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Author Topic:   Pistons coming apart
Bracer
Member
posted May 23, 2004 08:43 PM
I have a 377 that 2 sets of Keith Black pistons have come apart by the top ring gap. Could the intake being milled wrong cause this problem?


Okie11m
Member
posted May 23, 2004 08:54 PM
Wrong ring gap.


chomme
Member
posted May 23, 2004 09:41 PM
KB pistons are very prone to that.....I would buy a different brand.


stockcar5
Member
posted May 24, 2004 11:04 AM
if it were me i'd get some speed pros or wisecos.

luke

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sixwillwin
Member
posted May 24, 2004 11:22 AM
KBs are fine, some people are just too stupid to gap the top ring like the instructions sheet with the pistons says too.
Gap them per KB specs and they are awesome pistons.


Raz_900
Member
posted May 24, 2004 01:04 PM
^^^^^

I'm with Six. If you gap the top ring correctly (yes... about .030" - .034" in a 4" bore 350) they'll last forever.

My block last year had a hairline crack/pits in #8 and overheated every night. I'd even run it at 270+ degrees for 15 laps. Pistons look perfect and will be reused in a backup motor.

KB's are far better than Sealed Power/Speed Pro's. Unless you're going to pay for good forged pieces (not the $350-$400 sportman's) they're the best you can get.

Oh.. and you should pull 2 degrees or so of timing compared to 'regular' pistons. Say 34 instead of 36.

ford5
Member
posted May 24, 2004 05:04 PM
I guess I'm just STUPID then.....only been building engines for 25 years, so I don't know nothing....... never won any races either....LMAO! KB pistons are PRONE to that even with excessive ring gap....... been there done that. spend a bit more and get SRPs or equal, you won't regret it.


sixwillwin
Member
posted May 25, 2004 08:23 AM
ford5, I didnt mean to imply that you were stupid, I was referring to chomme's comment on people not using the recommended top ring gap. Everyone has opinions on the best parts for their motors, different things work for different people, for instance I cant afford the top dollar parts, so I prefer the KBs as a good alternate, I hate all the Eagle ****, but have had great luck with the Scat stuff, go figure, just luck maybe. Have had my overall best, longest lasting cranks and rods be right from GM. Peace.....

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stockcar5
Member
posted May 25, 2004 03:30 PM
im wondering how KB's are better than speed pros? not trying to start anything or be a smarta$$....i just thought there were made out of the same cast aluminum with the silicon alloy content??? i know you can get the cheap speed pros in non machined...is that whats better?

i agree with six...when you find something that works for you...guys will stick with it no matter what anyone says. i like speed pros cause i had motors spinning 7200 and have 60 nights on them. i also like wisecos, but i dont know if they are considered "good" forged pieces or not???

luke

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Ego Racing
Member
posted May 26, 2004 10:05 AM
I have a friend who runs KB's in a drag car. The engine was built as a torture test for some parts to see what they would take. Over 2 Years now in a 11 second 3300lb Monte Carlo with 200HP NOS and all he dose is once a year rings and bearings. He is running a 20 under CAST Chevy crankshaft that he machined and polished the whole thing to remove stress risers. After his work the crank losses about 8-10lbs. The car leaves the line with the front wheels 2 feet in the air.


Raz_900
Member
posted May 26, 2004 11:37 AM
quote:
Originally posted by stockcar5:
im wondering how KB's are better than speed pros? not trying to start anything or be a smarta$$....i just thought there were made out of the same cast aluminum with the silicon alloy content??? i know you can get the cheap speed pros in non machined...is that whats better?


Speed Pro's use a 16.5% silicon T5 heat treat alloy. KB's use a 18% silicon T6 heat treat alloy. The higher silicon content (needs to be 16% to be called hypereutectic) gives better anti scuffing and expansion properties among other things. The T6 heat treat process gives added strength to the finished product (ever wonder how the skirt of a KB could be so thin and not break?) but makes the alloy more difficult to machine.

Also, KB's have drilled oil return holes like a 'good' forged piston has. I haven't seen one of the new Speed Pro fully machined pieces first hand, but I believe they're still all slotted in the oil ring.

And yes, I will agree the KB design is prone to breaking the top ring land. Just like every other piston is prone to blowing a hole through it. There's nothing wrong with the design, just the first area of weakness. The piston didn't break off by itself, someone messed up the timing, gapped the rings wrong, overheated it or ran it lean. Take a KB with proper clearance and ring gap (.0025"-.003" piston - wall and .032"-.033" ring gap on a 4" bore 350) and I'll bet it lasts just as long or longer compared to any other piston under $500 in the same conditions. Run it lean and ANY piston is going to fail. The just fail in a different ways. How many KBs have you heard of blowing holes through them, breaking skirts or pulling the pin bosses outta them?



stockcar5
Member
posted May 26, 2004 11:40 AM
a buddy of mine blew a hole in his kb's. he found out later he was around 16 to 1 compression with 110 fuel...OPPS!!!
of course i've done the same...just not as bad. i was at 13.5 to 1


thanks for the info raz.

luke

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Bracer
Member
posted May 27, 2004 03:16 PM
I called KB and they told me to gap the rings at 34. That is what I did. Would extra air cause this problem from a leaky intake?


jammin
Administrator
posted May 27, 2004 04:02 PM
I go with the speed pros for claimer motors...
They have been extremely good to me....
If you break a speed pro hyper..you have done something...

jam

engineman
Member
posted May 29, 2004 07:51 AM
I agree with Jammin, I use Speed Pros in just about all of my claimers, but KB's are a great piston too,their just a pain in the **** !

However if you are running a 4.125 bore with a KB I dont care what KB says, I would gap it more like .036 to .038! thats right thats what I said,and I have seen no loss in power, it gives room for screwing things up a little, and a 4" bore I run around .032 gap,I also Bolt a Torque plate to the block before I gap them...makes a diff. I realize that most people dont have a TQ plate laying around, but you can use a head bolted to the block and flip it upside down......a real pain in the *** , but try it both ways and you will see! I have seen as much as .006 change in ring gap with the plate bolted on as opposed to off.
the reason a KB needs so much more top ring gap is cuz the top ring land is so much higher than most other hyp. pistons the ring sees a lot more heat. Just gap the krap out of em and never look back! or switch to Speed Pro!

Mark T.
Mark's Racing Engines
www.northtexasracing.com
engineman@northtexasracing.com

[This message has been edited by engineman (edited May 29, 2004).]

UNVRNO
Member
posted May 29, 2004 08:07 AM
Above someone asked why kbs are better.


Kbs are lighter pistons, and offer more compression with less dome due to the higher top ring location. they come with good spiral locks, so if your running floated rods thats good. they do cost about a hundred more than speed pros, so its a choice to be made.

Speed pros are tough as can be, but are also very heavy. the locks they come with are junk so if your running floated rods youll spend another 35 bucks on good locks.



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