I have an 89 stang w a 347 stroker in it w 185cc tfs heads good up to .600 lift and 1.7 rockers, and my cams lift is .579/.579 w a 114 lobe seperatioin. would anyone bw able to tell me if this would clear or how to check if it would clear? thanks!!
Checking piston to valve clearance is an extremely important part of engine building. Most aftermarket forged pistons have plenty of room for valve lifts up to .650” but it only takes a few minutes of your time to double check and eliminate the risk of bending every valve in your engine or busting every piston. Also, different rocker arm ratios, spring pressures, and cam timing settings can all affect clearance. Better safe than sorry.
Keep in mind there are two types of clearance: depth clearance and radial clearance. Depth clearance is generally what we think of when piston/valve clearance is checked. However when valve lift causes the valve to use the valve relief in the piston, you need to make sure there’s enough radial clearance around the valve head to ensure the outside lip of the valve doesn’t contact the wall of the valve relief.
To check radial clearance, I use modeling clay on the face of the piston. Place the clay down inside the valve relief itself and mould it to the shape so that you’re filling the valve relief with clay. Add more clay and build it up so that it’s a clay extension of the valve relief. I usually add about 1/4” of clay so that it gives me a good valve impression.
Now bolt your cylinder head on. You don’t have to torque it down, but make sure it’s snug to eliminate the head from lifting off the block. An important note is that if your pistons come out of the hole at TDC, make sure you use an old head gasket of the same compressed thickness that you're going to use when you finally assemble the engine. If not, your quench distance will be a negative number and you will not be able to turn the engine over.
After the head is bolted on, put a lifter on the intake and exhaust lobes of the cylinder you’re checking. Install your pushrods and rocker arms. If you’re using a hydraulic cam, you will have to use a solid lifter so that there’s no chance of the pushrod collapsing the plunger in the hydraulic lifter. Even with checking springs installed, there’s no reason to take a chance. Adjust your rocker arms so that there is no valve lash (make sure the lifter is on the heel of the cam lobe). If you don’t have the actual pushrods that you’re going to use, use a pushrod length checker. You don’t have to do both valves simultaneously; you can do one valve then do the other.
Now rotate the engine through 2 complete revolutions. Remove the rocker arms and pushrods and unbolt the head. When you remove the head, you will see an imprint of the valves in the clay that you put on the piston. At this point, you will be able to see the radial clearance between the valve and the valve relief and you’ll also be able to get a good idea of your depth clearance as well. If the clay was not disturbed, you can use a set of dial calipers or a machinist’s rule to measure the amount of depth clearance that you have.
If you want to double check your depth clearance measurements, remove the clay from the head and piston and make sure everything’s clean. Mount your degree wheel and find true TDC. Bolt the cylinder head on (with gasket if needed) and set up your valvetrain components as before.
The most critical area on a crank rotation is when the valves are open at the same time during valve overlap. This generally occurs from 15 degrees BTDC to 15 degrees ATDC.
Install checking springs on the valves that you will be checking clearances. Also, mount a dial indicator so that the plunger tip sits on the spring retainer.
Turn the engine until the piston is on its power stroke. Keep turning and when the piston starts to come up on the exhaust stroke, stop at 15 degrees BTDC. Depending on your camshaft grind, the valves may or may not have started to open at this point. If not, rotate another 2 degrees and check.
You will then be making a “map” of the valve clearance of each valve. At every 5 degree interval, zero out the dial indicator, then push down on the rocker arm tip with your finger. Keep your eye on the dial indicator and count how far you can open the valve with your finger until it contacts the piston. Do this every 5 degrees for each valve until the piston is at 15 degrees ATDC. The distance that you see on the dial indicator is the piston/valve depth clearance.
Keep in mind here that if you didn’t use a head gasket, you can add the compressed thickness of the head gasket to your results. Also, if you’re running a solid lifter cam, you can add valve lash to your results.
If you’ve used the clay method and the dial indicator method to check clearances, you can use them against each other for verification.
Also remember that when you change cam timing, it affects piston/valve clearances. So if you retard/advance your camshaft, be sure and recheck the clearances. Retarding the camshaft will give you more intake clearance but will take away exhaust valve clearance. Advancing the camshaft does just the opposite.
Different builders have different clearances that they like to see. I like to see at least .060” on the intake valve and at least .100” on the exhaust valve. .050”-.060” radial clearance is also ideal.