I was going to stay out of this one, because it seemed pretty obvious and straightforward, but Stephen brings up a valid point (although I disagree with it).
Here is exactly what the rule says:
"Engines may be bored to a maximum of .040 inch over standard bore size. Factory oversize replacement pistons or their exact equivalent shall be used. Cast or forged equivalent pistons shall provide the same dome/dish/valve relief configuration, ring thickness and spacing, pin height relationship, weight, and compression ratio as factory replacement oversize pistons. Piston rings are unrestricted.
Stephen, your point appears to be that if there never was a factory oversize piston, then there can never be an "exact equivalent" within the aftermarket (how can you be "equal" to something that doesn't exist?). Read that way, you are absolutely correct. However, you cannot take that one sentence in a vacuum. For example, how can that one sentence be applicable if the car came with cast pistons and you decided to install forged? That's because the rules allow the mod of forged pistons, even though they are not the "exact equaivalent".
When a modification is allowed, there are always implied modifications that go along with it. If you are allowed to overbore the engine, and balancing and blueprinting is allowed, there is no plausible and safe way to properly build an engine without appropriate oversize pistons. Since aftermarket pistons are allowed, I contend that aftermarket oversize pistons are also allowed, as long as the remaining features match the factory pistons.
As an aside, if you guys are running .020" oversize pistons in a .040" overbore, you're giving up a crapload of power and longevity. You're much better off using a .020" overbore block and making those rings seal better. Beside, you're running a large risk of cracking pistons and rings and/or damaging the block.
As to running longer (not wider) rings to make up a high piston-to-bore clearance, that's a horrible practice. By allowing that kind of p-t-b clearance your rings are not stabilized, will not seal to the piston properly, will not be adequately pressed into the bore, and will break much more easily. Any 'increased displacement' you get from doing this is more than overcome by decreased combustion chamber sealing and increased drag from chattering rings and pistons. If you're doing this you're much better off just not bothering to overbore the engine; you're simply pissing away time and money.
As to the clearance/service limit question, when an engine is properly built, the desired piston-to-bore clearance is maintained. Pistons (overbore or not) are designed and built based on the nominal bore diameter, not the service limits. Thus, when you buy/build overbore pistons, they will be .040" larger than stock nominal value, and you'll hone the bore to each piston and grind the rings to the desired end gap. These resulting bore values will be well within the +/- service limits of the original, plus the allowable .040". Unless you're driving this car to work every day, and/or you're running crappy oil in the thing, it is unlikely you will ever wear the cylinder bores to the point that the value is greater than the outside limits. My NX2000 had 11,000 total miles on it after nearly a decade of Showroom Stock racing. With the noted exception of the hole on the front of the block, the cylinder bores were nearly factory value.
[This message has been edited by grega (edited October 13, 2004).]
Here is exactly what the rule says:
"Engines may be bored to a maximum of .040 inch over standard bore size. Factory oversize replacement pistons or their exact equivalent shall be used. Cast or forged equivalent pistons shall provide the same dome/dish/valve relief configuration, ring thickness and spacing, pin height relationship, weight, and compression ratio as factory replacement oversize pistons. Piston rings are unrestricted.
Stephen, your point appears to be that if there never was a factory oversize piston, then there can never be an "exact equivalent" within the aftermarket (how can you be "equal" to something that doesn't exist?). Read that way, you are absolutely correct. However, you cannot take that one sentence in a vacuum. For example, how can that one sentence be applicable if the car came with cast pistons and you decided to install forged? That's because the rules allow the mod of forged pistons, even though they are not the "exact equaivalent".
When a modification is allowed, there are always implied modifications that go along with it. If you are allowed to overbore the engine, and balancing and blueprinting is allowed, there is no plausible and safe way to properly build an engine without appropriate oversize pistons. Since aftermarket pistons are allowed, I contend that aftermarket oversize pistons are also allowed, as long as the remaining features match the factory pistons.
As an aside, if you guys are running .020" oversize pistons in a .040" overbore, you're giving up a crapload of power and longevity. You're much better off using a .020" overbore block and making those rings seal better. Beside, you're running a large risk of cracking pistons and rings and/or damaging the block.
As to running longer (not wider) rings to make up a high piston-to-bore clearance, that's a horrible practice. By allowing that kind of p-t-b clearance your rings are not stabilized, will not seal to the piston properly, will not be adequately pressed into the bore, and will break much more easily. Any 'increased displacement' you get from doing this is more than overcome by decreased combustion chamber sealing and increased drag from chattering rings and pistons. If you're doing this you're much better off just not bothering to overbore the engine; you're simply pissing away time and money.
As to the clearance/service limit question, when an engine is properly built, the desired piston-to-bore clearance is maintained. Pistons (overbore or not) are designed and built based on the nominal bore diameter, not the service limits. Thus, when you buy/build overbore pistons, they will be .040" larger than stock nominal value, and you'll hone the bore to each piston and grind the rings to the desired end gap. These resulting bore values will be well within the +/- service limits of the original, plus the allowable .040". Unless you're driving this car to work every day, and/or you're running crappy oil in the thing, it is unlikely you will ever wear the cylinder bores to the point that the value is greater than the outside limits. My NX2000 had 11,000 total miles on it after nearly a decade of Showroom Stock racing. With the noted exception of the hole on the front of the block, the cylinder bores were nearly factory value.
[This message has been edited by grega (edited October 13, 2004).]
