Just looking for some opinions/experiences from anyone concerning ring gaps and hypereutectic pistons (specifically - Keith Blacks).

I have a 460 block I bored 60 over and have a bore size approx. 4.422". I'm using standard economy moly rings, nothing fancy. Engine is going in custom dually 4x4 setup weighing in at about 7500lbs dry. Will do some pulling and bogging, but mostly highway/city time. From this info, KB recommends from .029 to .035 gap depending on how much towing I will do, http://www.kb-silvolite.com/speclear.htm

However, if I ask my machinist, he insists I use .022 for the top and no larger. He claims to have no idea why KB recommends such a large gap. Furthermore, most every engine builder in town agrees that .030 is too big. One guy said he would send a set of rings back if they measured that much out of the box and he uses KB all the time. Most seem to like .004 per inch of bore, which would put me about .018. Of course, they all claim to have been doing this for 30+ yrs and I should trust them.

I email KB and here is what they said, "We do recommend a larger gap on the top ring only because the hyper metal tends to transmit heat away at a slower rate than more conventional alloys. The extra silicon tend to keep more useable energy in the combustion chamber causing the top land and ring to run hotter. In addition we have moved the ring higher in the piston for improved efficiency. The silicon tends to act like a coating that would be applied for more efficiency. It is also suggested that you retard timing about 4 deg.. because of the efficiency of the alloy. Remember detonation can destroy any motor vary quickly."

My biggest concern is not the loss of power from a large gap, but a large gap can permit the hot gases to escape down the bore, possibly burning the oil film from the cylinder wall. This will result in scoring or micro-welding. Also, I hear that a large gap may cause a big mess under the hood due to blow-by. Of course, I am also concerned I get a big enough gap to prevent butting.

Anybody been here before? Better yet, anyone who has torn-down an engine to check for butting, just to see? Why do so many builders disregard the manufactures recommendations and then tell their clients to do the same? Yet, they seem to get away with it.

Anyway, I'm stuck until I become confident about a decision. I would value any input on end gaps from someone running hyper pistons.

 

Unless your machinist guarantees to replace your pistons...I'd run what KB recommends. You need more ring-gap in a hyper piston or you'll end up breaking the top ring land. If it really worries you that much spend the extra money and get forged. I'm running hypers in my blown Vette with a 180 HP shot of NOS to boot...so far no problems, and I did gap them to KB specs. Deen

 

As you have probably found out by now KB suggest .0065" per inch
of bore.

KB says they run the top ring higher then the norm, plus hyper. piston do not transfer heat as well as a forged piston, there for
requiring more end gap.

We use KB pistons in alot of out race engine, there a good deal.

Tim

www.tmeyerinc.com

 

Yep, KB does recommend .0065 per inch of bore for normally aspirated engines. Unfortunately, they seem to be the only ones who do this. Hence my trouble. On one hand, KB says .029 minimum. On the other hand, my trusted machinist says .022 maximum. Anyone out there NOT do what KB says? Thanks for the replies all.

 

On a blown vette with nitro, KB recommends .0080 per inch of bore. Same as a heavy tow vehicle http://www.kb-silvolite.com/speclear.htm . On the blown vette with nitro, I can see high temps that would close that gap. I'm not so sure about a heavy puller. The only diff is KB recommends a larger bore size for the vette. Whatever I do, nobody will guarantee my pistons. If I follow the machinist, he would give me more sympathy than KB would if something should happen. Forged pistons are expensive and noisey when cold. Plus I would have to re-bore my block to fit them and they are an over kill in this engine. I already have hypers and a bored block. I appreciate your thoughts, Deen, Thanks.

 

Hypers are not recommended for nitrous applications because they can shatter* at the high frequency that nitrous creates. They can withstand very high cylinder pressures otherwise. (* from the SA-Design book on Nitrous)

 

Thanks for the tip, I never heard that about nitrous. I was aware that hyper pistons will crack before distorting. Kinda like glass. Anyway, KB continues to recommend their hypers for nitrous and Deen (see above post) has a blown vette with nitrous. Apparently, he has no problems.

I read this at http://www.kb-silvolite.com/page05.htm ,"Piston to Bore Clearance for KB Performance Pistons were dyno tested at wide open throttle with .0015", .0020", .0035" and .0045" piston-to-bore clearance. After 7-1/2 hours, the pistons were examined and all looked as new, except the tops had normal deposit color. Even with 320 degrees oil temperature, the inside of the piston remained shiny and completely clean. Excess clearance has been shown to be safe with KB pistons (no reported cracks in four years)." By John Erb Chief Engineer, KB Pistons.

Furthermore, KB recommends a large bore and ring gap for nitrous use, ie .0080 per inch bore http://www.kb-silvolite.com/speclear.htm . Now, it may be the case when using nitrous, that if the bore size and/or the ring gap were too small, the piston could explode.

Personnaly, if I were to build a nitrous/blown setup, I would ONLY use forged. Nothing is as tough as forging. I know this from experience. Compare a cheap case-hardened forged chisel to those expensive 'crapsman' powdered metal cast crap from S#@Rs. I spend more in gas returning tools, than the initial cost of some others.

 

My comments about not using Hypers with nitrous is also repeated in SA-Design's book on Engine Blueprinting. I'm not trying to scare anyone away from Hypers but the cost difference to go forged is not that great - for an extra margin of safety if you ever plan to spray. Otherwise go with the Hypers.
Mike