I need help for a 468 with compression ratio. The block is a 4.310 bore, the heads are 119cc or thereabouts. The piston is KB-257, 4.0 in stroke, 6.135" rod, .250" dome, camshaft:
294-302 adv. duration
254-260 at .050"
.629/.629" lift
108 lobe sep, 104 int cnt
The pistons are rated at 10.2 to 1 compression, with 119cc heads. But it doesn't say if this is at 0 deck height or .025" in the hole. I would like to make this at least 10.2 to 1 or even 10.5 to 1. With the camshaft, I think I need a little more.

I forgot to ask, do the Keith Black's need the file fit rings, or can I go with a standard Moly ring? Summit lists these as the 1/16 rings and Doug Herbert, who I ordered them from said they had the 5/64 ring. Competiton Products list them as 5/64 also. :confused:
Too many big blocks, too little time!! ;)

KB uses .040" quench for calculating the SCR. So it could be .025 deck with a .015 gasket or zero deck with a .040" gasket or anything in between. KB's CR numbers are usually accurate. The cam would like more compression but will work OK with what you have. Not having all the specs, I estimate the DCR at 7.79. If the heads were milled to about 112cc, the DCR goes to 8.28. Again, this is just an estimate, you'll need more accurate measurements to get more accurate results.

I don't know the thickness of the ring but standard fit rings will work since you have to open them up way beyond any other piston's requirements. No point in making more work for yourself.

i have the same setup only i'm only .030 over. i would order the file fit rings and double check with K-b about the top ring gap. mike

well if memory serves me right there was two designs of the keith black pistons, part number, i believe 257 you have are the 5/64 ring and the other was the 207 that take the 1/16th ring............they are the same pistons they both sport the .250 dome and both are rated at 10.2 to 1 compression with 119cc head.

now the last time i ordered 1/16 rings from doug herbert they cost me 127 shipped(ouch)

if you have the pistons in hand just check the top ring land and measure.......that way you know for sure what pistons you got.

as far as compression goes.....on a open chambered head every .006 that you mill off is about 1cc so if you was to mill say .030 off your head that would drop down to 114cc and roughly every 1cc is about a .1 in compression bump. combine that with a steel shimmed headgasket (thats if your deck height is .015 or more) you could reach close to 10.8 to 1 or so.

of course you wont know for sure til some measure ments are taken and you decide how much to mill.if any that is, just keep in mind to run at least .035" between the piston and head.......to keep the piston from smacking the head........

i have ran deck heights of .015-.012 (on one of my blocks) with a .020" steel shimmed head gasket and didnt see no signs of piston hitting head........and that was turning it to 7000 rpms..........the .035" clearence is just from what i have read and understood in writting about for bbc so since i can remember i keep my clearence around there..........of course they have said .060 for motors with alum. rods......i have yet to build one.......

anyway hope this helps..........

I will throw one other thing in here for you. If this is a rebuild, do not assume the block, rods or crank are correct. I had decided to get out the micrometer and go over everthing - my results don't add up. In my case, I have a decked block measured at 9.789" finished. Crank is 3.76", pistons are supposed to have 1.770 Comp ht, rods are supposed to be 6.135" - according to this I am supposed to have piston/deck ht of .004". Guess what, I don't. The piston/deck ht installed is .016". I lost .012" somewhere. Crank is .010" under, rods are reconditioned. Either the pistons comp ht is only 1.758, the rods are 6.123" or the crank is offset ground or a combination of the above.
Point is, if you want a certain compression, don't assume it is correct, you have to measure the assembled unit.

Okay, assuming I want 10.5 to 1 compression, which should I have done? Should I have the heads milled and/or the block decked? What would give me the best results? What head gasket to run? I am also worried about piston to valve clearance. If I run the steel shim gasket, will I have enough? I don't have the pistons in hand, as I just ordered them from Herbert and they have to get them from Keith Black direct. Said should be no longer than a week.

What about having the block decked to 0 deck height? Is this achievable in a big block application? Or will I run into too many clearance issues?

I forgot to mention Thanks for all the replies so far! smile.gif

i emailed KB about piston to valve clearance. they are SUPPOSED to be generous. my engine builder claimed it was no problem with my xtreme 274H grind. but i would always double check.

you should have tons of room with your small pistons the domes you have on your pistons are small.......they have plenty of room with the valve relieves. you wont have to worry......im running a 13 to 1 piston in my 122cc head big block with domes that are just over .550 tall and i had to custom shape some of the corners on the dome to fit in my open chambered heads.and im running a solid .685 lift cam and i checked mine without head gaskets and zero lash.

to ease your mind when it comes time to check put your head on without the head gasket and use playdoo like i do on top of the pistons use alittle oil on the chamber of the head so the playdoo dont stick to the head when you take it off......set up the valve train and lash it. spin the motor over then take it apart and you can take a reading with your calipers......

as far as the compression goes....you will have to assemble the bottom end and check each piston at top dead center and record your readings. 99% of the time you will find from one end of the block and side to side of the block you will have a different readings on each hole......specially if you had any rods resized because that just lowered the pistons down the hole. most blocks are out of square anyway.

so if you took your measurements.(make sure you take your measurements just over the wrist pin, other wise your measurement will be off because of piston rock) and you have say .015 being your highest piston and .022 being your lowest piston on your right bank then you could shave off .015 on that side.

and if your measurents comes out to .013 being the highest piston on the left bank and your lowest .019 then you can shave off .013 on that bank....

and that would get you as close to zero deck then you can run a .039 head gasket.

but for the cost of decking you might as well just run a steel shimmed head gaskets.

now the only way most of your pistons will read the same height is if you had some very good machine work done......meaning the machine shop took measurements from the raw block to see how much it was out of square and milled the decks back straight off of the mains......and made each side and front to rear the same.

then all the rods was made to the same length thru resizing..

but in the majority of the cases most machine shops just mill the head flat to the oil rails and call it good .......and just resize what ever rods that need them........so therefore you come up with all kinds of deck heights for the pistons.......

now once you decide what you can live with on the short block (redeck it or just run still shimmed or start over and have the block all trued up and all the rods the same length) then you can attack the heads.....thru alittle caculating you can figure out how much if any to shave your heads to get your final compression ratio you can live with.

now in my own opinion i hate decking and milling(other then just to clean them up is ok)........because that means more chances that you may have to run shorter push rods and mill the intake side of the heads to make the intake fit.......