Offshoreonly.com - Bbc 454 gen 4 overhaul

Your 454 Mag came with forged TRW pistons.
The .030 over SpeedPros that are in it now are the correct "cross reference" piston (similar dome, etc to the originals).

I do see that somebody has bore-notched your block (this is good) to unshroud the valves. That's good for 5 free hp (more on a high rom motor).

Here's what you need to focus on for a good outcome..
Check your rod bolts. If they are already replaced with aftermarket (SPS or ARP), then just check the big ends for roundness. If ok, then just match the bearing size properly. If they are still running stock bolts, then press in new ARP's and have the big ends resized (due to the press-fit, you have to rework the big end anytime rod bolts are replaced.
Next - before you disassemble, check to see where the piston sits below/above the block deck at TDC (piston quench pad to block surface).
It may be .020" "down" in the hole, or .005 "up" out of the hole. You need to know this to select your head gasket.
You WANT 0.036-0.045" quench clearance (between the quench pad on the piston and the flat portion of the head).
For example, if you plan to use the FelPro .039" thick (compressed) gasket, then your piston can not be more than 0.006" IN THE HOLE, nor more than 0.003" OUT OF THE HOLE. If you are OUT more than that, you'll need to use a thicker gasket. If you are IN more than that, you'll either need to use a thinner gasket OR deck the block to get the distance where you want it. All of this is important. Especially if you plan to add 5 psi of boost. The tight quench clearance improves "burn" in the combustion chamber AND creates turbulence that prevents charge detonation. Very important.
If you set up your motor as described above, with UNCUT 118cc heads, at 030 over, you will be at a compression ratio of 9.1:1 (stock block usually has the pistons down 035 or so in the hole, netting a 8.75:1 CR).
At 9.1 CR, 5psi of boost will be the limit with a Crane 731 cam or similar. If you want more boost, you should plan on running flat-top pistons.
If you do NOT plan on supercharging, you can cut your heads to 115cc - .003" per cc on GM 088 heads - to 9.4:1 CR.
Back to the cam - a Crane 731 will require a thrust button against your timing cover, and hydraulic roller lifters. Also, make sure the cam gear and distributor gear are compatible. Some roller cams have a drive gear that works with the regular distributor gear. Billet cams require a bronze distributor gear. For lifters you have 2 choices ... run "tie bar" retrofit hydraulic roller lifters or machine your block for Gen 6 dogbone lifter locator plates. Using dogbones allows you to use the lighter and less expensive GM Gen 6 lifters on your MkIV block.
For rockers, don't buy Chinese rockers. Repeat that sentence 40 times. You are better served with 1960's style stamped steel L88 rockers than Chinese aluminum needle bearing roller rockers. But, you can't go wrong with Crane gold or any if the Comp rockers that fit your budget. Don't buy used ones, unless you plan to send them in for refurb.
Wait until all head/block machining is done before you measure for pushrod length.
Match the valve springs to the cam, shim the springs to the correct installed height.. Check to see if you have any interference issues between the valve locks and the top of the stem seals. Sometimes, you may need to have the tops of the guides machined deeper to lower the seals for clearance (use Viton seals). Keep in mind you are going from .480 ish lift to around .600
Valve prep is typical. Undercut the exhausts.
A little bowl blending on the heads, a standard 3 angle job on the seats. Don't mess with the floors or shortsighted radius on the ports. Just don't touch em.
Use a piston stop to properly locate the timing pointer and use timing tape on the balancer for future easy TDC reference. Do not trust the timing set to degree your cam. Use a degree wheel and set it up +4 advanced just like Crane recommends.
The factory GM aluminum dual plane intake is surprisingly good. But if you're planning on a new intake, the Edelbrock RPM AirGap is the one to beat. If you plan on running in salt water, you MUST use an intake with a bronze lined water crossover or at least one that is hard anodized.
Be sure to mock up your intake with USED intake intakes to see if you need to do anything weird to mate up w the heads properly - run a camera in the ports to see how you transition from the runner into the heads. This is because you may have a decked block and decked heads and may also need to shave some off the intake flanges to get the correct gasket crush..
Port matching from intake to heads is a bit overrated, but you should fix any major mismatches. Try not to cut anything off the floors of your intake ports on the heads.
Stock 454 Mag exhaust manifolds start spring the fun at 450hp. If you stay below 450hp, you can use em. If you want more, you'll need something better - GIL, CMI, Lightning, etc.
An 800 or 850 Holley will be great. Vacuum secondary is fine, mech is not necessary. Both can be tuned to perform properly.
Timing curve for ignition will need tweaked some. Basically, wide open throttle you need 36 degrees total timing. Idle, you need around 12 to 14. Bring it in however quick you can without pinging on a hot day.
You'll want to make sure your engine can get plenty of air to the carb. Boats are notorious for HOT engine compartments, which is your enemy for getting plenty if cool air to the carb. Sometimes extra vents/plumbing may be necessary. Best way to know is to do all your tuning/jetting with the hatch off or at least closed down on spacers to create a 4" air slot to feed the motor. Then when you have a speed/rpm baseline, shut the hatches tight and try again. If you lose ANY rpm, you need to figure out how to get more cool air to it.
Factory fuel line sizing may not be adequate for 500hp. Always run a mechanical fuel pressure gage from a pressure tap right up at the carb inlet. While tuning, keep an eye on it, and while wide open throttle, your pressure should remain steady and never drop any. If it weakens with additional rpm, then you need to find and address your restriction.

Ill probably come back and add more later.