<VV> Cracked Pistons

[MarK Durham]

James, I too am a experienced a&p having rebuilt many turbo aircraft
engines, and damage like yours was always accompanied by other damage and
obvious detonation or pre ignition. So I think Kevin's thought process is
sound. You are pushing the limits from multiple directions and you would be
better off by leaving the pistons alone and adjust compression by using
combinations of head and base gaskets. Mark Durham, Hauser Idaho, 62 coupe
with modified 64 110
On Jan 22, 2014 1:17 PM, "kevin nash" <wrokit at hotmail.com> wrote:

>
> >
> > 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 betwee
>  n
> >  #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
>
>
>
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