Fox T-Bird/Cougar Forums  

They will work ron,but as you said .530" lift with those 1.7s on flat tops will equal "smack".

They *should be* ok. Just clay it first before you start it

I have yet to see an SO motor with true flat top pistons...every one I've pulled apart has dished pistons with no valve reliefs...that's why the '86 GT's have 9.2:1 compression with the same heads as the SO...true flat-tops...ours are listed as 8.8:1 or 8.9:1 nominal...not having valve reliefs doesn't necessarily equal "flat top".

-Don

he has a 306, not a factory SO 302.

I have yet to see an SO motor with true flat top pistons...every one I've pulled apart has dished pistons with no valve reliefs...that's why the '86 GT's have 9.2:1 compression with the same heads as the SO...true flat-tops...ours are listed as 8.8:1 or 8.9:1 nominal...not having valve reliefs doesn't necessarily equal "flat top".

-Don

Piston to valve clearance is the relationship is just that, the distance from the valve to the piston during the most critical time in the engines cycle, that time is the "overlap".  No other time during the engines cycle is the piston closer to the valve than in the overlap. Overlap occurs at the end of the exhaust stroke and the beginning of the intake stroke. This is the time when both the intake and the exhaust valves are open simultaneously.

A lot of people have used the max lift number when calculating piston to valve clearance and that is probably the single worst method. When the valve reaches it's maximum lift point the the piston is actually furthest away traveling down the cylinder bore drawing the intake charge in. The minimum clearances commonly used are .080 on the intake valve and .100 on the exhaust valve. For people using an aluminum rod it's a good idea to  allow more clearance as the aluminum rods stretch when in operation.


To use the clay method take a 1/4th inch amount, cover the valve reliefs on your piston and rotate the engine one full cycle. Make sure it's 2 revolutions past top dead center. Take the head back off and peel the clay away. The best benefit of clay is that you can use an x-acto knife and cut it into sections and measure exactly where the thinnest area is, if there is any. Remember .080 and .100 minimum thickness.

The next method takes a little practice but is great if you dont want to take the heads off the car, although it will be necessary to replace the intake and exhaust valve springs on one of the cylinders. It's best to use a checking spring, but if you don't have a dedicated light weight spring, take a trip to your local hardware store. Remember the spring only has to have enough pressure to keep the valves fully closed and return an open valve back to it's closed postition.

Now you want to adjust the rockers to zero lash. What is important here is if you are using a hydraulic type cam, wether it be a hydraulic roller or flat tappet hydraulic cam. Do NOT preload the lifter. All you want to do is take all the movement out of the pushrod WITHOUT collapsing the plunger in the lifter. Make sure you do this adjustment when the lifter is on the base circle of the cam which is at top dead center on the compression stroke.

You will need feeler gauges set to 0.100 thousandth's. Start with your piston at top dead center and rotate one full revolution in normal operating rotation. The piston starts traveling back up the bore, the exhaust stroke starts to occur, and the exhaust valve will start opening. Watching the valve train you will notice the exhaust valve opening till it's fully open. As you are approaching top dead center you will notice the intake valve starting to open before the exhaust valve is fully closed. This is the area of overlap, usually about 10-15 degress before and after top dead center. Both the intake and exhaust valves are open and the piston is near the top.  During this overlap point is when you want to measure the piston to valve clearance.

The piston is so close to to the top that you can push down on the nose of the rocker and actually feel the valve making contact with the piston. Now; with your feeler gauge set to 0.100, insert the feeler gauge between the valve stem tip and the rocker arm. If you can insert the gauge at 0.80 for the intake and 0.100 for the exhaust then you have enough piston to valve clearance.

It may take you some trial and error  before you can identify the prescise overlap period at which you need to check the clearances, but this is a good way for someone that does not want to take off the heads.

I highly suggest you go through the steps above several times, it is VERY easy to miss the overlap, resulting in very unaccurate measurements. Work slow and this process can be accomplished by just about anyone.

Final note, if your measurements are say less than 0.10, for example there is .090 clearance on the intake you can retard the camshaft and play around with that but remember that will also affect performace charactersitics of the cam.