The 170 I6 is the rarest factory Bronco engine — 1966 only, and almost every survivor has had it pulled. Treat a numbers-matching 170 truck as a collector piece to preserve, and treat a 170 in a driver as the starting point for a 302 swap. It is not a platform you build power on.
The 1966 Bronco shipped standard with the 170 cubic inch (2.8L) inline-six, a derivative of the Falcon economy engine. Output was roughly 105 hp at 4,400 rpm and 158 lb-ft at 2,400 rpm, fed by a single-barrel Holley/Autolite carburetor and backed by a 3-speed manual. Ford replaced it with the larger 200 I6 for 1967, then offered the 289/302 V8 as the engine most surviving Broncos now wear.
Key specs:
Decades of swaps mean a 1966 title doesn't guarantee a 170 is still under the hood -- plenty of these trucks picked up a 200 I6 or a 302 V8 along the way. The 170 and the later 200 share the same basic block architecture and look nearly identical at a glance, so don't assume from displacement badges or shop lore. The reliable check is a direct bore measurement: 3.50" bores to a 170, 3.68" to a 200. A machine shop or engine builder can confirm this in minutes with a bore gauge during a compression test or teardown. If you can't verify and the numbers matter to you -- for a numbers-matching restoration, they should -- pull the casting number off the block below the cylinder head and cross-reference it against a Ford small-six casting number list before you order parts or commit to "170-specific" pricing.
The 170 was engineered for light Falcon road duty, not for moving a 3,000-plus pound 4x4 on tall gearing. The integral-log cylinder head is the core limitation: the intake ports are cast into the head and cannot be replaced with a better manifold, so the engine cannot breathe past its factory ceiling no matter what carburetor you bolt on. Paired with the Dana 20's modest 2.03:1 low range (covered in the Dana 20 transfer case guide), the 170 leaves you lugging on climbs and downshifting constantly. Add 31" or larger tires without regearing the axles and the truck becomes genuinely slow.
A 170 rebuild is viable but not well supported. Standard wear parts — rings, bearings, a timing set, gaskets, a water pump — are shared with the broader Ford small-six family and remain available through suppliers like Summit and Rock Auto. The hard parts are what bite: a cracked integral-log head, a worn cam, or a spun crank can leave you hunting salvage yards, because nobody reproduces 170-specific castings. Budget a machine-shop rebuild at $2,000–$3,500 depending on what the block needs, and accept that you are spending it for originality, not capability.
If you do keep it running, the maintenance basics still apply: a proper ignition and carburetor tune is the highest-value work you can do, and the same approach in the tune-up guide — fresh points or a breakerless conversion, correct timing, a clean carburetor — wakes up a tired six as much as a tired V8.
Before committing to a rebuild, run a compression test cold and find out what's actually wrong. Pull all six plugs, ground the coil wire so it can't fire, and crank each cylinder through 4-5 compression strokes with the throttle held wide open. A healthy 170 reads 130-150 psi across all six cylinders with less than 10% spread between the highest and lowest reading. Anything under 100 psi, or a spread past 15%, points to worn rings or valve trouble worth a real teardown rather than a gasket-and-tune refresh.
If one or two cylinders read low, add a small squirt of oil into the low cylinder through the plug hole and retest. A reading that climbs noticeably with the oil added points to worn rings, since the oil briefly reseals the ring gap; a reading that barely moves points to a valve or head-gasket problem instead. Given how scarce good 170 heads are, a compression test that turns up valve damage is often what actually decides whether you rebuild the original engine or move straight to the 302 swap below.
For a driver-grade 170 truck, the 302 Windsor is the established answer. The early Bronco was factory-engineered around the small-block Ford V8, so motor mounts, the Dana 20 adapter, and exhaust routing are all known quantities. Two routes:
Either swap also means upgrading the cooling system and the charging system to V8 spec — plan those alongside the engine, not after.
Written and maintained by an AZ wheeler and driveway wrencher. Always cross-reference your factory service manual — modifications affect vehicle safety and warranty. Work at your own risk.
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