What is the maximum safe valve lift

[CobraEd]

I have a 70 302 with factory pistons with stock valve reliefs cut in them. I have Edelbrock Performer RPM heads and a Crane flat tappet cam that has .488 lift and 108 LSA.

I think I may have a lobe problem (another whole enigmatic story) and so I am thinking I need a new cam. I am thinking of a roller cam with 107 LSA.

What do you all believe is the maximum safe lift I can go to on the new cam with my piston/head combination ?

Thanks
Ed


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[520SC]

You definitely want to get on the phone with the camshaft manufacturer of your choice to discuss your options.

[olddog]

Asking for opinions on the net for a measurement of clearance that needs to be measured is not a good way to go here. You can tally votes, take an average, or what ever statistics you want to use, but you should really just measure it.

Set a piston at top dead center. Take the intake and exhaust springs off on that cylinder. Carefully sit both valves down on top of the piston. Measure the valve stem height. Pull it all the way up and measure again. Subtract the difference. Repeat on the other valve. Then work with a cam manufacturer or builder.

Call Brent and let him verify what I told you or correct what I said, if I'm wrong (never trust the net completely). Then let him pick the lobes and special order the cam for you. I expect his guidance will be worth every penny he tacks onto what you could do for yourself.

Has your block been decked, the main bearings aligned bored, the heads milled, the pistons notched, who's rods, head gasket thickness, valves ground, seats replaced? All these things can impact the piston to valve clearance. Builders always check this even if they built a hundred engines like it cause there are so many variables.

[FWB]

safe?........about a hundred thou. before it kisses the piston

[CHANMADD]

Max lift is to the point that the coils of the spring don't touch when the Valve is fully open.On the Edelbrock heads it's normally around 0.575 to 0.600.

[Gaz64]

I would think about a full engine rebuild if a camshaft change was part of the recipe, especially if a roller cam is a choice.

There are too many variables of minimum clearance to consider, and they all have to checked as part of the dummy build process.

Coil bind, retainer to seal, rocker to rocker stud, pushrod length, rocker geometry, connecting rods to camshaft,
valve to piston clearance at 10 BTDC and 10 ATDC as well as TDC, valve cover clearance.

All engines are a bit different, and so the clearance you have on yours now needs to be accurately measured first.

Gary

[Dwight]

I rebuilt a '89 Mustang 5.0 years ago. I used Holley heads and 1.7 roller rockers with a .509 lift cam. Total lift would be about .540. I clay checked the clearance and had more than 120 thousands on both valves.

BE SURE to check your push rod length. Later I bought another brand of aluminum heads and did not check the push rod length because I was swapping aluminum heads for aluminum heads. Should be the same. NOPE. Ate up the valve guides within a thousand miles.

Dwight