Sky65

I copied the information below from a post on another forum. Very interesting information and valuable if you are building an engine. The poster is a gentleman that goes by Techinspector. The entire thread is here.
http://www.hotrodders.com/forum/cams...-466842-3.html


Tom

Ummm, probably not. Your motor is the result of being produced in a mass production shop where being "close" is acceptable. It won't cost you much to have a machine shop measure the main bearing bore for size and parallelism and the block deck height measured on all four corners. During machining operations at the factory, it is common to have a small chip of metal left in the register from the previous operation by the machine operator and resulting in the wrong machining of the block. This is what we do as hot rodders, find these errors and correct them. Your snappy answer tends to alert me that you may have a bit of an attitude, so I would advise you to soften up a bit and pay attention to those who have leagues more talent and experience than you do. You don't know what you don't know.

Here is a photo of a fellow measuring the block deck height (the measurement from the centerline of the main bearing bore to the block deck where the heads bolt on)......
http://jamisonequipment.com/sites/de...s/BHM-24-1.gif

I'll help you with a few of the common meanings of engine building terms that you may not be familiar with......

BLOCK DECK HEIGHT: Distance from the centerline of the main bearing bore to the block decks where the heads bolt on. The blueprint dimension for this spec on a small block Chevy would be 9.025". Don't just call this "deck height", because then it can be confused with PISTON DECK HEIGHT.
http://image.superchevy.com/f/538997...eck-height.jpg

PISTON DECK HEIGHT: Distance from the crown of the piston to the top of the bore in the block with the piston at top dead center. The blueprint dimension for this spec would be 0.025". Shown here as "DECK CLEARANCE"...
http://www.grumpysperformance.com/pistondp3.jpg

PISTON COMPRESSION HEIGHT: Distance from the centerline of the wrist pin to the crown of the piston. The blueprint dimension for this spec on a standard 350 piston would be 1.560". Be very careful when choosing pistons, as manufacturers also produce a "REBUILDER SPECIAL" that has a reduced piston compression height of less than 1.560", making it difficult to engineer in a tight squish/quench to maximize power and minimize detonation on pump gas without cutting the BLOCK DECK HEIGHT. Shown here as "PISTON HEIGHT"....
http://www.grumpysperformance.com/pistondp3.jpg

SQUISH/QUENCH: The clearance between the top of the piston crown and the underside of the cylinder head with the piston at top dead center. It is the sum of adding together the piston deck height and the compressed head gasket thickness. Most hot rodders will agree that a range of 0.035" to 0.045" will work to minimize detonation on pump gas and make the most power. The mixture that is between the piston crown and the underside of the cylinder head is squeezed out as the piston ascends to the top of its travel in the bore and the mixture is blown across the bore into the combustion chamber. This "jet" action of the mixture breaks up large droplets of fuel that will not burn and homogenizes the mixture above the piston so that there is a complete burn of the mixture, maximizing horsepower and minimizing detonation on pump gas.

STACK: The sum of the height of the parts that are fitted into the block. If a stock 350 crank has a radius of 1.740", a stock 350 rod has a center to center length of 5.7" and a stock 350 piston has a piston compression height of 1.560", then all three of these values added together would yield a "stack" of 9.000". When this stack is fitted into a 9.025" block deck height, then the crown of the piston will be down in the bore by 0.025" with the piston at top dead center. Then, when a steel shim head gasket with a compressed thickness of 0.015" is added to seal the bores, the resultant squish/quench is 0.040", right in the middle of the spec that we like to use. All aluminum head manufacturers recommend a thicker "composition" head gasket than this though, and that's where we begin to run into problems. If the piston deck height is 0.025" and the composition gasket is 0.040" thick, then the squish/quench becomes 0.065", way out of the 0.035" to 0.045" spec that we want in the motor.

With a stock block having a block deck height of 9.025", we would need to cut the block decks 0.025" shorter to use the composition gasket and keep the squish/quench where we want it. Either that or use a piston with a taller piston compression height, such as the pistons offered by Skip White on ebay. He has made a deal with Wiseco to produce pistons for him that are 0.015" taller, measuring 1.575" piston compression height. Using these pistons, you would need to cut only 0.010" from the block decks to achieve the tight squish/quench with a composition gasket. The less material you take off the block decks, the better off you will be as far as lining up the heads and sealing the intake manifold to them. In some cases though, you just can't help taking a healthy cut on the block to equalize the block deck height on all four corners. Each block is different.

SWCDuke

I disagree with some of these statements based on my own experience. The Flint machining process was amazingly accurate. Main bearing and cam bearing bores are usually spot on. Decks are usually level (and so are head mating surfaces), but also usually above the nominal 9.025" nominal height by as much as .015" and different side to side by up to .010". This is an issue when you are restoring and original numbers matching block and do not want to disturb the stamped data. If the driver's side is high it can be machined down and different thickness head gaskets can also be used on each side to achieve near nominal .025" deck clearance with little or no variation between sides.

Good luck tryilng to find a machine shop that will make all the requisite measurements. Most are "production shops" that align bore, deck, and do other usually unnecessary machining operations. A lot or numbers matching blocks have been ruined, and the Web is full or horror stories. Machine shops have to be very carefully managed.

In another thread the discussion got into quench clearance. It's a way overrated subject unless you are building a racing engine and want the last tenth of a point of compression that can be tolerated on whatever fuel you are using.

Typical production small blocks of the era had quench clearance ranging from about .040" to .060", and the Chevrolet Power Manual recommends .035" minimum, but I know of drag racing engines that were assembled with literally zero quench clearance.

When Chevrolet got lots of customer detonation complaints on 340/360 HP engines the solution was to add a second .018" production gasket, which cured the detonation but increased quench clearance by .018". How 'bout that! Double gasketing was implemented about halfway through the '62 model year and continued to the end of '63 production.

Also, C. F. Taylor states in his text book that the benefit of additional detonation resistance does not show up until quench clearance is less than .005 times bore diameter, which would be .020" on a 327.

I prefer to work with guys who can do their own disassembly and assembly and make the requisite measurements and manage the machine shop very carefully, and I have written two papers that discuss specific issues on block and head prep and another on how to manage the compression ratio.

Base small blocks should be in the range of 9.5-10:1 and SHP small blocks should be in the range of 10-10.5:1 to operate on modern pump premium, usually with a more aggressive spark advance map than OE. That's true measured compression ratio, and I've never run across a situation where the above ranges could not be achieved with the proper combination of piston selection and head gasket thickness.

Duke