KB piston

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Mat-it
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KB piston

Post by Mat-it »

I have a '95 351w with TW 170 heads, truck/tow application. I need a 20cc+- dish hyper piston with healthy valve relief to maintain a ratio of around 9:1. I'm looking at the KB379 piston with 15cc stepped dish. The problem is UEM does not list the relief depths, it only states "for sportsman N heads". I'm trying to figure out from photos how different they would be from the "twisted" reliefs so my question is would 3-5cc of material removal be enough to successfully notch this piston to work with the wedge heads?

On the side, I have not looked at forged pistons because I've read their noisy on startup and not really needed for the street but are there brands/alloys that can be set tighter with maybe offset pins to make them run quiet?
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Re: KB piston

Post by BillK »

Mat,
I thought the Twisted Wedge heads needed a special valve relief piston ? I seem to remember a customer having a problem trying to run them with a non TW piston. K-B's tech department has always been pretty decent to deal with. Call them and ask.

As far as forged pistons being noisy on cold starts that is an old wives tale as far as I am concerned. And there are different forging alloys some of which have very low expansion and are not all different than any other piston.

But you have to make sure they will work with your TW heads. I am almost certain that several manufacturers have pistons with the angled reliefs for the TW heads.
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Mat-it
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Re: KB piston

Post by Mat-it »

Yes I have backed myself into a corner with these heads. My 4.00" bores were good so only honed them. A lot of the wedge specific pistons I see are for over-bored strokers or have small dishes/reliefs. I was convinced through other forums that these heads would work over stock pistons but my "custom" cam is a bit too much.

There is definitely more selection in the forged area so if they aren't going to make my truck into a diesel I will search for a forged solution.

Thx
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Re: KB piston

Post by rustbucket79 »

Look for forged pistons made with 4032 material, it is a higher silicon content material designed to expand less therefore having tighter cold clearances. They will have some noise for a minute or so until they expand.
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Re: KB piston

Post by Adger Smith »

I think I still have a sample piston (JE) that I used for the angles of the TW heads. When I cut non TW pistons to fit TW heads.
You just have to be careful with the notch depth and valve size/clearance of valve on some pistons.
big valve cut deep doesn't work on some non TW pistons.
Your welcome to it if you want to cut a set of pistons.
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Re: KB piston

Post by Schurkey »

The two engines I own that have the KB Hyper pistons in them, both knock for ten or twenty seconds until the pistons get some heat in them. The pistons fit tight, I don't know why they knock.

400 SBC, and 455 Olds.
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Re: KB piston

Post by Adger Smith »

They knock because of the taper ground into the skirts. The tight is at the tip of the skirt.
Measure a KB just below the oil land and it has an abnormal amount of clearance and taper toward the skirt tips.
That large cold clearance is necessary because the expansion is greater in that area of piston mass.
That is normal construction/sizing with the Hyper material.
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Mat-it
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Re: KB piston

Post by Mat-it »

Thanks for the offer Adger but I'm still not sure which way I'm going to go. I've found some 4032 pistons from DSS that only take off 2 thou of CH and have an 18cc dish designed for wedge heads. I'm waiting to here back on their weights to see how close they are to stock, as now balance enters the equation. Maybe not an issue for a truck? but that'll be another post.
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Re: KB piston

Post by travis »

DSS, or autotec (racetec) pistons. Although I’m not sure if racetec is still in business or it’s just that their website is down. And I would definitely get it balanced unless you are using a stock replacement piston. Even on a low rpm motor there is a very noticeable difference in operating smoothness.
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