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Re-balance with piston change?

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Re-balance with piston change?

Postby gasman5.0 » Thu Nov 20, 2008 11:27 am

I have a motor that the builder accidently installed the incorrect pistons. They are SRP-206063 -28cc. The motor is 9.5" 4.030 x 4.00" with a max rpm 7300-7400 shift,it's internally balanced and has an ATI super balancer. The solution we came up is switch the pistons out to some offerings from Keith Black part #750 pistons -15.8cc and keep the existing rods.
My builder said that with the KB piston,pin,rings we are looking at 2 grams lighter than the srp stuff the motor was balanced with.
He say's we will not need to re-balance with only 2 grams out. I personaly don't see a need with only 2 grams but I'm no engine builder, I sure would like here what some of you engine builders think. I guess I'm looking for peace of mind.

Thanks Darrin
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Postby rustbucket79 » Thu Nov 20, 2008 11:40 am

I wouldn't rebalance, you've just created a very minor overbalance situation, nothing your engine will ever know.
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Postby Baprace » Thu Nov 20, 2008 12:17 pm

Gasman5.0 , the 2 grams will not be noticed and because it is reciprocating weight you would only correct the crankshaft 50 % or 2 grams for two pistons.
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Postby PackardV8 » Thu Nov 20, 2008 12:48 pm

However, definitely equalize the combined weight of the piston, rings, pin and locks assemblies. KB stuff is pretty good, but, as someone here said, "If you didn't check it, it's wrong."

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Postby gasman5.0 » Thu Nov 20, 2008 1:53 pm

Thanks for the feedback, I have some peace of mind... :) I did a search and the KB 2618 forgings pistons didn't have any negative feedback. The only item I found was attention be paid to top ring end gap. They want to see it opened up .0040" X bore dia.

Thanks Darrin
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Postby Baprace » Thu Nov 20, 2008 7:23 pm

gasman 5.0 , the ring end gap sounds too tight for a blown car, why not recheck your figures or better yet call the ring manufacturer's tech line, I would have figured about .0055 to .006 per inch of bore. JMO

your figures are .0040 x 4 inch which in my mind equals .016
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Postby gasman5.0 » Fri Nov 21, 2008 9:42 am

This motor isn't blown, it's n/a and will see 150 shot nitrous on a very few occasions. The info end gap is what keith black has posted on their site, they like to see .0040 n/a & .0050 nitrous. The one thing I've read is be extra careful with the KB piston's and too tight end gap. I'll double check with a real person before I gap em'.
It's a 4.030 bore by the way.

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Postby Wolfplace » Fri Nov 21, 2008 12:48 pm

gasman5.0 wrote:This motor isn't blown, it's n/a and will see 150 shot nitrous on a very few occasions. The info end gap is what keith black has posted on their site, they like to see .0040 n/a & .0050 nitrous. The one thing I've read is be extra careful with the KB piston's and too tight end gap. I'll double check with a real person before I gap em'.
It's a 4.030 bore by the way.

Darrin

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That is just the Hypereutectic ones not the forged
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Postby gasman5.0 » Fri Nov 21, 2008 1:45 pm

Just finished talking with the tech guy@ KB & recommended a .021-.022 top ring end gap. With a 4.030 bore that equates to .0053"-.0055"multplier, which does not match anything on their chart.
Baplace recommended .0055-.006 or .022-.024 end gap.
On a N/A street/strip motor 550-600hp on motor @ 7k where would you gap them with 2618 piston? And what changes should be made for 150 shot.

http://www.kb-silvolite.com/clearance_pop.php

That brings me to note that the published grams piston weight info. is also incorrect for the kb-750 pistons. According to my engine builder.
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Postby jdavis » Fri Nov 21, 2008 2:29 pm

Make the builder put em on the scales in front of you so you can see the difference with your own eyes.

That ring end gap is what I run on a whole bunch of 4.030" bore sbc's. For nitrous I would look at the .024 number more than .022. Although its hard to tell without good info about the piston ring material. The thing with rings is everyone gets hung up on how tight they can make the gap, all i'm going to say is set them too tight and it will put a hurting on your finances. The principle is thermal deformation of the material, the hotter it gets the more it grows with nitrous or a blower its going to get hotter than n/a. Or in the case of the hyper kb pistons, the design creates a pretty high temp in the piston ring and so it takes a larger gap as well.
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Postby gasman5.0 » Fri Nov 21, 2008 2:54 pm

jdavis
I'm going weight them before they go in. I'm leaning towards a little larger end gap myself. I don't plan on spraying alot, but plans change. Just like you said to tight when it gets hot $$$$$.
By the way these are a 2618 forged material NOT hyper's.
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