2014 08:49:48 -0800
> From: James Cuneo <jamescuneo at hotmail.com>
> Subject: <VV> Cracked Pistons
> To: "virtualvairs at corvair.org" <virtualvairs at corvair.org>
> Message-ID: <BAY173-W14B7CF97DA042D51C1D867CDA70 at phx.gbl>
> Content-Type: text/plain; charset="iso-8859-1"
> 
> 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.

 
James- I think that the D-cut in the pistons is causing the problem, and the reason it keeps happening on #5 is that it is "probably" the LEAST ping prone cylinder on your engine (I know how silly this sounds)- keep in mind that the safeguard will keep your timing on all cylinders as advanced as you have set your base timing, and retard from that- the most ping prone cylinders will get the most retard, the least ping prone cylinders get retarded the least. The one that is breaking pistons is therefore seeing the most cylinder pressure, and has its timing more advanced than the other cylinders- the fact that the safeguard hasn't indicated knock doesn't mean anything- it just means that the borderline knock condition didn't last long enough for the light to come on in the display- I've seen that very same behavior with my safeguard.
Also, I know for a fact that those Clarks Hi-Tech pistons are supposed to be as strong as forged- un- modified anyways. A few years ago I had some very LOUD pings at over 20psi on my turbo on those Hi Tech pistons- I took the engine apart shortly after that, and found ZERO damage to the rings or to the pistons. The engine set-up had a compression ratio of 8.2:1, but because it also had thermal barriers in the combustion chamber, "acted" as though
the compression ratio was 8.8:1- the point is that I'm fairly confident that my engine saw cylinder pressures quite a bit higher than yours, with no piston damage. All of the head gasket surfaces in both heads were really bad, but not leaking yet- the shop that repaired the heads said it looked like the heads were "dancing around"- moving with respect to the gaskets.
I think if you want to stop your piston breakage problem, don't d-cut them for a Turbo application, no matter what.
Kevin Nash
 
 
 		 	   		  


More information about the VirtualVairs mailing list