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help needed with valve clearance on a shaved p79


Guest smoov280zx

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Guest smoov280zx

On a p79, shaved .080 with flattops and a .475 lift cam, would the pistons need have valve reliefs cut into their tops?

 

Part 2:

 

If the head is shaved .080, the camtowers shimmed .075 and the valve springs shimmed .080, would the .005 difference in the springs and towers have any kind of bad effect on the the lashpads or camlobes? I would think something in there would be adjustable by at least .005, but i could be wrong. I just want to know if i would need .005 thicker lashpads than normally needed to accompany that cam. Thanks for any input.

 

Mitch :confused:

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Guest V8wannabe

Question #1...don't take a chance, put some clay/putty on the piston, bolt on the head and turn it over 1 rotation. take the head off the block and the putty will tell you how much clearance you have

Question #2 I doubt that .005 will hurt anything. You want to watch that the cam does not travel off the pad. I checked mine with a black marker, cover the wipe pad on the rocker with ink, assemble the cam & rocker arm, rotate the cam and it should remove the ink where it is striking the arm...the closer to centre the better. Just make sure it's not running off the end. good luck!

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The .080" shaved head will clear the flattop just fine with .480" lift but just to be sure, do the clay test.

 

And I don't know why you are shimming the valve springs .080"? you only need to do that if your valve seats been shaved or using longer valves. Or you will bind the valve springs with .480" cam. .480"+.080"=.560"

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Guest smoov280zx

I am using the 280 valves, which i read to be .080 longer, that why i'm shimming the springs. I'm trying to keep everything consistant so i won't have weird size lashpads or anything else.

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Guest smoov280zx

whats the difference between shims and valve spring seats? The haynes manual only shows spring seats, but the the p79 mod page says the heads already come with one shim under each spring. Is he talking about spring seats?

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You want the valves shimmed to yield the specified valve spring installed ht (and pressure). You put in the longer valves so that you wouldn't have to buy huge lash pads. I think the 280 valves may be .100" longer than the 280ZX P79 valves and not 0.080" (check to confirm this). If so, you'll need .100" of added shims to get the spring pressure correct. You are trying to raise the valve assy (including lash pad) by the same amount you raised (shimmed) the cam towers in order to preserve the geometry.

 

The clay method is one way to check piston-to-valve clearance, or you can install light springs on an intake and exhaust valve, rotate piston to TDC, and use a dial indicator to see how much room is there when the valve is depressed to touch the piston top. DAW

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