Frequently Asked Questions

• Valve height: measure from the machined surface the cam-cover gasket sits on to the valve tip, with the springs installed after the valve job — and give this spec to your machinist (find another if he won’t listen).
• Oil restrictor: in the head between cylinders 2 and 3 — a ~2″ tube with an hourglass crimp (~1/8″ ID) and an .080″ hole drilled in the crimp. Both sizes are critical to oil flow and pressure to the cam and lash adjusters; it’s fragile and easily damaged during cleaning.
• Valve angle (30 vs 45): refers to the valve face/seat. Only the second-design 1969 head uses 45°; all earlier heads are 30°. (A 3- or 5-angle valve job improves flow.)
• Cam followers: replacements have a rough surface that must be machined smooth (e.g., by hand on a fine sharpening stone, ~30–45 min each with cutting oil).
• Lifters: use new ones (a slightly different design without the top-hole pin — don’t mix old and new).
• Pistons/overbore: 230 pistons are available; 250 pistons are hard to find. Stay at .030″ if possible (up to .060″ has been done). Rings and bearings are industry-standard six-cylinder sizes.
• Driveshaft/crossmember: the same across 1967–69 (only the THM400 used a different crossmember).

For an automatic, a 200-4R overdrive fits better than a 700R4 (dual-pattern bellhousing, needs only a small crossmember move; the 700R4 is Chevy-pattern only). A steeper rear gear works well with either engine. The OHC-6 revs higher but stops making real power around 4500 RPM.

• Cam/bearings: the cam rides directly on the aluminum housing — there are no replaceable cam bearings, and the housing rarely wears even past 200,000 miles. Wiped cams come from oil starvation, so make sure the head’s oil restrictor is intact.
• Valves: do NOT tip the valves (you’ll get interference between the follower and spring retainer; lash adjusters compensate for stem height). Valve stem height is critical — have it measured, not eyeballed.
• Compression: keep CR just under 10:1 for the street; 11:1+ really wakes it up. A killer combo is a 1966–67 4-bbl head on a 1968–69 short block.
• Valve springs: if running singles, match the Sprint dual-spring specs; with a 1-bbl head and valves you’re near coil bind on a 4-bbl cam.

Valve stem lengths (machinists often grind these by eye — don’t):

66 1-bbl  4.9     67 1-bbl  4.9     67 4-bbl  4.99
68 1-bbl  4.8     68 4-bbl  4.9
69 1-bbl  4.8     69 4-bbl  4.9

Dual exhaust (Sprint splitter look): the factory used a Y-pipe off the dual-outlet manifold. A good DIY route: buy over-the-counter V8 dual exhaust — the crossover muffler, front resonators, and V8 tailpipes (the dual splitters) — but not the V8 front pipes; install it with proper hangers, then have a muffler shop custom-bend the two front pipes (max 1-7/8″ dia) up to the resonators. Or copy the factory axle-tube pattern in 2–2-1/4″ pipe.

About the engine: the OHC-6’s belt is reliable — it’s the oiling system that wipes out camshafts (top-end oil starvation). It came as a 1-bbl base (9.5:1) and a 4-bbl “Sprint” (10.5:1, dual valve springs, the first production Quadrajet, high-lift cam, dual exhaust manifolds, 215–230 hp). It was 230 cid in 1966–67 and 250 cid in 1968–69; 1968 blocks are the best to build.

The automatic is a Super Turbine 300 (closer to a TH350 than a Powerglide), in air-cooled (OHC-6) and water-cooled (V8) versions. The manual uses a 3.10:1-first transmission and a longer z-bar. Speed parts are available from Clifford (cams, headers, Weber kits); usable power is ~3000–4500 RPM even though it revs to 6500–7000.