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Old 11-28-2008, 11:11 AM
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deck hight

how to measure deck hight? Will a taller or shorter hight Chang push rod length? 350 chevy

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Old 11-28-2008, 01:21 PM
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using a dial indicator. the ends will sit flat on your block with the needle pointing down in to the cylinder. turn your crank to reach tdc and whatever the dial indicator says is your deck height. I THINK. lol, thats what i was told to do for mine
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Old 11-28-2008, 08:04 PM
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Originally Posted by tomthecomic
how to measure deck hight? Will a taller or shorter hight Chang push rod length? 350 chevy
Down and dirty, cheap way. Use a 12" steel rule and a feeler gauge set. Turn the crank to where it looks like the piston is coming up to TDC by eyeballing the piston, but is not at the top of its stroke yet. Lay the edge of the rule (stand the rule up) against the block about 1/4" from the edge of the bore at the 3:00 O'Clock or 9:00 O'Clock position as you're standing at the side of the motor. Run feeler gauges between the edge of the rule and the piston crown. Keep changin' blades until you get a snug fit. Write down the combined thickness. Turn the crank just a few degrees and repeat the measuring process. Write it down. Repeat several times. As the piston comes to exact TDC, the measurement will get smaller and smaller. Once the measurement begins to get larger, you know you have passed TDC and the piston is headed down in the bore. Your smallest measurement recorded will be the correct piston deck height.

If you cut the block decks or mill the head, yes, that moves the rocker arm/pushrod pivot point closer to the cam and will require a shorter pushrod to maintain the same geometry. Changing the piston deck height with a taller or shorter piston compression height will not change the rocker/pushrod relationship however.
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Old 11-28-2008, 11:30 PM
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You measure deck height from the crankshaft centerline, to do this accurately the crank has to be removed and the distance from the crank journals to the deck needs to be measured with a height gauge on a surface plate or a large micrometer with a ball end adapter.

Of course no one does it this way unless you are squaring the block up, most just check the depth of the piston from the top of the deck and go from there. With this method you are assuming the block is square and the crank centerline is straight and parallel to the deck as well as the length of the rods are identical, pin heights are the same etc.

When you check the distance the piston is down from the deck just remember there are running clearances to account for in your measurement, crank clearance, piston pin clearance, piston rock at TDC etc. Make sure you check all the bores since the stroke can be out a few thou from throw to throw....piston pin heights vary too.

In other words don't get carried away getting the pistons "zero" decked when the engine is static, everything grows at rpm and all those clearances stack up. Also that deck is a different shape at running temperature than the bore since it is a plate of cast iron and you don't want the rings riding in that area by having the pistons coming up above deck height at rpm.

Be conservative, you can always run a thinner head gasket to get compression or more squish in the chamber.

As for pushrod length, it depends on if you machine the deck a lot or not, if all you take off the deck is a cleanup cut and your running a regular hydraulic cam there is a tolerance built into the hydraulic lifters that will allow no changes to your valve train. Do a lot of machining on the deck and realign the crank bore and things can get out of line.

Thats why you check it during mockup for proper geometry of the valve train. For the most part though a typical cleanup of the block and touch up on the crank centerline with a hone with a normal hydraulic cam and you won't need special adjustable pushrods.

It really depends on what you do and how much you do to it and what you intend to do. In other words,,,we need more info to make a recommendation.

Last edited by 4 Jaw Chuck; 11-28-2008 at 11:36 PM.
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Old 11-29-2008, 10:30 AM
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good lord I don't know if I need to know now. I was hoping I could use a yardstick, gum and a hammer. I don't know how I would use that together. Thanks I will give it a whorl after an order from summit.
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