Need some PTV advice/luck

[A-drain]

Just needing some help on piston to valve clearance. I'm helping my buddy build a 351. We've watched videos and read other forums on how to do this but the truth is we aren't pros and still pretty lost on the subject. I was really hoping to get lucky enough that someone here is running the EXACT SAME combo and lemme know if it clears or if we are doomed so here's the combo...

A 89 351 block
Speed pro hypereutectic pistons #H336cp40
Comp cam XE274
Afr1338 heads.
1.6 rocker ratio.

Any help is appreciated thanks!

[Tiggie]

No help from me on running that combo, but maybe someone else might chime in.

My first question is have you checked pushrod length? We should be able to get reasonably close on PTV with pushrods that are close but I'd highly suggest checking pushrod length if running aftermarket heads, a decked block, and/or aftermarket rockers. An ounce of prevention is worth a pound of cure!

Is the engine still able to be checked apart, or have you buttoned it all up? Apart, get you some clay and put a 1/4 inch or so on top of your pistons. Install the head, set your valvetrain, and rotate it over a time or two then take apart and see how thick the clay is where (and if) the valve contacted.

If it's already together and you don't want to pull the head, go get a pair of lightweight springs you can compress with your fingers that are about the size of valve springs. Swap them in for a pair of valvesprings, get zero lash on rockers. Rotate until you see the exhaust open fully, begin to close, and the intake begin to open before the exhaust is closed. You can push the rockers at this point with your fingers and feel/look for smallest amount each valve will move as it contacts the piston (do them one at a time). Use your feeler gauge to check the gap between the rocker tip and the depressed valve stem tip.

[gadget73]

Do you have valve reliefs in the pistons? That can make all the difference.

When in doubt, stuff a wad of clay in there, bolt the head on, and rotate the engine with a breaker bar. Measure the dents in the clay and you'll know. If it stops dead, then you have less than 0

Pushrod length doesn't actually affect piston to valve clearance, other than if they are way too short the valves don't open all the way. Not really the way to fix it. There is basically a right length, and everything else is wrong. For pedestal rockers, you want the length that gets you .020-.060 lifter preload, and ideally you want to check the contact pattern on the valve stem to make sure you have a narrow centered contact patch. Stud rockers actually work the same, but checking the contact patch is a little more critical since you can adjust almost any pushrod to have the right preload. That doesn't mean the geometry is proper though. Off too far and the valve will side load and eat the guide up over time.

But yeah, any time you change things, that needs to be checked and confirmed. Also a little tidbit I learned the hard way, what you measure on a pushrod isn't what Comp will sell you. If you need to change the pushrod length, buy their pushrod length checker tool and order what the tool says you need.

P2V clearance is really more about duration than lift. If the exhaust valve stays open too long the piston can smash it. Its not going to smash at max lift though.