1967-1972 C10 Action Line Spark Plug Gap & Ignition Timing

At-a-glance tune-up specs

For 1967-1972 Action Line C10 trucks with factory points-style ignition (HEI didn't appear until 1975+):

These are factory specifications for stock points ignition. HEI conversions and aftermarket distributors require different settings — see below.

Modern unleaded fuel adjustments

The factory initial timing values were calibrated for leaded fuel with octane rating 95-100. Modern pump premium is 91-93 octane and behaves differently:

• Reduce initial timing 2-4 degrees from the factory spec on modern pump premium.
• A stock-cam SBC 350 truck is fine at 4-6° BTDC initial on pump premium (vs the factory 8°).
• Watch for detonation under load — if you hear pinging, retard timing 2 more degrees.

If you've converted to HEI or an aftermarket performance distributor, the timing curve is different and the initial timing recommendation changes. Most HEI installations recommend 6-10° initial with a more aggressive vacuum advance curve.

I6 vs V8 timing differences

The Chevrolet inline-six engines (250 and 292) use lower initial timing than the SBC because the long-stroke I6 design has a slower flame propagation and benefits from less aggressive ignition advance. The factory spec of 4° BTDC for the I6 is reasonable; modern unleaded doesn't require much retard from this number — 2° BTDC is acceptable, but 0° (TDC) is too retarded for good response.

The 292 I6 (HD truck engine) is essentially a tall-deck variant of the 250. Same timing spec — 4° BTDC initial — but the 292 has more displacement and slightly different idle target (550 vs 600 rpm).

Setting timing — procedure

Standard for any Chevrolet engine:

1. Warm the engine to operating temperature. Cold-engine timing is meaningless.
2. Disconnect the vacuum advance hose at the distributor and plug it temporarily.
3. Connect a timing light to the #1 spark plug wire. Cylinder #1 on a SBC or BBC is the front-most cylinder on the driver's side. On the I6, cylinder #1 is the front cylinder (closest to the front of the truck).
4. Aim the timing light at the timing pointer on the harmonic balancer.
5. Adjust the distributor by rotating it clockwise (retard) or counter-clockwise (advance) until the timing mark lines up with the spec.
6. Lock the distributor clamping bolt.
7. Reconnect the vacuum advance hose.
8. Verify idle RPM with a tachometer. Adjust the carburetor idle screw if needed.

Spark plug heat range notes

C10 trucks were used hard — towing, hauling, urban delivery — and the spark plugs were chosen for the duty cycle:

• R44T / R44TS — middle heat range. Standard on most V8 applications. Best for trucks doing mixed driving (combination of stop-and-go and highway).
• R45T / R45TS — slightly hotter. Used on lighter-duty applications. Sometimes spec'd for the 307 or low-output 350 in a truck driven mostly stop-and-go.
• R43T — slightly cooler. Used on high-output applications or trucks doing lots of towing/hauling.
• R46T — for the 250 I6, slightly hotter than the V8 standard.

For a stock-spec truck driven normally, the factory plug heat range is right. If you're chasing fouling problems (plug fouls quickly in city driving = too cold a plug; plug overheats in highway driving = too hot a plug), step the heat range one notch in the appropriate direction.

When to deviate

Use the engine builder's or distributor manufacturer's spec instead if you have:

• HEI conversion — different timing curve; verify against the HEI manufacturer's instructions.
• MSD or other aftermarket ignition — different specs entirely.
• Aftermarket cam (RV cam, performance street cam) — total advance changes.
• Forced induction (turbo, supercharger) — substantially different ignition map.
• Modern fuel injection — entirely different ignition strategy.

Common mistakes

1. Setting timing without disconnecting vacuum advance. The vacuum advance line adds timing under load; the factory spec is for base initial timing without vacuum advance contribution.
2. Setting timing on a cold engine. Timing changes with temperature; always at operating temp.
3. Running factory leaded-fuel timing on modern unleaded. Causes detonation. Reduce 2-4 degrees.
4. Wrong spark plug heat range. Hotter than spec causes pre-ignition; cooler fouls in stop-and-go.
5. Forgetting to verify the timing pointer is correct. A previous owner may have rotated the harmonic balancer or the timing pointer; verify TDC mark accuracy with a piston-stop tool before trusting the timing pointer.

A reminder on safety

These are research-derived starting values, not factory shop manual data for your specific Action Line C10. Always verify against the actual factory service manual for your specific year and engine — tune-up specs vary across the run. Modern unleaded fuel requires adjusting timing values; running factory leaded-fuel timing on pump premium causes engine damage from detonation.