This is the second time in a year this #2 piston has broke.Any Ideas?

 

Either insufficient ring end-gap or severe detonation.

 

Or the cylinder wall ridge was not removed when the first piston went in (new ring hitting ridge).

 

Is that the compression ring gap at the top of the break? Seems tight. I can't see any ridge on the other cylinder, is there one?

 

I saw that too. If there is any taper at the bottom of the bore, it usually gets even tighter.

 

Plus it will tighten up even more when it's hot.......

 

The joys of hypereutectics! They work great unless you encounter any detonation at all. And with that dome in there I'm assuming you have some compression.

What brand of piston?

I strongly suggest getting some forged pistons. It appears Hypers just can't handle what you're doing with it.

Now why #2 each time? Could be intake distribution and running slightly leaner in that hole? #2 usually runs fairly cool since water pump feeds into the block right in front of it. Either way, I'd verify end gaps etc. KB hypers need a lot more...the cheaper *claimer* brands are just junk....the sealed power brand might be a little better.....but I just wouldn't trust them. Forged pistons are pretty cheap for a small block these days.

Time to hone block with TQ plates and make sure all is straight.


JIM

 

I have a slightly different take than Jim, I agree that forged would be better but the hypers can work as long as everything is right. That ring gap looks tight, who built the motor and what is the top gap supposed to be? I "think" the hypers need extra gap.

What is the compression of the motor ? Do you know the cc's of the dome of the piston and combustion chamber? 0 deck? how thick is the head gasket?

How much timing do you run?

My last thought is probably not applicable; but on a heavy nitrous motor that is running rich the fuel will get under the top ring (right at the intake valve) and when it ignites it will lift the top land and break the piston.

 

Oh I agree....as I said....IF everything is right...AND they are good ones...they can work. I mean I've sprayed 150 shots on many a stock cast piston and never hurt one yet. Hypers are better than stock cast.

The new LS-7 uses Mahle hypers....but again..you have an ecm and a myriad of controls and failsafes to keep it out of trouble. Plus I will bet the Mahle's are a much better piston than what we normally see in the aftermarket rebuild game.

Good point on the N20....if it was involved here.....it would follow your thinking on that piston as well as the thoughts on mixture distribution. That adjacent piston is pretty black looking.


JIM

 

what were the symptoms before-during failure?

 

Thanks for the responses.Let me go back 3 years when the engine was put in.The engine was built as a 69 350-350 all numbers matching including the correct Vin # on the block pad.This was an expensive engine in a large part because of the correct numbers which of course doesnt do anything for the quality of whats inside.The engine builder is in Wisconsin and has a good reputation.Compression is 10.5-1 instead of the 11-1 factory.When I put the engine in I used a B1 vac. can on the distributor and set the total timing to 34*all in by 2800.The engine ran great but to be on the safe side I started bumping up the timing until I got a slight ping.I took another total timing reading and found it was at 38*.That gave me a limit at least for that tank of fuel which was BP premium.I then set it back to 34*total.I went back in and removed the vac can and restricted the rod so the total vac adv.was 14*.So I showed a Max. timing of 48*with vacumm connected.Reason for backing up was unknown fuel and 10.5-1 comp.and not knowing if the owner would hear ping from bad fuel.The engine great for 2500 miles.Then it happened-The guy was out cruising ran it to 4000 in 4th gear stayed there for a couple of min. slowed down and felt a miss.The spark plug was smashed so off came the head and there is the pic of what I saw.The plugs are med. brown and he said he did not hear any pinging.So I called the engine builder and he said maybe a bad piston.He said send that piston back along with #4 and he would set up a new piston and send them back.The pistons are Keith Black so I called them and they said check the ring gap before I sent the pistons to the builder.The gap was correct(pretty sure it was .026) at all points in the cylinder even at the top.Sent the pistons out.Pistons came back,I removed the #2 piston top ring and put it in the cylinder and the gap was good.Put #4 back in its hole and #2 and set up timing the same and drove the car.It ran super and I thought problem solved-bad piston.500 miles later Spring now the guy is out doing the same-slows down after a little 4000 rpm cruise and the miss is back.Same thing #2 broken for the second time.I dont see signs of detonation looking at the plugs or tops of pistons.No ridge at top-gap is in spec.I dont have a bore gage so I used the piston to push the ring lower in the cylinder and the ring gap was the same.I dont know the cc of the head or piston dome.No nitrous.No symtoms before only a smashed plug afterwards.I checked #2 and #4 and #1 cylinder plugs after driving pretty hard before I returned the car and they looked good not lean.The ignition is a Pertronix inside the old distributor-do you think it could advance timing in #2 at high rpms and cause severe detonation?I think that might be reaching pretty far to find blame.I think I have touched on everyones response-Thanks and please let me know if you have more input or need more info to help.Sorry for the long post.

 

I'm not sure but I kind of see something like scratch or something in the valve pocket. Did you check valve to piston clearance ? Maybe the cam was changed. What about deckheigth, gasket tichness, ... in combination with that.

I don't think that it is detonation, since there is not melting on the edges of the break, usually detonation is accompagnied with a lott of heat production.

Yves

 

what is the piston to head clearance?

 

From the pics the ring gap looks very tight.......I would check it just to be sure.....

 

I agree if there is no debris between the ring and cylinder pushing that gap closed it's a no brainer, ring butted together and broke the land

 

What does the cylinder wall look like [in line with the failure area]? Is there any scuffing? The ring gap looks pretty tight, but that may be a result of the failure. Check bore roundness at the top of the cylinder (if bore is undamaged). From the picture, it sure looks like the ring was binding up a little and putting load on the top edge of the piston.

 

Another vote for ring gap butting together. Maybe cylinder wasn't honed with torque plate? Check for block cracks at that location. .026 is correct gap for usage, unless you're using NOS, then they need to be .032.

I had one bore job where one piston had enough taper that it hung at the bottom of the bore if you dropped it in and it fell with it's own weight.

Breakage is right next to headbolt hole, suspicious.

 

I didnt check the piston to valve clearance but was assured by the engine builder that it wasnt an issue.
As far as the ring gap -the ring in the pic is from the first broken piston.The gap does appear close but thats probably because there is some crap in the gap.I took that ring off the broken piston and put it back in the cylinder at different spots and it measured .026.
Update-I just got off the phone with the engine builder.The engine is back in his shop in Wisconsin.He said he has fully disasembled the engine and has found nothing.He said he found no sign of detonation and #2 cylinder is round and straight.He said every thing looks good and the only conclusion he can come to is another piston failure. He has decided at his expense to install a set of forged Weisco pistons.

 

I have seen this happen before with KB hypers. My machinist won't even build the motor if you want to use them.

 

Smart and honest builder you have there. I'd still hone it with a TQ plate to go along with the new forged pistons. Double check balance....those Wiseco's are likely to be a lot lighter.



JIM