How to Replace a Mechanical Fuel Pump on a Small-Block Chevy (and Keep the Pushrod From Falling)

The first time I swapped a fuel pump on a small-block Chevy, I spent forty minutes lying on a cold floor trying to get the pump arm to slide under a pushrod I couldn’t see. Every time I pulled the pump back out to look, the rod had dropped again and was sitting right where the arm needed to go. A buddy wandered over, pulled a short bolt out of the front of the block, threaded in a longer one with his fingers, and the new pump went on in about thirty seconds. He didn’t say a word. He didn’t have to.

I’m Jay, and this is the guide I wish I’d had that day. It covers how to replace a mechanical fuel pump on a small-block Chevy, the 265 through 400 family that went into everything from Bel Airs and Novas to C10 pickups, plus the big-block differences and the tests that tell you whether the pump is really the problem before you buy one.

The Short Version

If you only read one section, read this one. On a small-block Chevy, the pump is driven by a pushrod that rides on an eccentric at the front of the camshaft. When you pull the old pump, that pushrod falls down and blocks the new pump’s arm. The fix is a hole Chevrolet left in the front of the block.

1. Prove the pump is bad. At the carburetor inlet you want roughly 4 to 7 psi depending on the carb, and at least a pint of fuel in 30 seconds or less at idle. Low numbers can also be a clogged filter, a kinked line, or a cracked rubber hose sucking air, so check those too.

2. Prep. Engine cool, battery negative off, fire extinguisher within reach. Disconnect the lines at the pump and plug the inlet hose from the tank so it doesn’t siphon onto the floor.

3. Hold the pushrod. On the passenger side of the front of the block there’s a short 3/8-16 plug bolt, usually with a copper washer, that lines up with the pushrod bore. Pull it and thread a 3/8-16 bolt about 2 inches long in by hand until it stops against the pushrod.

4. Swap the pump. Remove the two 3/8-16 pump bolts, pull the pump and gasket, scrape the face clean, and install the new pump with a new gasket so the arm goes under the end of the pushrod. Torque the bolts to 25 to 35 ft-lb.

5. Remove the long bolt. Put the original short plug back with sealer on the threads. Forget this step and the long bolt holds the pushrod against the eccentric while the engine runs, which wears it out in a hurry.

6. Start and check. Reconnect the lines, crank in short bursts until the carb fills, check for leaks, and pull the dipstick and smell it.

The rest of this article covers the why, the parts, the gotchas, and what to do on a big-block.

How a Small-Block Chevy Fuel Pump Works

The stock pump is a single-action diaphragm pump. A rocker arm sticks out of the top of the pump body and into the block. Above it, a steel pushrod rides in a bore that runs up to an eccentric on the camshaft. As the cam turns, the eccentric pushes the rod down, the rod pushes the rocker arm, and the arm pulls the diaphragm down against a spring. That draws fuel in from the tank through a check valve. When the eccentric comes back around, the spring pushes the diaphragm up and sends fuel out the other check valve toward the carburetor.

That spring is the reason your pressure is what it is. The pump only pushes as hard as the spring pushes. Once the carb’s float closes the needle, the diaphragm just sits there and the arm moves without doing much. That’s normal, and it’s why a mechanical pump doesn’t need a return line on most cars.

On a small-block, the pump bolts to the block through a mounting plate. The plate is held on with two small 1/4-20 bolts, and the pump bolts on top of it with two 3/8-16 bolts. Pull the plate and the pushrod slides out the bottom of its bore. The rod itself is a simple piece: 1/2 inch in diameter and 5.75 inches long on a standard small-block. The Chevrolet Performance part is 3704817, and ARP sells one as 134-8701.

Here’s the part that trips people up. The pushrod isn’t attached to anything. While the old pump is in place, the pump arm holds the rod up against the eccentric. The moment you pull the pump, gravity takes over and the rod drops down in its bore until the lower end is sitting where the new pump’s arm needs to go. You can try to push the arm up under it, but you’re working blind and the rod usually wins.

A side note for anybody who owns a Ford or a Mopar too: most V8s from those camps drive the pump arm right off an eccentric on the front of the cam, with no separate pushrod. The falling-rod problem is mostly a Chevy thing.

Is It Really the Fuel Pump?

