At what amount of valve lift does a person need to be concerned about piston-to-valve clearance? (This would be on a 9.5:1 302 with flat-top pistons)
The cam duration has more of an effect on piston-to-valve clearance. Think about where the piston is when the valves are at full lift. No where near TDC. My current cam has .545/.555 lift but only 266* duration, it's not even close to the piston. Previous cam only had .520 lift but 300* duration. I had to widen the piston valve reliefs.
Thanks for the reply. So what do you think about a .493 intake/.500 exhaust lift with a 218°/224° (at 050") duration? I'm looking at a Comp Extreme Energy XE262H.
All other dimensions being stock...The P/V clearance will be fine with that cam. As stated duration has alot to do with it...Intake valve chases the piston down the bore on intake stroke.This is where the problem arises with clearance...As I am sure you know...Milling heads/decking the block. comp height of pistons all play into this, soo...Yeah...Be certain to check it all out.
Well it's kind'a hard to check with the heads on and the engine in the car. The pistons have valve reliefs, but I don't know how deep they are.
You're gonna have the intake off anyhow, just pull one head and clay it. If you don't wanna take a head off to check you could buy one of those cheap camera scopes that Harbor Freight always has on sale for $69 and peek in the spark plug hole.
If you already have a bore scope?.. bmc's method is sound and works well. I prefer to use lightweight checking springs but another very easy method to save you from all the typical headeaches associated with this check is to use a lever style valve spring compressor. http://www.jegs.com/i/Manley/660/41830/10002/-1.. Rotate the engine towards and just short of the end of the exhaust stroke where the exhaust valve is just starting to get close to the piston crown. Then use the levered compressor to try and "bump the piston" as you chase it past TDC into the intake stroke. In case the valve will actually contact the piston.. which for whatever my opinion is worth is very unlikely to occur with such a small cam like this one.. you MUST be careful not to pry too hard on the valve spring compressor. Slow and progressive increases in valve lift will allow you to easily feel the piston contact before you really need to worry about tipping the valve head over and most flat tappet valve springs aren't going to need 20" biceps to gett'er done anyways. Turn the crank by hand to sneak up on TDC and chase the piston a little ways past TDC just to make sure you have around .100" thou clearance. Either back up the crank or go though the engines full cycle to repeat the same procedure for the intake valve and you have your answer after removing only a single spark plug, valve cover, and pair of rockers for that cylinder. These simple types of compressors are very cheap/easily made and also available at local chain parts stores. Sears has them too. PS. you can also pull both rockers and check back and forth between them as you rotate the crank to establish that there is no issue with PTV clearance during the overlap period. Gets it all done with less crank rotation needed.
"Flat Top" is a somewhat ambiguous term when it comes to SBF pistons. Some aftermarket flat tops have 4 valve reliefs and some have two. The depth of the reliefs varies by manufacturer as well. I typically find that Ford OEM pistons have shallower reliefs than do Speed Pro or KB Silvolite hypers. Some 5.0 Ford OEM pistons had no relief at all ('86?). Throw deck clearance, milled heads and head gaskets into the mix and you can see why physically checking it is the only way to be sure. I only bring this up because one person can experience insufficient clearance while another can have plenty running the same cam and rocker ratio. That's where the internet forums can get you into trouble.
Yep '86 Stang HO, plus '86 & '87 HO used in the LSC Lincolns... Run real nice with a set of Twisted Wedge heads(at least if the gasket isn't blown)...
The pistons are aftermarket. I don't remember the brand. I also don't remember if there are 2 or 4 valve reliefs. I know a guy who has a bore scope, so I think I will try checking it that way. If that doesn't work out for me, I may try groberts suggestion. If the engine wasn't already in the car, I wouldn't have a problem pulling off a head, but at this point, I really want to avoid it.
The other issue to watch out for when running larger than stock valve sizes(2.02" and above) is radial clearance in the relief pockets. Be sure to check that clearance in those areas is at least .060" due to piston rock and expansion characteristics. More is better of course but I would not allow less than that amount of wiggle room as things tend to move around more the looser the motor gets over time and the springs start to lose some of their control during over-rev.
