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Old 04-30-2003, 06:20 PM   #1 (permalink)
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piston to deck clearance

i have an early 70s 302 motor at home and it has original factory pistons in it. i measured the deck height and it ranged from .031"- .036". the motor im building is a 302 and the pistons im ordering are keith black hyper pistons. they list all their 302 pistons to have a compression height of 1.608". to figure my approx deck clearance i use this formula. half stroke- 1.5" + rod length- 5.090" + 1.608"= 8.198". 302 deck height for C8OE is 8.206". subtract 8.198 from 8.206 and i get .008" deck clearance. is this gonna be right for the motor im building or did i miss something? the block has never been decked. is it the fact that 302s from the factory ran shorter compression heights than the aftermarket of today?
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Old 04-30-2003, 06:33 PM   #2 (permalink)
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piston to deck clearance

You have done the math to a block that C8OE is 8.206 but is it really those numbers! You wont know till you do a mock up and measure it. You can do all the math you want tell you check it. Yeah in a perfect world you would be .006 in the hole. The block aint near perfect. I just decked a 351 one side the pistons were .037 at one end and .032 in the hole at the other on one bank. the other side was .021 in the hole across the board. So until to check you aint going to know.
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Old 05-01-2003, 12:50 AM   #3 (permalink)
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piston to deck clearance

I am 100% with coolfalcon start with a good foundation that YOU know what is what start with a crank grinder who does index grinding this will make sure the crankpins are 90 degrees apart next find a machinist who uses a line hone and bores on a machine that sets up on the mains do not let them bore with a deck bar have the decks lightly skined and trued recon rods or use after market test assemble eng check each cyl for deck height and then send eng back to machinist to put the final where you want it some of these factory machine jobs make you wonder how they every got it to run cylinders so slanted it takes a .060 over to true them

[ This Message was edited by: Foxfire on 5/1/03 12:51pm ]
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Old 05-01-2003, 07:46 AM   #4 (permalink)
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piston to deck clearance

For a few years in the early 70s, the factory deck height on the 302 was increased to 8.229 from 8.206. My Mexican 302 had the taller deck when I got it. Now it has a shorter one.

The guys are completely right about mocking up your bottom end to make the measurements. Tolerance stacking can have an adverse effect on the final compression ratio. My block also had a drift on one side of about .005 front to back. The other side was almost dead on. Decked the best side first, then flipped the block 90 to do the other. Now it's all squared up and every piston is down .005.

Good luck with your project.
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Old 05-01-2003, 08:52 AM   #5 (permalink)
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piston to deck clearance

Just for future info....when Ford lists a deck height...it is a nominal value that is not to be machined below during their machining of the blocks. This means that they will be above it in most all blocks you check.

So, you can't take the static lengths of the rod, the compression height and 1/2 the stroke and then go deck the block the difference between the two and expect it to be at zero deck height when assembled. You can only get an accurate value to deck the clock by mocking the assemby using a rod, piston, bearsing and the crank.
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