Block deck height
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| + | ==Overview== | ||
| + | The block deck height is one of the first things that need to be known in order to calculate the compression rsatio of an engine. The following text reveals how to measure the block deck height. | ||
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| + | ==Deck heights defined== | ||
First, know that there are two deck heights on a motor. Block deck height is the measurement from the centerline of the main bearing bore to the flat part of the block deck where the heads bolt on. On a small block Chevy, this dimension is usually given as 9.025" from the factory. The block deck height is not to be confused with '''''piston''''' deck height, which is the measurement from the piston crown to the block deck where the heads bolt on, with the piston at top dead center. This dimension could measure zero or it could be as much as 0.045" "down the hole" if "rebuilder" pistons were used. Rebuilder pistons have a shorter compression height (usually 1.54" compression height or '''''CH''''') than stock or quality replacement pistons (usually 1.56" CH), if the block decks were uncut. | First, know that there are two deck heights on a motor. Block deck height is the measurement from the centerline of the main bearing bore to the flat part of the block deck where the heads bolt on. On a small block Chevy, this dimension is usually given as 9.025" from the factory. The block deck height is not to be confused with '''''piston''''' deck height, which is the measurement from the piston crown to the block deck where the heads bolt on, with the piston at top dead center. This dimension could measure zero or it could be as much as 0.045" "down the hole" if "rebuilder" pistons were used. Rebuilder pistons have a shorter compression height (usually 1.54" compression height or '''''CH''''') than stock or quality replacement pistons (usually 1.56" CH), if the block decks were uncut. | ||
An uncut production SBC block deck height is nominally 9.025". When combining a 0.045" piston deck height with a 0.040" compressed gasket, the result is a 0.085" '''[[quench]]''' distance, which will have little effect in controlling detonation. Quench is recommended at no less than 0.035" to 0.045", so a zero piston deck height with 0.040" gasket puts the squish in the right place (0.040"). This isn't the only way a 0.040" quench can be reached, obviously. A better way (and cheaper from the machine shop labor standpoint) is to use a thinner head gasket and keeping the deck as thick as possible. This is easy to do with the 4" nominal bore SBC because of the abundance of '''[[head gasket]]s''' available for that engine. | An uncut production SBC block deck height is nominally 9.025". When combining a 0.045" piston deck height with a 0.040" compressed gasket, the result is a 0.085" '''[[quench]]''' distance, which will have little effect in controlling detonation. Quench is recommended at no less than 0.035" to 0.045", so a zero piston deck height with 0.040" gasket puts the squish in the right place (0.040"). This isn't the only way a 0.040" quench can be reached, obviously. A better way (and cheaper from the machine shop labor standpoint) is to use a thinner head gasket and keeping the deck as thick as possible. This is easy to do with the 4" nominal bore SBC because of the abundance of '''[[head gasket]]s''' available for that engine. | ||
| + | ==Measuring block deck height== | ||
To measure the block deck height on a small block Chevy is relatively easy. The only tools you need are a 12" dial caliper and a pencil and paper. Look for the caliper on eBay for a deal. | To measure the block deck height on a small block Chevy is relatively easy. The only tools you need are a 12" dial caliper and a pencil and paper. Look for the caliper on eBay for a deal. | ||
| + | ==SBC main bore diameter== | ||
The main bearing bore diameter (without bearings installed in caps or block, caps snugged down) of a large journal (LJ) SBC is 2.6402"-2.6410". The small journal SBC is 2.4906"-2.4916", the SBC 400 is 2.8406"-2.8415". Dividing that by 2 gives the radius of the main bearing bore. For the LJ the radius is 1.3201"-1.3205. There is room to slide a 12" dial caliper down into the bore of #1, #7, #2 and #8 and snag one end of the caliper jaw on the edge of the main saddle, with the other jaw on the block deck at the edge of the cylinder. Measuring an undecked production LJ block in this manner might give you a reading of 7.7045". Adding the main bearing bore radius to 7.7045" results in a deck height of 9.0246"-9.025". | The main bearing bore diameter (without bearings installed in caps or block, caps snugged down) of a large journal (LJ) SBC is 2.6402"-2.6410". The small journal SBC is 2.4906"-2.4916", the SBC 400 is 2.8406"-2.8415". Dividing that by 2 gives the radius of the main bearing bore. For the LJ the radius is 1.3201"-1.3205. There is room to slide a 12" dial caliper down into the bore of #1, #7, #2 and #8 and snag one end of the caliper jaw on the edge of the main saddle, with the other jaw on the block deck at the edge of the cylinder. Measuring an undecked production LJ block in this manner might give you a reading of 7.7045". Adding the main bearing bore radius to 7.7045" results in a deck height of 9.0246"-9.025". | ||
| − | If you have an engine other than a small block Chevy, you need to find the main bearing bore of the motor or snug a cap onto the saddle without a bearing and measure the diameter, then divide that figure in half for the radius. | + | If you have an engine other than a small block Chevy, you need to find the main bearing bore of the motor or snug a cap onto the saddle without a bearing and measure the diameter, then divide that figure in half for the radius. The remainder of the calculations are exactly the same as stated above for the SBC. |
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[[Category:Engine]] | [[Category:Engine]] | ||
| + | [[Category:GM]] | ||
| + | [[Category:Undeveloped articles]] | ||
| + | [[Category:Undeveloped Engine articles]] | ||
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