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Old 12-10-2011, 07:57 PM
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Piston clearence.. take a look

hey guys,
in the process of building my first hp motor. almost ready to check to piston to valve clearences and piston to head. need to get a few more things but thought while im waiting i would have you guys give some input.. do you think this will clear at top dead center number 1 looks like it is pretty high over the deck.. the pistons are kb 25.5cc dome and the cam is a 663 680 lift solid roller with about 246 255 duration... hopefully i dont sound dumb but have done my research and have been taking my time with this build. let me know what you think with the clearence.
thanks
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Old 12-10-2011, 08:52 PM
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I wish I had a better answer for you. But nobody on here can tell you if it`ll clear. You have to check it. Even if someone was running a identical combo and it worked for them don`t mean it will for you. When cam sizes get bigger after a certain point checking it is mandatory. Cheap way to check it is play dough and talcum powder. The powder prevents it from sticking.
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Old 12-11-2011, 10:39 AM
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Clear?????

Hi,i dont think the pistons will clear the head,im THINKING that your connecting rods are too long.hoping some of our experts will chime in....
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Old 12-11-2011, 03:45 PM
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What head? Those pistons will clear most aftermkt. and GM rect. port open chamber heads but not some of the GM oval ports.
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Old 12-11-2011, 05:14 PM
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It looks almost flush to the deck to me. You look at the flat part, not the dome. Get a junk set of head gaskets that are as thin or thinner than the set you are going to run, and a set of $9 checking springs. Torque it down and see how it fits. You will get various opinions on what is an acceptable amount of clearance. Some will say a minimum of .100" for both intake and exhaust but that is just crap. The intake can be way tighter. The exhaust is always the valve you have to worry about. When the exhaust valve is fully open, the piston is at or near bdc and QUICKLY coming back up the bore. The exhaust valve starts to close and the piston is hauling *** trying to smack it as it's pushing the exhaust gases out. The piston will reach tdc and the exhaust valve is still a little bit off its seat and hasn't closed yet. Run a STRONG set of springs along with quality valvetrain components. Run titanium retainers if you can, shaft rockers, good lifters, etc, etc. Anything that can reduce valvetrain mass and/or make the valvetrain more stabile will be of benefit to you. Beehive springs are very light, aluminum rockers, chromoly pushrods should be considered as well, if you are on a budget. I'd shoot for a minimum of .035" intake clearance and .080" exhaust. If you have ample intake clearance but are tight on the exhaust you can always advance the cam 2 degrees or so to free up some room.
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Old 12-11-2011, 06:45 PM
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Im running after market aluminum heads. And the rods are stock length.
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Old 12-11-2011, 08:36 PM
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Like DoubleVision said, those are things you are going to have to check for yourself to see. Too many variables in play(what size valves, how much has the block and/or heads been cut, is the cam going to be advanced when you degree it, and on and on....).
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Old 12-12-2011, 05:45 AM
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I use plain ole kids clay with a little oil no head gasket and 0 lash turn it over pull head off and check . you will be fine with a .100 on the ex. I like .80 on the intake probably can be less, but .035 is way to close for me to each his own.
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Old 12-12-2011, 06:05 AM
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If you're unsure about the actual dome and how it fits the chamber here is what I do.
Blue up with a marker or some Dykem the head around the chamber. Put the head on the engine and attach just enough bolts to keep it in place no gasket required. Reaching up the bore gently scribe the bore diameter onto the head.
Remove the head.
Then take a piston and set the dome of the piston into the chamber and move it around in a circular pattern not allowing it to lift off the head. If nothing hits, the edge of the piston will be able to cover the scribe line all the way around as you move it.
If it hits you'll know where to remove material. Once assembled you're guaranteed a minimum of the .04 gasket thickness as clearance.
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Old 12-15-2011, 03:21 PM
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This is really pretty easy, and you dont even need clay. Take two valve springs off of the same cylinder and replace them with super weak springs from the hardware store. they need to be just strong enough to hold the valves up. Put the head on with an old gasket and rotate the engine while looking through the spark plug hole. you'll be able to see the piston coming up as the valves move. When they get to their closest point, stop rotating the engine and mount a dial indicator to the top of the valve. Push down on the valve until it touches the piston and record the number. BOOM, there's your clearance. i've done it this way lotsa times, easier than clay I think. With a dome that big and a .680 lift, you'll probably have to fly cut. No biggie. If you have good heads, that thing should tear ***. Compression rules.
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Old 12-15-2011, 05:07 PM
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Quote:
Originally Posted by 3 window
This is really pretty easy, and you dont even need clay. Take two valve springs off of the same cylinder and replace them with super weak springs from the hardware store. they need to be just strong enough to hold the valves up. Put the head on with an old gasket and rotate the engine while looking through the spark plug hole. you'll be able to see the piston coming up as the valves move. When they get to their closest point, stop rotating the engine and mount a dial indicator to the top of the valve. Push down on the valve until it touches the piston and record the number. BOOM, there's your clearance. i've done it this way lotsa times, easier than clay I think. With a dome that big and a .680 lift, you'll probably have to fly cut. No biggie. If you have good heads, that thing should tear ***. Compression rules.
Ive done it the some way. Works great for me.
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Old 12-15-2011, 05:28 PM
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If I need to fly cut is this something a guy who's building his first motor can do correctly???
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Old 12-15-2011, 06:12 PM
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Take the pistons to a shop. They need to be chucked up on a bridgeport normally and cut. It takes longer to set up the machine than it does to cut the pistons. Tell the shop how close the pistons and valves were when you measured and they'll take enough off. For instance, if you were at .040, they need to take .060 or .040 off the valve pocket. No closer than .080. Really simple.
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