Late AMC 304s have a certain personality. They came from the tail end of the V8 era, back when emissions gear, lean tuning, and tired carburetors could make a perfectly decent engine seem half asleep. Pull one apart today and you might find clean factory parts, old machine work, or a surprise from somebody’s weekend in 1996. It’s part of the deal with these engines.

The goal for most 1979–1981 AMC 304 rebuilds is pretty simple: good oil pressure, clean compression, and a V8 that starts cleanly. This is where Block Kit OH90042B, also known as the AM 304 79-81 M block kit, comes in. It gives the short-block side of the project a foundation before the rest of the engine gets any attention.

What Kind of AMC 304 Are You Working With?

A 1979–1981 AMC 304 belongs to the late stretch of AMC V8 production. The basic design still traces back to the same compact, cast-iron AMC V8 family, with a 3.75-inch bore, 3.44-inch stroke, hydraulic lifters, and the familiar low-rpm pull people still like in CJs, Wagoneers, J-trucks, and AMC cars.

These later engines were built during the emissions-heavy years. Factory horsepower numbers had already moved downward from the early V8 days, and the parts around the engine often did as much damage to the driving experience as the engine itself. A sticky choke, tired distributor, weak fuel pump, or vacuum leak can make a 304 seem worse than it is.

The first look should be patient. Check the casting and the parts bolted to it. Check for old machine work. Some of these engines have already been bored, turned, patched, swapped, or “freshened up” with whatever parts happened to be on a shelf.

What Does the OH90042B Block Kit Handle?

The OH90042B block kit covers the short-block pieces that take the hardest wear over time. It includes pistons, rings, rod bearings, main bearings, oil pump parts, and overhaul gaskets. It’s a useful package for a 1979–1981 AMC 304 that needs its base health brought back.

A good rebuild starts down low. Rings seal the cylinders. Bearings control crankshaft movement and oil clearance. Pistons need to match the bore after the machine shop has measured the block. The oil pump parts need real attention because AMC V8 oil pressure depends heavily on the front cover and pump area.

This is also where engine kits earn their keep. Piecing together an older AMC rebuild one part number at a time can work, but it leaves more room for mismatched parts and last-minute issues. A block kit gives the project a cleaner starting point.

How Should Early-’80s AMC Changes Affect Your Parts Choices?

The 1979–1981 304 did share its bones with earlier AMC V8s, but the later years deserve their own parts check. Emissions-era equipment and decades of repairs can create differences from one engine bay to another. A late CJ 304 and an AMC passenger-car 304 may share the same basic heart, but the pieces around the engine can tell different stories.

Part selection should come from inspection rather than habit. The year range points you toward the right family of parts. The engine’s actual condition gives the final answer.

The block needs real measurements. The crank needs the same treatment. The timing cover deserves more than a glance, especially around the oil pump cavity. The cylinder heads, intake surfaces, and valley pan area need the same kind of patience. A 304 rebuild can go very well when the builder treats the teardown as a fact-finding job instead of a race to the parts cart.

Why Measure Before Ordering the Final Kit Setup?

The OH90042B kit offers piston and bearing options for different machining needs. This is a big help on older engines because “standard size” becomes a guess unless the block and crank have been checked.

A cylinder with taper may need boring. A crank journal with wear may need to be turned. A block that already had work done years ago may require an oversize piston from the start. The machine shop decides those things with tools, not optimism.

This part of the rebuild can feel slow, but it saves money. Nobody wants a fresh box of standard pistons on the bench while the block needs .030 over. Nobody wants standard bearings after the crank measures .010 under. A good AMC 304 rebuild rewards the builder with proper measurements.

Where Do Late AMC 304 Rebuilds Usually Get Tricky?

The oil pump area is the spot most AMC V8 builders watch closely. The pump lives in the timing cover, and wear in that cover can hurt oil pressure even with fresh gears. A Melling K-85 oil pump repair kit can help restore the pump parts, but the cover itself still needs a careful look.

The distributor gear and cam gear relationship also deserves respect. Poor alignment can chew up gears and turn a new build into a bad afternoon. The timing cover, dowels, cam gear, distributor gear, and oil pump should all work together cleanly.

The intake valley pan is another AMC detail that can trip people up. The valley pan gasket helps with intake sealing and oil control under the intake. Install it poorly and the engine may reward you with leaks, vacuum issues, or oil where you had planned on air and fuel. Old engines have a way of grading your work quickly.

What Upgrades Make Sense During a 304 Rebuild?

A dependable 304 usually benefits from mild, practical upgrades. A fresh timing set can sharpen throttle response. A stock-style or mild cam can keep vacuum and idle quality in a good place. Clean ignition parts can make the engine start and run like somebody actually cared during assembly.

The fuel and cooling systems deserve attention during the same project. A rebuilt carburetor alongside clean fuel lines and a healthy radiator can do more for a street-driven 304 than a flashy part that does nothing for daily use. Old hoses and crusty passages have ruined plenty of fresh engines. Cheap parts can become expensive quickly.

Restraint works well here. A late AMC 304 can be a nice cruiser engine, trail engine, or weekend V8 without turning the whole vehicle into a fussy project. Compression, oil pressure, timing, and fuel delivery need to agree. Suddenly, the engine starts to seem alive again.

Northern Auto Parts Has Your Rebuild Kits

A 1979–1981 AMC 304 rebuild goes best when the prep work drives the parts choices. Confirm the engine, measure the block, inspect the crank, check the timing cover, and pay attention to the oil pump area before the final order.

Block Kit OH90042B gives builders the short-block pieces needed for a solid late AMC 304 refresh. The kit covers the pistons, rings, bearings, oil pump parts, and gaskets that help bring compression and oil control back where they belong.

Northern Auto Parts has supplied rebuild parts and engine kits for classic engines for more than 40 years. The catalog supports projects that involve an old AMC V8, a few mystery bolts, and a vehicle still deserving of more miles.