Thread: Custom Piston ?

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I have just purchased a 396 Engine actually Block, Heads, Crank, Rods, and a set of new Speed Pro Pistons came with this deal. The motor has all of the machine work done and new cam bearings installed. Block came bored .040 and the Speed Pro's are .040 as well. The feller I bought this from is a guy right here on CHR. He had the machine work done some time ago ( 2 years I believe) When he had the block done he did not have the pistons at that time, seems this always has a bad ending and this time is going to be no different. The .040 Bore job should have been ( If my math is correct) 4.134 as the stock bore on a 396 is 4.094. Well I had the block miked today and it turned out to be 4.140, the Speed Pro's are L2240NF which turn out to be a forged piston. I was a might worried about the .006 clearance in the block so I called Speed Pro and they tell me that Piston Clearence should be .0015 on this piston. Man that seems awful small for a forged piston, but Speed Pro insists this is the correct #, and it is stated on the piston box as well.

So it looks like I am hosed on this set of pistons. So the question I have is ---Can I buy a set of pistons to fit my bores at 4.140. I have searched and searched the net for any and keep coming up empty. These piston websites have very little information about any thing like this. Anyone know if I can and two from whom?

(I can always have the block bored to .060 but if at all possible I would like to leave a little room for down the road.

Thanks Roland

You could run Wiseco forged pistons at 0.006" clearance. Here is a blurb copied from their website....

Question: What kind of piston-to-wall clearance can I run?

Answer: The factors that affect this are cylinder wall thickness, whether the block is filled, the overall compression height of the piston, piston material and thickness, and whether a marine engine is to see fresh-water cooling. Most small blocks get 0.004" piston to wall clearance and most big blocks get 0.005" due to the use of our 2618 high-strength alloy. For heavy blower and nitrous applications, Wiseco recommends adding .001 to the standard clearance. Special note: Clearance numbers are obtained from measuring the largest diameter of the piston, typically at the bottom of the skirt. All measurements should be taken 90 degrees from the pin centerline.

The question is, which costs less, forged pistons or cutting the bore to +0.060" and some lesser cost kind of pistons.

well were did they measure the skirts at? most of the time the bore will be the same for over bore the piston has a built in piston to wall size so a piston with less piston to wall will be bigger for that bore size .so if you want a bigger piston to wall you have to go + on the piston wall bore so let say on a4.600 bore if you want to add 001 then its 4.601 . take the pistons knurl or coat the skirts then if you want to use them thats a piston that may have a open oil return slots under the oil ring support rail like a cast piston so they can be fitted with less piston to wall. if your looking at the piston a drilled piston can run more piston to wall . the piston may make abit more sound till warm but the piston with slots can fold the piston skirts in or crack in time i put then in at 002 .the bores are going to move with out deck plates so 001.5 i would not put them in there less with deck plates the 2618 can be used at 003.5 to .004.5 bore on 4.000bore to.004.5 and4032 can be used at4.000bore to 4.200 .002to.002.5 this is a base line for je /srp and others i used many and for use they can be open up more by 001to003 min bore size if a blower /marine pistons.if you want them knurled and file fit then to bores i can do the pistons if you want them done on a machine made for the job. if no one has one around your neck of the woods. jerry may add to this as well i know many that will not fit that piston at min with out deck plates i bore hone my share of blocks and air cooled stuff with and with out deck plates. i never set them in it min bore size less its a cast piston for stock use and some times i add .0005to.0007 that.

Well----There's a little more to it than the apparent .006 differance between the official overbore size and what the block mikes---it is standard that the clearance is in the piston not the cylinder---other words a piston for bore size "X" are "X-.00Y"
example----a piston for the stated .040 over 396 would be 4.134- "Y" or probably about 4.130----a piston for a 4.140 bore would mike in at 4.140-"Y" or around 4.136ish

We only do finish bore sizes with pistons in hand----

Morning Pat!!!!typing and drinking coffee or I might have beat you to post and I wanted to add that you could knurl the skirts for him!!!!!

Originally Posted by jerry clayton

Morning Pat!!!!typing and drinking coffee or I might have beat you to post and I wanted to add that you could knurl the skirts for him!!!!!

back at you jerry and now i am off to work .honing blocks i have more to hone then i want right now . sure would be nice for a older ck10 my arm is falling off

Know what you mean---I once counted up 14 to do and thats when I bought the Sunnen stuff