A lot of good fuel pumps get thrown away because a car stumbled on the highway on a hot day. Before you spend money, spend twenty minutes testing. The 73-87 Chevy truck service write-up I lean on puts it bluntly: never replace a fuel pump without doing the pressure test. I agree with that.

Symptoms that point at the pump

• Engine starves at highway speed or under load, then recovers when you back off
• Fuel leaking from the small vent hole in the pump body
• Oil level rising on the dipstick, or oil that smells like gasoline
• A new tick or clack from the pump area, which can be a worn arm or a loose pump

That vent hole and the oil smell matter more than people think. If the diaphragm tears, fuel can leak out the vent or into the crankcase. Gas in the oil thins it out and hurts the bearings, so if you smell fuel on the dipstick, change the oil and filter along with the pump. Don’t just top it off and drive.

Symptoms that usually aren’t the pump

A car that dies when you turn the key from START to RUN is an ignition problem, not fuel. A car that runs fine but shows empty on the gauge is a sender or gauge problem, and we covered that in how to test a fuel sending unit on a classic car. Run it out of gas a time or two because the gauge lied and you’ll start suspecting the pump for no reason, so rule out the gauge first.

Heat soak is the other big one. If the car only acts up after a long hot drive and a short stop, look at how close the fuel lines and pump run to the exhaust. A pump is not going to fix a line routed against a header tube. And if the engine itself is running hot, sort that out first. Our guide on tracking down overheating on a classic small-block walks through it.

The pressure test

Get a 0 to 15 psi fuel pressure gauge and tee it in at the carburetor inlet. Pressure at the pump doesn’t tell you what the carb is seeing. Start the engine and let it idle. The truck service text lists 5 to 9 psi for V8 engines, but what you really care about is what your carburetor wants. These are typical ranges, and your carb’s own spec sheet beats any table:

Carburetor	Typical fuel pressure at the inlet
Rochester Quadrajet	5.5-7 psi
Rochester 2-jet (2GC, 2G)	4-6 psi
Holley 4150 / 4160	5-7 psi
Edelbrock (AFB style)	5-6 psi
Carter WCFB	4-5.5 psi
Autolite 4100	4.5-5.5 psi
Motorcraft 2100	4-5.5 psi

If you see pressure that bounces around at idle, or numbers well under spec, and the filter and lines are good, the pump is weak. If you see high pressure with a stock-style pump, look for a mismatched aftermarket pump or a carb needle and seat problem before you blame anything else.

The volume test

Pressure alone can fool you. A tired pump can make 5 psi with the line dead-headed and still not move enough fuel at wide-open throttle. So run a volume test. Disconnect the line at the carb, point it into a clean, approved container, and run the engine at idle. The same truck service write-up says one pint should come out in 30 seconds or less. If it takes longer, check for a restriction in the line or a clogged filter before you condemn the pump.

Check the suction side

This is the one I see missed most. The line from the tank to the pump is under suction, not pressure. If a rubber hose on that side is cracked, it won’t drip. It will just pull air, and the pump will pump bubbles. On a lot of older Chevys there’s a short rubber hose at the frame rail and another at the pump inlet. If they’re hard, shiny, or cracked when you bend them, replace them. For a carbureted car, SAE J30R7 hose is the low-pressure carbureted fuel hose, and J30R9 is rated for fuel injection pressure and generally holds up better to today’s ethanol blends. I just buy the R9. It costs a few dollars more and I don’t have to think about it.

Parts and Tools

Picking a pump

For a stock or mild street engine, a stock-style replacement is fine. The Carter M6624, for example, is listed for a long run of small-block cars and trucks, including 1969-1977 Chevelles and 1967-1980 Camaros. Carter rates it at 40 gph average and 45 gph maximum, with pressure from 2 to 9 psi, a 3/8 inch hose barb inlet, and a 5/8-18 threaded outlet that takes the factory steel line. The 73-87 truck write-up notes that stock-style pumps are serviced by replacement only, so don’t plan on rebuilding one.

If you’ve built the engine up, the Edelbrock 1721 Performer RPM pump is a common step up for small-blocks. Edelbrock rates it at 110 gph and 6 psi, says it supports engines up to 600 hp, and says it doesn’t need a regulator. It has 3/8 inch NPT ports, so you’ll need fittings, and the lower housing can be rotated in 60 degree steps so the ports line up with your lines.

