p/v clearance threory-- bogus or not? what do you think?

[DRASTiK]

I was installing a set of heads on my car this weekend, and having all kinds of trouble getting the valve spring off the head. My question is this: If the distance the valve travels to the piston at TDC is 1.89", and the lift of my cam is .544. Is it safe to say that my p/v clearance for that valve is 1.346"? or am I just WAYYY of base here?

[Capri306]

Unfortunately, you're the latter: waaaay off. The reasons?

1. The math for clearance is a fuzzy mess. You have to include all sorts of measurements when you wanna find out how far the piston actually is from the valve (head gasket thickness, piston height, etc) and

2. The valve TIMING is more important to determine exactly if and/or when the valves are going to hit the piston.....well, hopefully they don't, but if you're in for some flycutting, this is your bag. This is why a lot of guys wonder what the maximum lift you can put in a stock 5.0 is, and there really isn't an answer because every cam has differently timed events. Think of it this way. When your exhaust (or intake) valve is full-open, is it right at TDC, a little after, or a little before? This is where it gets fuzzy. Oh well, hope I didn't confuse or discourage ya. Later!

[DRASTiK]

No discouragement here. Thanks for the reasoning. I figured that little formula seemed a little too simple to be accurate. That's why I made the thread.

I've got an air compressor and I know that you can use compressed air to keep the valve from opening while compressing the spring.
My concern is how exactly to go about doing this. Do I fill it with air, then stop the air without releasing the air of course, then compress the spring and hope the valve stays closed or do I keep the air going while I compress the spring? I was trying this earlier, but the valve kept opening and the air would leak out the ports to the intake.
I also tried using nylon rope to hold the valve closed, but had no luck there either. Maybe I wasn't trying hard enough for fear of mangling a valve. I've got weak springs to use while checking the clearance, and a spring compressor, but they do me no good if I can't get the current springs off the head. Any thoughts?

[andy669]

Dave, do you have access to any speciality tools? The best way that I've found to check P to V clearance (after the heads are already on) is to find the point in the camshaft timing events where the valve is going to be closest to the piston. (This is the fuzzy area that Capri mentioned) The intake valve will actually chase the piston down somewhat, and without knowing the timing events of your cam I cant determine where the closest point will be. If you are able to determine this point, put a dial indicator on the retainer and measure the distance it travels when you push down on the valve. This is where the special tool comes in if you are using some real springs. It will hook on to the poly lock on the rockerarm for a pivot point, giving you lots of leverage on the retainer.

Or you could just pull them back off and clay them! Obviously you havent fired it up, so the gaskets should be ok to reuse.
andy

[DRASTiK]

I think I'm gonna try and get the test springs on, and then attempt to find the closest point, and measure it that way. Correct me (again) if I'm wrong, but that point will either be just before, or just after the piston reaches it's highest point? If so, then I'll just measure once with every little movement of the crank in that whole area. I just wanna make sure I get it right before I fire this thing up because I can't afford to buy these parts again!! thanks very much for the help so far. I should have time to mess with it tomorrow night after work, then I'll post my results or problems, whichever I come out with

[Jeff Chambers]

If you can't afford to replace the parts because you didn't check PV properly, then you can't afford to NOT do the PV check properly. If you've increased cam lift or duration, went to heads with a larger valve, gone to a thinner head gasket, or whatever, then the only sensible thing to do is to check it properly. From the way it sounds, you're planning on using your hydraulic lifters to check it as it is? WRONG! You need to make a solid lifter by removing the plungers from a few old lifters and turning them into solid lifters by flipping the internals or filling the lifter with washers. Otherwise, you'll collapse the lifter while you're trying to do the PV and won't get anywhere near the correct measurement.

TAKE THE TIME TO PULL A HEAD AND CHECK PV CLEARANCE CAREFULLY AND PROPERLY IF YOU'VE GOT ANY QUESTION AT ALL.

At most it'll cost you a few more hours and the cost of some new gaskets. But that's a bunch cheaper than new pushrods, lifters, valves, etc. that can be ruined by not doing it right.

When you check it, you want to maintain .100" clearance on the intake valve and .125" on the exhaust valve. Be sure to check for interference at the radius of the eyebrows as well since the valve may contact here before it runs out of room in the depth of the relief. You also want to maintain .040" clearance between the piston and the closed portion of the combustion chamber.

[DRASTiK]

Thanks Jeff. I'm sorry if I gave the wrong impression. The reason I made this thread is to make sure I get it done correctly. I was fed some wrong info and it's my fault for not analyzing it before I believed it. You would think that as much bad advice that is circulating around the car scene, that I would've been smarter than that, and I'm kinda pissed because I wasn't. But that's neither here nor there. I don't mind the work, I enjoy taking motors apart and putting them back together, as long as I'm not rushed (which I'm not). It was just a stupid mistake that I didn't catch at the right time. Thanks again to all.

[Jeff Chambers]

No apologies needed. Just don't want to see anyone mess up their ride.

[DRASTiK]