What's the stock valve spring pocket size and can stock heads handle a cam with .520" advertised lift given the correct springs?

sorry i don't recall the pocket size, but i'm running 98s36 Crane springs and they fit in the pockets without any machine work. There a single spring with a dampner. I'm running a Crane Powermax 286H cam with .534/553 lift on cast iron 781 heads. I love it. No spring bind, lots of clearence for the seals, slightly choppy idle and good power.

Typical BBC valve springs are 1.487". They are the low performance single spring with dampener. No low performance production spring either in BBC or SBC can handle lifts higher than .500"

Using better springs to allow more lift is just part of the story. Factory rockers won't work. You'll need long slot rockers or roller rockers. Total valve lift is limited to the valve guide length. If you're adding more lift then the valve guide height must be checked to make sure the spring retainer doesn't make contact.

I use base model Merlin heads. They come with 1.550" single springs that are good for .600" lift. The spring pockets can be enlarged enough for 1.625" springs.

Some of the factory smog low perf springs coil bind at not much more
than .400" lift. You'll find them on '70's oval port heads.So any replacement cam will require a spring upgrade. The factory LS-6 spring was designed for .520" lift.
and can go to .550" lift. The l-88/Ls-7 stuff is good for .600+
lift. Retainer to guide clearance should be checked. factory
rockers are good for .530". Longer slot rockers are recomended.
There are many aftermarket replacement springs that drop right on with no machining, except maybe retainer to guide clearance.

Thanks, I'm about to order the new valvetrain and get whatever machinework is needed done. I'm going with an XE268 cam that has .515/.520" advertised duration. I know better than to think the stock springs could handle that cam so I'm ordering the springs recommeded by Comp for that cam. It's a 1.487" dual so I was just trying to find out if I'm going to need to enlarge the spring pockets.

That should all fit, no problem. You won't have a clearance problem between retainers & guides, or any other hidden gotchas.

Let us know how it runs... I'm considering upgrading the cam in my 74 454 convertible.

you do mean .515/.520 Lift at .50 right?? that's fairly tame for a BBC. It should have good vacuum, idle and torque.

Sorry, I didn't catch that typo, it should be .515"/.520" advertised lift.

The cam's advertised specs are 268/280 (.224/.230 @ .050) and .515/.520 with 1.7 rockers with a 110* LSA. It says slightly rough idle so I figure vacuum will suffer a little but I shouldn't need a resivoir. I'm going to be keeping the power steering so I figure pump mounting will be the hardest problem. I figure that cam will work well with the set up of the engine, (396 .030" over, 9:1, '71 402 large runner oval port heads, 113cc chambers, Performer RPM AG, and a 750 carb). I would have gone with the XE274 but I don't think that it would like the compression ratio and bleed off too much cylinder pressure at low speeds. I have a set of the hooker SC headers and a manual trans so I'm not worried about the need for a stall converter, just trying to get as much power I can out of the compression ratio I'm stuck with.

My cam has 226/236 duration and the power brakes still operate fine. The P/S look like a major PITA to mount the pump, so i installed an S10 manual gearbox. Even while driving with the extra weight in the front, it's still fairly easy to turn, plus i love the road feel . The only downside is when at a complete stand still it takes a bit of grunting to turn. Guess it all depends what you can live with

That sounds like a good cam, although personally I would have gone with the 274. A 268 in a big block is pretty mild.

You've got what looks to me like a real good street combo, if you're using the 396 closed-chamber pistons and the 292 heads. That will be very tolerant of modern gas and should put you back hard into the seat if youcan get it to hook up

That cam should be OK with power brakes. I've had a 280 Magnum in a 396 before, and the PBs were happy.

I actually want the XE274 but didn't think I had enough compression to run that big of a cam. I haven't ruled it out yet though, I might just go ahead with that choice if you guys think it will run. Worst case I'll have to have the heads milled a little to decrease chamber cc's.

my comp ratio is only about 9.25-1 with my 286 cam. Still runs great. Then again you are running smaller cubes.