I’m not sure which post I talked about my issues with this customer’s motor so I’ll start a new thread. My complaint was how easily these KB 107 pistons came apart. No, I did not build the motor. I just inherited a guys misfortune on a car he just purchased. I have had problems with these pistons before with 383. So, to all the KB fans out there that always say “ the motor was built wrong”, here’s my findings. The ring end gap on the other cly was .035, which is plenty big enough. My piston the wall was about .004. I think they are designed to run tighter than than because they are supposed to be low expansion pistons. There was no signs of scuffing on the skirts. There was no signs of detonation either at 9.25 CR. My machine shop said he “could see no apparent problems. These pistons will just come apart.” Incidentally, I called THE most respected Mopar guru in the world looking for pistons availability and his words were,” those pistons are f***ing junk made out of melted down beer cans. At least I feel I’m in good company.

looks like some debris was run thru it at some point. i've beat the snot out of kb's for years without issue. believe what you want.

Curios was this on an nos motor? I have a RB engine in the garage with a set of the hypereutectic pistons.

ARENT Those the HYPER U CRACKTICK pistons??

I

If those pistons had .004 side clearance, the engine was assembled wrong. those should have around .0015 Detonation usually starts to lift the edges of the ring lands, similar to your photos. Since you don't know the whole story behind the damage to this engine, why trash KB pistons? I have used them over the years many many times with great results. Engine overheating, excessive ignition timing, poor fuel, etc. etc. could all be factors. Maybe a forged piston would have taken the stress better, no doubt, But I got a couple forged pistons on my shelf with lifted ring lands, do I blame the piston company? Nope I blame the failed fuel pressure regulator that was the root cause of the failure.

Wish I could say how many passes our old low buck 13-1 compression iron mill had. Was last freshened in the late 90s, but mostly sits under the bench until called to duty. I know I’ve put around 100 on it between our “fun” engine projects. My dad raced it way back when and the short block was also ran in a dirt modified before we got it . Was originally built as a low buck “claimer” engine. Would I build a “race” engine with them? ….probably not, but here I am!

What are all of those small looking gouge marks on the top of the pistons? I've also been using KB pistons on customer cars and so far so good.

Those pistons look like they’ve been rattled to death to me, but that could be debris damage from the broken ring land. What do the other piston tops look like? What do the rod bearings and the bottom of the main caps look like?

Hyper-eutectics are prone to breaking ring lands when they’re detonated.

In the late 90s my dad built a 440 using those pistons. Only got 2 races on the engine before a piston came apart while warming up the car on the track. Another friend racer of ours with a small block chevy broke a motor around the same time frame with the same pistons. Didn't hear much good about them so we always stayed away from them from that point. Than my wife was running her dad's car around 2014-15 and broke a motor in the burn out. Tore it apart and it had KB hyper in it as well. The engine had several years on it, but what was left wasn't pretty. Couldn't even find the 1 piston as it was in about a million pieces.

Debri ?

You mean the top of the junky a$$ piston. Lmfao. FN mint. Ha ha ha

The way that ring land broke looks like someone stuck something solid in the ring groove (rings butted together) and pried it up and snapped it off. I also would have rounded off the edges of the valve relief. Oh and that is too much piston clearance. Don't follow the directions don't complain when it fails.

"The ring end gap on the other cly was .035, which is plenty big enough."
Did that piston come apart?

I get it some don't like them and won't run them , which is ok.

But I have seen these used and pounded for thousands of passes/miles. Hell two of my personal engines have them. They have been getting beaten since the late 90's/ early 2000's.

Melted down beer cans... THE most world renowned sb mopar GURU...WOW! Just wow....

What's the bore size and how much for the other 7?

I hate to see that, my KB107s were awesome. 4 drag weeks in the mid-low 11s, several thousand miles on top of that. Then I sold the motor and it has since been sold again(in a car). As far as I know they are still kicking.

Ya... .004" on a hyper is idiotic... that would be plenty for a 2618 forged piston

Was the piston dropped during or prior to assembly? That would be my guess.

Shhh dont disturb those who already knows it all with comon sence.

I built a 440 16 years ago for my Boss using some KB pistons. Set the quench at .035 and compression at 9.4-1. I set the timing at 34° and it's been fed whatever is at the pump all these years. My Boss pasted away about 4 years ago and his Grandson has the truck it's in now. Still running hard! My guess would be detonation or preassemble damage. You can't hear detonation a lot of the time.