Hi guys,
Wondering about the implications associated with swapping cast pisons for forged or hyper. This would be on an engine that has never been run since re-build. Should it be re-balanced or am I safe to just swap them out???

Mike

Mike,
It depends on which pistons you get. Have your balance shop weigh them. If the new ones are within a gram or two of the original ones, then you will probably never notice it. if there is a substantial difference, then yes, the assembly should be rebalanced.

------------------
Bill Koustenis
Advanced Automotive Machine
Waldorf Md

1971 Heavy Chevy - original owner
Team Chevelle #100

Like Bill said it depends on the pistons you get.One word of caution tho.If you get TRW/Sealed Power Forged Piston you might get away with the Piston to Wall clearence.Many of these pistons will work well with .0015 to .003 clearence..... However some other forged piston brands are designed for race use and will require .006 to .010 wall clearence.If you used these pistons with the typical .001 to .0015 cast piston clearence they will gall and tear up the pistons and cyl wall badly.You have to be carefull as some of these Race pistons are dished or flat tops pistons....So make sure to check what clearences these new pistons are designed to use.

So there may be more issues than just balancing.They will likely effect the deck height also.Many of the typical cast pistons are purposly made with too much deck clearence...So they will still fit if the block is decked several times.Most of the better quality cast and forged pistons are made for a purpose built engine that gets machined to specific dimentions.



------------------
1969 El Camino
69 LS1 335 HP 427
( not the origional engine)
M21 12 Bolt 3:07's.
Daily Driver work truck.

Many thanks fellas, sure appreciate the advice. Gotta get my dues paid again soon considering all the great info have been receiving lately...

cheers,
Mike S.

I would like to clear up a real big misconception about piston clearances. Basically, when you bore a block, you do not use a different dimension for a forged piston compared to a cast piston. The clearance is actually "built in" to the piston. If for instance you take a 350 Chevy that has been bored .030" oversize. You bore the block to 4.030 period, thats it. The typical cast piston will measure 4.0285 to 4.029 ... giving you about a .001 to .0015 piston to wall clearance. A "typical" forged piston (if there is such a thing) will measure 4.026 ... giving you a .004" piston to wall clearance.
But the main point is...the clearance is actually in the piston dimension, not in the bore size. Now there are instances that you will want to run more than the clearance recommended by the piston manufacturer, then you would make the bores bigger. But 99% of the time, you finish the block exactly to size.
Hope this helps,

------------------
Bill Koustenis
Advanced Automotive Machine
Waldorf Md

1971 Heavy Chevy - original owner
Team Chevelle #100

BillK,

My Machinist said the same thing as he stood in front of me with one of my new 496 ROSS forged pistons in his hand with a 4" to 5" mike as he was miking it!!!

He said, "see, the clearance is on the piston."

pdq67

PS., is it "mike" or "mic"?????

The tool is called a micrometer. So I guess you would say,"mic". That is unless you are in a habit naming all of your tools. Then I guess you could call it Steve. http://www.chevelles.com/forum/biggrin.gif
This brings up a question I have. I want to have my 454 block checked and have the machine work done. So, if I have it Magnufluxed, etc. and it checks out sound, then how do I know what size pistons I need? What I mean is...can the machine shop
determine what size overbore is needed without cutting into it? I was under the impression that you needed to have your pistons in the hands of the machinist so he could measure them for exact dimensions and then bore the block to have the piston fit exactly. I mean what happens if you have 1-2 pistons in the bunch that are .001 smaller than the others. I'm just curious how close are the tolerances on aftermarket pistons nowadays and what someone does to make each and every clearance is as close to perfect as they can be.
I hope that loooong question isn't too confusing and that I don't sound more ignorant than I already am. Thanks!

------------------
Justin Wray
Fathom Blue 1970 Malibu 1/8th mile best of 8.48 at "The Track that Time Forgot", George Ray's Wildcat Hot Rod Dragstrip

Justin,
Tolerances on new pistons today are pretty good. They are usually all exactly the same. Unless a block is in real good condition with hardly any wear, we tell our customers to let us bore the block first to see what will clean it up. Then we get the pistons and measure them just to double check, before we finish hone the block. On most stock rebuilds, it is pretty safe to just bore and hone the block to the ecact oversize, and we do this a lot. However, you should always check the pistons before final assembly.
There are also situations that require more clearance than "normal" such as filled blocks, marine engines, engines running boost or a bunch or laughing gas etc. On anything other than a stock engine, it is always best to talk to the actual piston manufacturer to get thier recommendations.
Hope this helps,

------------------
Bill Koustenis
Advanced Automotive Machine
Waldorf Md

1971 Heavy Chevy - original owner
Team Chevelle #100

BillK,

Just curious on something.How much metal on average is removed during the honeing process ? Is it common to use a multi stone process or straight to the finish grit ?

Al.

[This message has been edited by 69LS1 (edited 01-12-2003).]

Well, you've set my mind at ease a bit, BillK, on aftermarket tolerances. Thanks. I'm going to take the block to the machinist (it's in pretty good shape, miiiiight actually just need a honing) and see if he tells me anything different. If it needs bored...I'll let him bore it, but no hone. If it needs just a hone, then I'll see about getting the pistons before he does that to it.
This is for a low 11-second 454 application, btw. Looking at Speed-Pro forged pistons.
Hope I didn't steal someone's topic here. Sorry if I did. http://www.chevelles.com/forum/frown.gif

------------------
Justin Wray
Fathom Blue 1970 Malibu 1/8th mile best of 8.48 at "The Track that Time Forgot", George Ray's Wildcat Hot Rod Dragstrip

Al,
We try to leave .003" for honing on most jobs. If we are going to use a torque plate we might leave a bit more. We use three steps...a rough hone out to about .0005" Then a fine stone, followed by about 10 strokes with a plateau finishing stone, which is actually a fine abrasive "brush"
Different shops have different ideas on the final finish. Most of the ring manufacturers are now recommending the very fine finish that the plateau tool gives. We have had very good luck with it...no complaints of rings not sealing and the engines turn over noticably easier while assembling them. But...if you ask 10 different machinists, you will probably get ten different answers on final finish.

------------------
Bill Koustenis
Advanced Automotive Machine
Waldorf Md

1971 Heavy Chevy - original owner
Team Chevelle #100

[This message has been edited by BillK (edited 01-12-2003).]

bill,
whats your opinion on using the torque plates on a street engine that is raced 4-5 times a year? is it worth thw extra $? thanx mike

------------------
1969 SS 396

Thanks BillK,

As far as Bad67SS questions goes I still would think some caution would be warrented in his selection of what pistons to use.With so many piston mfg's around offering quite a selection of different pistons for different needs.If his is a street engine, A piston haveing .008 to .010 clearence would likely not be a wise choice for typical street.Yet a Forged or Hyper piston with a more typical closer clearence would work fine.

Al