Race pumps are a different animal. The Holley 12-327-11 is rated at 110 gph and 6-1/2 to 8 psi, which is more than an Edelbrock or Carter AFB style carb wants. Holley does sell repair kits for that one, which is nice. If you go that route, plan on a regulator and a gauge. More pump is not better if it pushes the float needle off its seat.

One more thing: if your old pump has three lines instead of two, the third is a vapor return that sends hot fuel back to the tank. Some trucks were built that way. Buy a pump with the same outlet so you don’t lose that circuit.

The rest of the list

• New pump gasket, and a mounting plate gasket if you’re pulling the plate
• Gasket sealer for the mounting surfaces
• A 3/8-16 bolt about 2 inches long (a coarse-thread bolt from the hardware store is fine)
• Thread sealer for the original plug bolt
• Heavy grease, as a backup way to hold the pushrod
• New fuel pushrod if yours is worn (more on that below)
• New inlet filter for the carb, and new rubber hose and clamps for the suction side
• Flare-nut (line) wrenches so you don’t round off the steel line fitting
• 9/16 inch wrench or socket for the plug bolt and pump bolts
• A torque wrench that reads in the 25 to 35 ft-lb range
• Drain pan, rags, safety glasses, gloves, and a fire extinguisher
• A short bolt, plug, or vise grips with a smooth jaw to block off the inlet hose

If you’re doing a concours-type restoration, National Parts Depot sells an OE-correct fastener kit (C-3901-100AK) that includes the 3/8-16 x 1-3/4 inch pump bolts with split washers and 1/4-20 x 5/8 inch plate bolts with star washers. That’s a nice detail when the judges crawl under the car.

Step by Step: Replacing the Pump

1. Prep the car

Work on a cold engine. Hot exhaust and dripping gasoline are a bad mix. Disconnect the negative battery cable so nobody bumps the key. Have the extinguisher close and keep any shop heater, water heater, or droplight away from the job. If the car has been sitting, take a good look at the old pump for wet spots before you put a wrench on it, so you know where fuel is going to come from.

2. Disconnect the lines

The outlet is the steel line that runs up to the carburetor. Use a flare-nut wrench and hold the pump fitting with a second wrench so you don’t twist the line. The inlet is usually a rubber hose from the tank line. Pull it off and plug it right away, because if the tank sits higher than the pump, fuel will siphon out until the tank is empty. I’ve watched it happen. It’s a lot of gas.

3. Hold the pushrod with the bolt trick

Look at the front face of the block on the passenger side, near the timing cover and roughly level with the pump. You’ll find a couple of threaded holes left over from the early small-blocks, which bolted motor mounts to the front of the block. One of them has a short 3/8-16 bolt in it, about 1/2 inch long, often with a copper washer. That hole is drilled through to the fuel pump pushrod bore, which is why it has to stay plugged and sealed.

Write-ups don’t agree on whether it’s the upper or lower hole. The 73-87 truck procedure says the upper bolt on the right front mounting boss. An old hand on the Corvette Forum calls it the lower right hole. Don’t argue with either one. Find the short plug bolt that isn’t holding a bracket on, and that’s your hole.

Pull the plug bolt and thread your 2 inch bolt in by hand until it stops firmly against the pushrod. Finger tight is enough. Don’t crank on it with a wrench. You’re holding the rod in place, not clamping it.

If you can’t get to that hole, and on some cars with big accessory brackets it’s a pain, there are other ways. The factory chassis service manual’s approach, according to owners who’ve quoted it, is heavy grease on the pushrod to make it stick in its bore. Some people put the greased rod in the freezer for half an hour first so the grease firms up. Others slide a long screwdriver up to hold the rod and let the pump arm take its place as the pump goes in. Corvette Forum members even posted a page from a ’66 service manual showing a pickup tool with little mechanical fingers doing the job. Some owners say it works fine, and others say it slips because the rod is so smooth.

There’s also the engine position trick. A few veterans put the engine at #1 TDC on the compression stroke before they start, because that puts the eccentric on its low side on their engines and makes everything easier. The surer test is just feel. If the new pump fights you, turn the engine over by hand a half turn and try again.

