Frequently Asked Questions

Block: Bore .060 over (413 cubes), hone to a #625 Sunnen finish with .003 piston clearance, minimum deck. Unshroud the valves at the bore, confirm main saddle alignment within .001, remove casting flash, re-tap/deburr, tap oil galleries for threaded plugs, drill an .042 hole in the right-rear plug for distributor-gear cooling, chamfer the oil-filter and oil-pump holes, and elongate the front oil-return hole for fast drainage. Clean with hot water and dish soap (rifle rod for galleries, 12-ga brush for lifter galleries, 9mm for the rest), then WD-40, re-clean, blow dry, and oil the cylinders immediately.

Crank: Use an early 350 crank (lighter, thinner throws), magnaflux, heat-treat (case harden), shot-peen, straighten. Grind .010 mains, offset-grind the rods .015 for more stroke (.020 undersize to get the offset), cross-drill, chamfer the oil holes, micro-polish; run .002 main clearance. A late 400 crank also works (avoid very late ones — different flange).

Pistons: Lighten the forged 400 pistons (machine the back of the dome or the pin-boss area; thin-wall tool-steel pins help). Moly rings only with the #625 finish; end gap .014 top, .018 second.

Rods: Upgrade to at least ’73–74 SD rods or Carrillo (you’ll rev past 7000). Run .002–.0025 clearance, racing bearings, grooved uppers; torque rods by stretch to +.005–.008.

Oil pump: Milodon 455 H.O. pump, tack-weld the pickup; Loctite the cover screws.

Windage tray: Stock ’65–73 4/5 tray (full-length if possible), modified with cut openings + perforated metal welded in to fling oil into the pan (worth 10–15 hp).

Cam: Call Bob Cook at Competition Cams (800-999-0853) and spec it to your combo — he won’t let you over-cam it.

Heads: Any big-valve early Pontiac head with 2.11″ intake / 1.77″ exhaust valves and screwed-in studs. Good D-port castings: 16, 12, 13, 62, 48 (72cc chambers → ~9.7–10:1). On a street engine don’t fully port — just open the bowl under the valve and blend; gasket-match the ports. New bronze guides, hardened exhaust seats, multi-angle valve job, spring dampers, a late baffled valve cover. Don’t use factory (thick) head gaskets; use head studs.

Intake: A Torker or (preferably) a Doug Nash, with a 750 Holley and the thickest carb spacer your hood allows. Third choice: a gasket-matched early stock manifold with the #7 runner relieved.

Ignition: MSD #8563 distributor, 6AL box #6420, Blaster coil #8202, and a Soft Touch Rev Control #8738 (or 2-Step #8739). Big plug wires, stock-heat-range AC plugs gapped .035. Keep total timing 34–38°, all in around 4000 RPM (not 1800 — that just rattles and pings).

Headers: 4-tube if they install cleanly, but 3-tube “Tri-Y” headers are easier and make great low-end torque; use a crossover and a slightly smaller tube at the muffler.

Flywheel/balancing: For a 4-speed, a neutral aluminum flywheel with the engine internally balanced gives a softer launch and quicker revs. The stock damper is fine — torque it to 160 ft-lbs.

Nitrous: Build a strong engine and add mild nitrous rather than over-spraying a mediocre one.

• “Safe at 160”: after the Corvair “unsafe at any speed” era, the 400 got chassis reinforcements not on the Camaro or 350 — triple-welded rear spring shackles and lengthened front control arms (rear attach point ~13″ behind the axle) for much more strength.
• Ram Air engine: built to hold together to ~8000 rpm with special anti-pump-up lifters, but redlined at 5100 only because oil pooled under the valve covers faster than it could drain (he blew a valve cover proving it).
• Fuel at sustained high rpm: the float valve opening is too small; substituting the 1/8″ seat and float from a Buick Roadmaster largely fixed it. He also added an electric pump at the tank as a booster. Fuel use was heavy — ~5 gallons over four 1/4-mile runs; ~8 mpg with the 4.11 axle, ~12 with the 3.08.
• Linkage: the Corvette logarithmic throttle linkage (only up to 1/2 throttle until the last 1/8″) made it far more drivable, especially in the rain.
• Top speed: with the 3.08 he saw 165 (redline 150) in a short safe blast, with plenty left — though turbulence made the ride rough; better aero was wanted.
• Cam: the Ram Air cam had more overlap and ~.100″ more lift, requiring doubled valve springs — which is why bolting just the Ram Air intake onto a standard 400 doesn’t transform it (though the RA intake alone adds ~15 hp from better volumetric efficiency).

In the accessory catalog, a “Sport Options” heading actually grouped all the Sprint, 350, 350 H.O., and 400 models — so effectively every Firebird except the base one-barrel OHC-6 was a “Sport Option.” In common use this got shortened to just the “400” option. It’s referenced in the 1968 Pontiac Sales Manual.