Cracked #4 Piston on 393W - Opinions Please

[Outlaw Bronco]

Timing is 4 degrees at 3500 & 3 degrees at 4500 @ WOT.


I didn't watch the AFR this time, but I have watched it in the past and it was closer to 15.



Timing #s have to be a miss, do a little research on that.

15:1 afr will never live under a load for very long. It's perfectly fine to see that just crusing down the road though!

IMO it had some pre-ignition with that afr and away it went. Look on the bright side, your damage is very minimal to the eye, i have seen MUCH worse.

 

[AZ73]

I am shocked by the lack of residual damage. Also (caveat- I'm an engine assembler, not builder) I always thought detonation, (and failure) occurred closer to the edge of a valve in-dent where it's the thinnest on a small block Ford piston. His broke at a thick point. I'm leaning towards Airbur's ring gap issue.

 

[Ourobos]

What's your total timing under WOT? Base plus total centrifugal at say 4000G.

A stock Mustang EFI ECU will at 16 degrees, so at least 26. I dyno tune mine though, so it varies throughout the power band, no less than 26 though.

 

[Letsgocrawling]

From the pics, looks like a piston ring/cylinder wall clearence issue. only a ring issue would cause that piston damage. No detonation noted---but who knows. The top ring is broken--can't see the other end---or the builder gave it plenty of gap, strangely enough no sign of cylinder wall trauma either---but the pison is at tdc, could be jacked below. Chamber is well chingered wich means the valve job would probaly suck right now. Pull the slug out and look at it, if the cylinder wall looks ok, put the same piston back in it and run it. Piston weight does'nt matter, material is never removed from the piston for balancing. I would for sure inspect the valves and seats---maybe a touch up on that hole. Also the total timing number was'nt real. My 2 cents, hope for the best!
mark

 

[John Marinan]

From what I understand with hyper pistons, the actual cause of the break at the base of the valve pocket is caused by heat in the upper part of the cylinder. The ring gap closes because of the high temp and is jammed too tight causing it to blow the piston apart at it's weak point. there can be different reasons for the heat build up. a lot of builders make the ring gap a little bigger to prevent this from happening.

 

[strokers]

Detonation, ran the motor too lean wot air/fuel should be closer to 13. It has 20k miles on it if you had setup issues sure would have figured that out way sooner. If you have a Afr or 15 at wot you also need a tune.

 

[pcf_mark]

X2 on the rings being gapped too tight. If the wall is scored in another cylinder that would be my guess.

It could also be too much timing / heat in the cylinder. That neighboring cylinder has what looks like a very white exhaust valve. If so you have a lot of heat in the chamber. Too much timing or too little fuel?

 

[ryan287]

I did notice the water jacket in the upper right of the cylinder had a piece of silicon plugging the passage. Also, the main water port on the back of the head was over 50% clogged with corrosion. Maybe it was just overheating on that one cylinder, and I never heard any pinging suggesting detonation. My water temp was always rock solid at 190.

I'm hoping it wasn't timing or fuel starvation, but this next go around I'm going to be very aware of both.

All the valves do have a fairly white look to them, but the cylinders look almost wet with residue, like incomplete burning of fuel. Is that normal?

Thanks

 

[00gyrhed]

I had a chevy 350a buddy gave me back in the 80s because it was "blown up" in his words. It had a 12:1 CR. It had identical damage. Knocked off a big part of the piston. no cylinder scouring, or valve or head damage. I put only new 8.5:1 compression ration pistons in it and transplanted it. It is still running today.

 

[ryan287]

I spoke with a very reputable engine builder today and he said a couple very informative things:

- my piston damage is typical of a hypereutectic piston with an improper ring gap.
- it is fine to bore a Windsor block to. 060, apparently the blocks on the Windsor have a lot more room than the 302's. News to me, I figured the only difference was the the deck heights.
- sleeving the cylinder is also an option which I hadn't considered. I was resistant to sleeving, but they have no reservations about sleeving even on turbo and supercharged engines.

Anybody have thoughts? I was about ready to build a new motor, or buy a Ford crate lower. This will cost about 1/2 to 3/4.

 

[TwoDalesDad]

.....jb weld.....great stuff

 

[Master Chief]

302 is also Windsor family. It sounds like he was differentiating between 351 Cleveland and 351 Windsor, which your 393 started life as.

My '69 302 is .040 over bore and I have no cooling issues which I hear sometimes occurs at .060.

 

[73azbronco]

Lots of good info on the KB piston site on top piston ring gap and hypertetics. Lesson learned, the gap on the top land is usually huge, on the 25-40thou regime, not a standard 17-20.

 

[73azbronco]

Anybody know why this happens? I noticed the water jacket right next to the cylinder was fairly occluded with a loose piece of sealant...thinking it may have caused a local overheating issue on the cylinder wall.

Engine has maybe 20k mi on it and has always ran perfectly - no overheating issues or anything. Happened under full throttle in 3rd at about 4000RPM.

All the pushrods are fine, piston connecting rod still fine, and valves seem fine. There's no sign of the missing piston chunk - it seems to have been chewed up and spit out the exhaust.

Thanks for your input

Just curious what the indication was that you had an issue?

 

[ryan287]

There was a huge racket like sand/rocks in the cylinder for about a hundred yards, then quieted down. Because the chunk of piston fragged, it basically smashed my spark plug, so it ran missing a cylinder. I figured it a was a pushrod, but none were bent when I took my valve cover off.

 

[ugly74]

Going .060 over may be okay, but i wouldn't, without sonic testing first to confirm how much material would be left.

 

[Ourobos]

Going .060 over may be okay, but i wouldn't, without sonic testing first to confirm how much material would be left.

Agreed. FWIW I ran a 71 block .060" half filled with hard blok with zero cooling issues.

 

[Timmy390]

Could always get that one cyl sleeved and not go any bigger bore than you have now.

Tim

 

[matts460]

Had this happen to #7 on my 390 FE in my 72 f100. My NEW engine builder clued me in on the Silvolite KB Hyperutectic CRAP pistons doing this from the ring gaps to tight and cylinder wall bore to piston clearance being to tight! Also found out I had 10:1 with my heads vs what I thought was just 9:1. I was always running 87 octane!! OOOPPS Anyway Speed pro makes really good Forged flat tops for good price!

 

[bmc69]

- sleeving the cylinder is also an option which I hadn't considered. I was resistant to sleeving, but they have no reservations about sleeving even on turbo and supercharged engines.

I've dry-sleeved many high-performance (or rare) engine blocks over the years and never had the first issue with any of them. As long as the job is done correctly, the results can be even stronger than the original was.