damage on pistons...

[Nacho-RT74]

Story behind this.

A friend of mine with a 73 Charger was looking some extra power. Then got KB240 pistons ( only plug and play HP pistons available for 400 ). He decided sleeve the engine and then get standart bore again. also Crank machined to 030

We got the short block and we proceed install the heads. Unfortunatelly when engine was assemblied noticed pistons where hiting the heads trying to spin crank... WHAT THE HELL ? YEAH, PISTON WERE OVER THE DECK but rarelly just on 2 cilinders, pistons 4 and 6. Anyway the fact is we noticed 1,3,5 and 7 pistons top point where lower than the other side. To make some tries we used metallic gasket on driver side and regular gasket on passenger side to tray to get them equal. Mounting rockers and pushrods, of course they where too long and need to get them shortened around 4 milimeters... 4 MILIMETERS... THATS A LOT!!!!

heads where also milled down but you never got that rods shortened just with milled down heads.

Got running the engine but never good. Engine was bad machined decking to much when installed the sleeves specially on passenger side, we did know that, but anyway we wanted to give a chance. Well car was runing just one week and decided to remove engine, dissasemble and ask to the machine shop for responsabilities. After months fighting with machine shop we got a new engine, standart bore now machined to 020 but regular pistons... My friend got car again but no on the way he wanted. Of course machine shop paid everything.

So after all this descirbed problems, what do you think caused this ?

pistons are number 2 and number 4.

rest of 6 pistons are perfect and the ones I will use on my engine. Already bought the new ones to complete the set.

[aifilaw]

sounds like either you got a set of pistons with different pistons in it.
bad quality control on the pistons pin-deck height
somehow bad rod height?
most likely would be they machined all the rod journals on the crankshaft and some were done off-center... which is a typical screwup for a bad machine shop.

[Nacho-RT74]

forgot to say pistons were bad placed ( valve reliefs ) the stupid guy of the machine shop trashed the installing instructions and when got them out of box and noticed where different pistons said WHAT IS THIS ? my friend went to machine shop and guy told him, THIS PISTONS ARE NOT TO THIS ENGINE... my friend told him... YES THEY ARE, CHECK FOR INSTRUCTIONS... OOOOPS I TRASHED IT. Never got the instructions sheet back. My friend told him, use your head and will get them right mounted. Of course the guy doens't have head/brain and were wrong placed on block. My friend started to asembly heads without check before the pistons and was that moment when notice the pistons were meeting with heads on 4 and 6 cilinders. Remove heads again and then also noticed the wrong placed pistons.

Reinstall the pistons on right places after be sure of that checking KB website, and still pistons meeting with heads where again the ones on cilinders 4 and 6, so I don't think is a piston or Rods related, probably crank bad machined.

Rods are all same number stamped and they were/are the stock ones.

Anyway the burnt pistons are 2 and 4, and the burnt area is the top one, toward to intake manifold I think.

[aifilaw]

I'd place my money on the poor machining job on the crankshaft

[Nacho-RT74]

yes I'd do that too.

But about the pistons... how they could get burnt on that way ?

[aifilaw]

its hard to tell from the pictures, but it kind of looks like the pistons were much too small for the bores and someone just put large rings in there to make up for it.
I'm not sure I understand your question about the pistons.

[Nacho-RT74]

Well rings are standart bore and they were gap trimmed on cilinder as KB recomends. the damage visible is MELTED material. Sorry is a cell phone camera ( 2 megapix ). Those cilinders were out of compression before removed, that's the reason why engine was removed after run one week.

[aifilaw]

melted pistons could be due to too much advance, weak pistons material (hypereuratic (   ), or cast pistons). It's possible that due to a lean condition, and/or too advanced timing that the damage could have been caused in 1 week... I'd say it was more likely than saying the manufacturer had a poor quality batch...

[firefighter3931]

  Those 2 cylinders were probably lean causing the meltdown. Hyper pistons are not very receptive to tuning errors. 


Ron

[no318]

Those 2 cylinders were probably lean causing the meltdown. Hyper pistons are not very receptive to tuning errors. 


Ron

I agree, too. 

Detonation causes damage to the edges of a piston like that and is caused by fuel igniting by the heat/pressure of combustion.  Lean mixture, vacuum leak, ignition timing too advanced, too much compression pressure for the octane fuel being used???

Hypereutectic pistons, like KB, are good, but don't handle detonation or pre-igntion very well.

[Nacho-RT74]

93 and 95 octanes are the only ones used here.

I was thinking also on that, detonation because bad timming, or specifically too much compresion and lot of heat, specially on this case as I tried to descirbe.