dfolse62

Newly bored etc, installed and during the first couple of test runs (just down the street, not the strip) the motor craters. I pulled the heads to find three broken pistons. These are Keith Black, I was told, that have eyebrows and an extra groove (indention machined in) from the eyebrows to the edge of the piston. I was told this is normal for valve clearance in case 2.02 valves are used.(I didn't) This is where the pistons broke. All on the intake valve side of the piston. The engine builder showed me a box full of this type piston, all broken exactly the same way.

He says this is caused by detonation. My total timing was at 38. Stock Hei distributer, vacuum advance, stock curve, didn't kick back against the starter.

Is this a piston with a known weak spot?

Oh yeah, it also broke two cylinder walls. I am not a happy camper and don't want to pay for the fix if crappy pistons were used. What do you guys think.

MotorHead

Most of these pistons break because there is not enough ring gap, KB pistons require more gap than normal because of high silicon in them ( or something like that can't remember exactly ) , got any pics of the pistons ? What heads do you have and what is the compression ratio?

Did a machine shop build the motor for you ? Some of theses guys will try and blame you when I fact it is their fault

dfolse62

Machine shop built it. It has the older 487X heads. I might be wrong on this number a little. The comp was supposed to be less than 10, 9.5 I believe. Someone here locally had mentioned ring gap and I don't know what it was. I Took the block back today and may never know what the gap was for sure. I am sure that it will be "my" fault!

ratflinger

Have them check the ring gap on the unbroken pistons. If it's a reputable house they will own up to undersized gaps. I'd go with forged pistons next time.

bojangies46

sounds to me you have to much cam for the pistons, valves hit pistons, did you check clearence valve to piston before you put it together, with heads on.

Scott Marzahl

From your description, that was my first thought. If the rings aren't gapped right, the photos on the KB page show that the the edge usually breaks off, not the middle of the piston.

Does this have a hight lift roller cam, whats the deck height and thickness of head gasket?

LFZ

same thing happen to me on a 383....KB pistons suck...there is a reason why they are cheap!

stingr69

Those hypers will fail more easily from detonation and or ring gaps that are too tight. Forged pistons will take more abuse but cost 2 times as much or more.

If the compression is well matched to the cam, and the ring gap is given extra care, and the RPM is not too high, you can use hypereutectic pistons and they will be fine. If you are not careful with any of the above, they can fail.

There IS a reason that forgings cost more.

-Mark.

Cory1970

Well we all know Keith Black’s are not the best, but they are definitely not the most expensive either.

I for one have been very happy with mine after years of abuse. For 4 years I have done nothing but pound on my motor every single time it is fired. I rarely drive my car for any purpose other than giving it he!!. I have not been easy on these pistons.

If you treat the motor right (read proper tuning) and don’t run nitrous, KB’s can give you years of life. I also spent ample time gapping my rings, checking valve to piston clearance, and measuring for custom push rods. Do it once do it right!



Here are my details (a little old school):

350ci bored 40 over

10.5:1 compression – the pistons are basically out of the hole (decked block) and the heads have been decked several times.

Double Humps w/ 2.02/1.90’s – ported by a profession builder

Edelbrock Air Gap RPM – ported at the same time as the heads were

Comp Cams XE284H (hydraulic flat tappet)- 240/246 durration on a 110 LSA
– Matched Springs - Comp
- Pro Magnum Roller Rockers – Comp
- Hi-Tech Push Rods (Custom Length) – Comp – Bent a set of the High Energy rods first

Malory Unilite & Malory HyFire 6AL - 38 degrees of timing, completely in @ 2800 rpm, rev limiter is set @ 6250 RPM.

650 BG Speed Demon (these actually flow closer to 680CFM) – I tuned it on the dyno with an O2 sensor.

Taking the time to set up your motor right and tune it properly is a good idea no matter what pistons you have. Yes, forged pistons will be more resilient and stand up to detonation for longer (good for nitrous & forced induction), but for everyday use KB’s are fine.

fauxrs2

I'm helping a friend with a big block build up - when we took the reciprocating assy to the machine shop (KB pistons) they said they wouldnt balance em with the KB's....

