Thread: Piston to valve clearance

Tweet

I am working on a 1985 Ford Mustang 5.0 H.O. and i am about to go from a stock cam to a x303 Duration int/exh 224 Lift int/exh .542 and 107 Lobe Seperation am i going to run in to Piston to valve problams. I'm going to change valve spring on stock heads and the right length pushrods. If so what is max lift on a 5.0 H.O. motor with stock pistons.

In case no one offers the magical answer, here is an explanation from Crane Cams on how to check piston/valve clearance as well as other aspects of a cam change...
How Does Installed Height Affect the Spring Tension?

Installed height is the determining factor of what the valve spring “Closed Tension” will be. The camshaft specification card, and the spring section of the catalog both show what the approximate tension a particular valve spring will exert if installed at a specific height. For example, spring part no. 99848 shows 105# @ 1.700”. This means that if this spring is installed at a height of 1.700” it should exert 105 pounds of tension with the valve closed.

How Do You Change Installed Height, and What Effect Does it Have?

The easiest way to lessen installed height is to insert a shim in the spring pocket below the valve spring. Another method is to use a different design valve spring retainer. Retainers with a deeper dish will have more installed height; with a shallower dish, less installed height. You can also use a valve lock that is designed to change the location where the retainer is positioned on the valve stem. Longer length valves can also be used.

The shorter the installed height, the higher the valve spring tension will be, and the less distance the spring can travel before reaching coil bind.

The taller the installed height, the less the valve spring tension and the further the spring can travel before coil bind occurs.

What is Valve Spring Coil Bind and How Does it Relate to Spring Travel and Valve Lift?

When the valve spring is compressed until its coils touch one another and can travel no further, it is said to be in coil bind. To measure this you must install the retainer in the valve spring, then compress the spring until it coil binds. Now measure from the bottom side of the retainer to the bottom of the spring. This measurement is the coil bind height. This can be done on the cylinder head with a spring compression tool (part number 99417-1), in a bench vise, or in a professional valve spring tester.

Using Figure 5, subtract the coil bind height “B” from the valve spring installed height “A”. The difference “C” is the maximum spring travel. The spring travel must always be at least .060” greater than the full lift of the valve. This safety margin of .060” (or more) is necessary to avoid the dangers of coil bind and over-stressing the spring.

If coil bind occurs, the resulting mechanical interference will severely damage the camshaft and valve train components.

How Do You Increase the Spring Travel?

The valve spring must have sufficient travel (plus .060” safety margin) to accommodate the amount of valve lift created by the camshaft and/or an increase in rocker arm ratio. To increase spring travel you can either raise the installed height (but this will lessen the spring tension), or change to a spring with additional travel. If there is not a standard diameter spring available with enough travel, then the cylinder heads will have to be machined and a larger spring installed.

Crane Cams offers some special valve springs in standard diameters which saves you from having to machine the cylinder heads. For example, a small block Chevrolet engine can use spring kit part no. 11309-1 to handle .550” to .600” valve lift. The 85-95 302 Ford hydraulic roller engines can use spring kit part no. 44308-1 to handle .550” lift.

Besides Coil Bind, What Other Types of Mechanical Interference Should You Look Out For?

When you increase the valve lift with a bigger cam or increased rocker arm ratio you must be sure that there is no interference between any of the moving parts. Some of the components that must be inspected for clearance are:

1. The distance from the bottom of the valve spring retainer and the top of the valve stem guide, or the top of the valve stem seal must be equal to the net valve lift of the valve plus at least .060” more for clearance.
2. When using rocker arms mounted on a stud, the length of the slot in the rocker arm body must be inspected to be sure it is long enough to avoid binding on the stud. The ends of the slot must be at least .060” away from the stud when the rocker is at full valve lift and when the valve is closed. – Crane Cams offers steel long slot and extra long slot rocker arms to relieve this interference problem. Aluminum roller rocker arms may be required to provide sufficient travel on larger lift camshafts.
3. The underside of the rocker arm body cannot touch the valve spring retainer. You will need at least .040” clearance to the retainer throughout the full movement of the rocker arm. If necessary, a different shape retainer or rocker arm design will be required. In some cases, installing a lash cap on the tip of the valve stem can provide the clearance required.
4. Valve to piston clearance must be checked to be sure there is sufficient clearance. The intake valve must have at least .100” clearance to the piston and at least .120” clearance on the exhaust valve.

What is a Quick Way to Check Valve to Piston Clearance on an Assembled Engine?

Low tension checking springs, Part Number 99881-2, must be used (instead of your normal spring) to mock up your valve train and to check the piston to valve clearance on the engine. Assemble the valve train and verify correct lifter preload or valve lash. By mounting a dial indicator on the cylinder head with the plunger’s tip on the valve spring retainer, you can quickly check the clearance.

Hand rotate the engine through a complete cycle (two rotations of the crankshaft), stopping at several points before and after Top Dead Center (T.D.C.) to check the valve clearance. The least amount of clearance will usually occur between 15 degrees before T.D.C. and 15 degrees after T.D.C. This also provides a graphic illustration that gross valve lift does not determine piston to valve clearance, as the piston is fairly far down in the cylinder when maximum valve lift is reached.

By pushing the rocker arm down with your finger, the valve will contact the piston. The amount of movement shown on the dial indicator is the valve clearance at that point of engine rotation. Rotate the crankshaft a few degrees and re-check the clearance. As the piston moves through this area, the dial indicator reading will lessen, then become larger as you rotate the engine past the critical point. The shortest reading you get is the actual valve to piston clearance.

What is the Critical Point of Crankshaft Rotation for Checking Valve to Piston Clearance?

The critical point for both valves is the “Overlap Period” as the exhaust cycle is ending and the intake cycle is beginning. You must start checking the clearance before and continue after T.D.C. on both the intake and exhaust to be sure you have the correct readings through the overlap period.

You can find all the tools required for checking valve to piston clearance (as well as degreeing a cam) in Crane Cams’ Tune-A-Cam Kit, Part Number 99030-1.

I've checked V/P clearance with a layer of putty on the piston.Bolt the head on, put pushrods in the one cylinder, and turn it over by hand. The valves will dent the putty, and you measure the thickness.

What R. Pope said That's the way my Grandpa taught me

That's a heck of a cam for a stock 302.

Originally posted by 76GMC1500
That's a heck of a cam for a stock 302.

Ya it is a pritty big cam for a stock 5.0 but since all is said and done gas is bad i want my 5.0 to be a hot rod but gas sucks right now and i know you cant have both gas and a hot rod or at least a 1985 mustang 5.0 cant have both but i am trying to find some thing thats in between.

Originally posted by Speed Freak
Ya it is a pritty big cam for a stock 5.0 but since all is said and done gas is bad i want my 5.0 to be a hot rod but gas sucks right now and i know you cant have both gas and a hot rod or at least a 1985 mustang 5.0 cant have both but i am trying to find some thing thats in between.

We pay to get them done, then we pay when they're done, $$$$ Money and time, time and money...

I mean you're motor is barely going to run with that cam and is going to absolutely guzzle gas. Look for something in the 212-224 degrees at .050" lift range. That will give you some more rpm for some more power and if you stick with the smaller cams in that range, you may even get better milage.