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What is normal practice when zero decking a block?

Does the machinist assemble the short block and take a measurement from cylinder deck to piston face at TDC, or does the machinist take into account the cranks stroke, rod length and piston height to determine how much to remove, without assembly?

The second theory would not be accurate, because the deck height is not always exactly the same on all production blocks, correct?
 

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Ideally, the engine is mocked up with the installation of the crankshaft and then the same piston/rod combo (ie, one piston & rod) being fit to all 4 corners of the block for the deck height measurement.

Paul
 

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Unless you have to have a specific minimum deck height for a rule of some sort, being within a thou in deck height wouldn't make a noticeable difference IMO. I calculated my total height at 10.286 with Speed Pros, Eagles, and a stock crank. Measured came in around a 1/2 a thou different. There was around 1/2 a thou variance from hole to hole as well. Granted, it could have been my measuring.

If it were a normal performance build, I'd calculate and have the deck cut a thou less. That way, you'll probably be between a 1/2 thou and thou and a half out of the hole.

I'm not a pro, just one guys opinion.
 

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Unfortunately, the practice of using one piston and rod assembly to check all four corners is fraught with problems IF, accuracy is the goal. IF, and it's a BIG IF, ALL the pistons have exactly the same compression height AND all the rods have exactly the same center to center measurement, AND all the crank throws have exactly the same stroke, you will have errors built in. That said, it's often "good enough"...if you'll settle for that. IMHO, the best way is to give your machinist the specific deck height you want...AFTER you do the work to figure it out. That way the block deck should be square to the mains and perpendicular to the bores. :)
 
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