The reason....

"When they fail you will be back here complaining its my fault, it happens way too often...so therefore I no longer touch any build using those cheap pieces of @#$% (his words)" he said buy a set of forged from anybody else, or Speed pro (i think it was) hypers and he would spin it up..."

In the end he bought forged....

No doubt if assembled properly they can deliver years of service...this machinist has just seen way too many people who dont...or decide that a mere 150hp shot should be fine on brittle hyper pistons

Cory1970

Wow, I have never heard of a shop turning down work b/c of KB's. KB's are not nearly as bad as people say. Some people seem to use forged pistons to make up for their inability to set up the motor properly.

Although I am pro KB on my little street NA SBC, I never put anything but forged in nitrous or forced induction motor. It would be stupid not too. On an NA set up you can control for almost every variable and prevent an instance where you would need forged pistons, but there is just too many things that can go wrong on a nitrous or forced induction set up.

I agree, sometimes the end users fault and the shop get blamed.

LFZ

My 383 was built by a VERY respectable engine builder that a few on this forum know who....my engine had a flat tappet 240/246 hydraulic cam, 10.1:1CR, AFR 195s, tuned very good at 36 degrees total, plugs always looked nice, and drove it for about a year at <2000miles, then oneday, cruising at 2700rpms, i get knock. Pistons were to blame. I would never buy KBs again, and recommend anyone building an engine to stay away from them..I now have a set of SRPs, a hydraulic roller, and no worries.

63mako

My machinist won't touch the motor if your using them either.

fauxrs2

This is a well respected shop in town so he isnt hurting for work. He made it clear that he took this position simply to avoid the issue of being blamed for either

1. lousy quality pistons
2. Lousy assembly by the owner

He made it clear that he didnt / wouldnt sell them or put them into any motor he built or worked on...purely self defense I guess.

V77Vette

box full of broke ones and a known issue he still used that brand

dfolse62

Sorry I was away last night. I do have pictures of the KB failures but I don't know how to post them yet. If you will look at the posted pictures though, mine are broken in that indention by the intake (large relief cut) The upper ring is exposed for about an inch. The three broken pistons are broken exactly the same. I don't have all the info in front of me, but it was a relatively mild comp cam. I can't see indication of valves hitting the piston but I guess they could. The machine shop assembled the engine. Like I said, it ran great, but should probably last longer than a half a mile (give or take 200 yards) It was built to be a highway cruiser.

dfolse62

Looking at the Keith Black site (United Engine and Machine Co.) under "Special Notice on Top Ring End Gap" That is exactly what three of the pistons looked like.

shafrs3

KB pistons are fine, I've had them for years, I'd use them again in a heartbeat.

As stated ring gaps are unique. It's probably the main reason they've developed a negative reputation, that and over zealous abuse. If you drag race often or consider yourself highly abusive to your equipment go with forged, they'll cause less damage when they bend/break.

Detonation, as opposed to pre-ignition, is an extreme condition that can hammer an engine to failure regardless of piston used.

gkull

I had KB's in my 355 ci. many years of abuse. I even had severe detonation when some kids playing a joke dumped 5 gallons of diesel fuel in my Vette. They told me later that they just wanted it to smoke and smell funny.

My Hyper piston even survived over 7500 and valves impacting all the pistons.

I'm not sure what happens if you do not have enough ring gap.

I would say than in order to break pistons and cylinder walls it would take something like hydrolicing the motor on startup with a cylinder full of fuel or water. So the piston and wall are damaged. then as you drive along the failure becomes catastrophic.

vetteaddic

Thats high on compression using iron heads,did you notice any pinging or dieseling?
I thought you said 10:9 comp,