Rudimentary valve clearance question.

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J

Johnny from Chicago

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I yanked my stock heads off my 383 yesterday and am replacing them with Edelbrocks 84cc head. I have zero deck and a single trough relief across the top of the piston 5cc. I am trying to find out my ultimate maximum valve clearance without buying a cam and installing it only to find I wasted my money. Of course I want the most lift I can get in there .600 would be great, duration and pattern I will leave to the cam engineers. I set #1 to tdc and rested the head on there without valve springs and pushed them in until they contacted the piston (they touch right in the trough) then I used a depth mic to see how much lift I got when I pulled them back up. Bad news .349intake and .372 ex. Does this make any sense? My old 516 heads were decked .010 and I was told by the machine shop I had .510 worth of clearance with them. I have been running a 484 cam but have been getting pump up at 5400 so I want a solid lifter double throw me down cam ( not triple since I only have 9.5:1). Any info?
 
Johnny,you are going about it all in the wrong way.Its true you only have some 350-375 lift on the valve when the piston is at TDC,but your valve is not open at TDC to the full lift.A stock piston and head can handle a 500+ cam with out really looking at it.What you need to do is have a .100 clearance on the intake and about 120 on the exhaust for the valves to not hit the pistons,BUT,this is done with the motor and valve gear installed and set and turning over,and being checked at its valve timing.There is a relation ship between the valve and piston position,when the piston is at TDC,one of two things is happening,its either finishing the exhuast stroke,and stating to open the intake,with the exhaust valve slightly open,or its starting the power stroke down,so the valves have very little lift.When the piston is about a inch down in the bore the valve willbe fully open then,and not at TDC.The Valvles open and close with the cam timing and the piston position.If you need more help,email me your number and i will explain,Mrmopartech
 
How deep are the valve notches? My 383 (piston .014 in the hole) didn't have enough valve clearance with a 292-.509 cam. only had about .070. That was with a steel shim head gasket (.018). I had flat top pistons without valve notches. With the Eddy heads though, who knows. Ya gotta put a cam of some kind in there and measure what you got and go from there.
 
I guess I'm going to have to sack up and buy the cam I want and make the motor eat it one way or another.
 
So I measured my piston to valve clearance with the only way that made me feel sure. I rotated the engine a couple of degrees at a time and used a depth mic to see how much it took to tap the piston with the checking springs installed . I had .220 intake and .230 exhaust with the new Edelbrocks on. I realize this is with my old cam, but I know I have so leeway now. (I quadruple checked this on #1 today and I'm triple checking it on the other cylinders tomorrow.)
 
according to what you measured it seems like that a .600 cam will fit. BUT WAIT! A .600 lift cam will have a lot more duration than your .484. which means the valve will be opening earlier. I wouldn't go with more than a .560 - .575 lift cam as a max. The duration is almost as important as lift when it comes to piston to valve.
 
Yea, a couple people have mentioned that duration will kill you quicker than lift. I was also thinking I was safe in the range you listed, but I play it safe and am considering around .540 or .550 with not too much more duration than the .484 cam. I will only have 10.1 compression after decking the heads, so I can't go too wild. I have also had time to check the other cylinders and they all check out near the same clearances .220-.230. Thanks for all the info.
 
John, your piston to valve clearance, like MrMopar said, is dependant on your cam choice. Each cam is different, and the newer fast rate lift cams will lesson this clearance. I would suggest you keep the [email protected] under 240, and the lift under .550. Your only definate clearance is to buy the cam, and install it, degree it and get it where you will run it, then put clay on the piston, install a head, and spin the engine over. You can do this without a gasket, because you know what the compressed gasket thickness is. BTW, you are installing the composite gaskets, not steel shims, right? Then you pull the head, and using a razor, cut each impression in half. The thickness of the clay is your minimum clearance. (A depth mic is very difficult to use, because you must find the spot where the piston and valve are the closest..It's not at max lift)
 
I am considering the comp cams xe solid .520 and .540 with 244 and 252 or the .525 .525 with 248 248. Yes I'm using a .040 gasket because of zero deck and not a steel shim. These cams were also what the comp techline recommended. I guess it's a 119.95 gamble, but I'm pretty sure of my readings since I did find the closest points were not at max lift just like you said. I also did get the exact same readings over a two day period of time checking several cylinders.
 
You gotta check it to be sure. I had stock pistons .040 down in a 383 and had my 452 heads shaved .060. With the .484 cam on a 104 centerline I had just over .100 piston to valve clearence which is at the limit. I like to use 2 light springs on my heads and push the valve down until it hits the piston. I check both of them every 5 degrees thru the whole overlap period. Good luck and dont forget the Eddy heads will also have the larger 2.14 and 1.81 valves. Ron
 
John, I just want to be sure you're checking them the right way. By your first description, you're not. It's a dynamic relationship, and the checking procedure also has to be. So you need to have the cam you're running, and the piston, and head, then put them together with the timing chain and turn it thru a few cycles to get the correct readings. Also, your cam's timing (where you installed it at) will also change the piston to valve clearances.
 
My first description definately was not right but since then I've done the same thing that 383 man described, spun it over and pushed the valve in until they touched the piston all the way around the overlap and lift period.
 
ok, just so long as you check...now you know what you're paying for at a good shop..lots of checking...
 
My back is killing me from the repeated off and on of the heads, the claying, the turning it over again and again, and my head hurts. I think it would have been easier to pull the engine and take it back to the machine shop.
 
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