4. Remove the old pump and look at the pushrod

Take out the two pump bolts and wiggle the pump off. It’s going to dribble fuel, so keep the pan underneath. Scrape the old gasket off the plate. Get every bit of it, because a leftover lump of gasket is how you get an oil leak at the pump.

Now is the time to look at the pushrod, especially if the old pump was putting out low pressure and the new one isn’t explained by anything else. Back off your holding bolt, remove the two small plate bolts, pull the plate, and the rod will slide out. Measure it. A stock rod is 5.75 inches. Look at both ends. If they’re mushroomed, cupped, or noticeably short, replace it. A worn rod shortens the pump’s stroke, and a short stroke means less fuel. The Chevrolet Performance rod was about fifteen bucks when I checked, so this is not a part to save money on.

If the engine has an aftermarket roller cam, ask the cam maker what pushrod to run. Companies like Comp and CNC Motorsports sell rods with bronze tips for steel billet cams, and some aftermarket blocks call for a rod about 0.200 inch longer than stock. Grease the rod, slide it back in, put the plate back on with a new gasket, and snug the 1/4-20 plate bolts. They thread into iron and don’t take much. One Camaro torque table lists the fuel pump plate at 32 to 40 inch-pounds. That’s inch-pounds, not foot-pounds. Then run your long bolt back in to hold the rod.

5. Install the new pump

Put a thin coat of gasket sealer on both sides of the new gasket and stick it to the pump. Check the pump’s ports against your lines before you bolt it up. If it’s a clockable pump like the Edelbrock, now is when you rotate the lower housing to line things up, following the instructions in the box.

Slide the pump in at a slight angle so the arm goes under the end of the pushrod, then straighten it up against the plate. Here’s the rule I follow: the pump should sit flat against the plate with hand pressure, or close to it, before you start the bolts. If it’s sitting cocked a quarter inch away and you’re thinking about drawing it in with the bolts, stop. Either the arm is on the wrong side of the rod, or the eccentric is on its high side. Pull it, check, turn the engine a half turn if needed, and try again. Forcing it with the bolts is how arms get bent and pumps get ruined.

Start both 3/8-16 bolts by hand, then snug them evenly. The torque sheet I use for small-blocks lists 25 to 35 ft-lb for the fuel pump bolts.

6. Pull the long bolt and plug the hole

This is the step that ruins pushrods. Back out the long bolt, put a little thread sealer on the original short plug bolt, and reinstall it with its washer. If you leave the long bolt in, it holds the pushrod against the eccentric and the engine will chew up the rod. One Corvette Forum member who’s used the bolt trick for forty years said it best: forget to swap it back and you won’t like that first startup noise. I hang a strip of tape on the steering wheel that says PLUG BOLT when I start the job, and it doesn’t come off until the short bolt is back in the block.

7. Reconnect the lines

The steel outlet line uses a flare fitting. Flares seal metal to metal, so don’t put sealant or tape on them. Start the nut by hand so you don’t cross-thread it, then snug it with the flare-nut wrench.

If your new pump has pipe-thread ports (NPT) and you’re installing fittings, Edelbrock’s instructions say to use a liquid Teflon thread sealant, not Teflon tape, and not to overtighten because you can crack the pump housing. Tape shreds and the bits end up in the carb. That’s good advice for any brand.

Put new hose on the suction side and use real fuel hose clamps. While you’re there, change the inlet filter at the carb. Carter recommends a new filter whenever you replace a pump, and it’s the cheapest insurance in the whole job. A new pump will push whatever junk was in the old one straight into the carb.

8. Prime and start

Reconnect the battery. The carburetor bowl may be empty or close to it, so crank in short bursts of 10 seconds or so, with a rest in between to let the starter cool. It can take a few tries for the pump to pull fuel all the way from the tank. I don’t pour gas down the carb on a hot engine to speed it up. If it won’t catch after a few bursts, go back and check for an air leak on the inlet side or a line that isn’t fully seated.

Once it’s running, watch every fitting for a few minutes. Look at the pump vent hole too. Then shut it off and check for drips under the car.

After the Swap: Test It Again

Put your gauge back on and repeat the pressure and volume tests. You should see steady pressure in the range your carb wants and a pint in 30 seconds or less. Write the numbers down and keep them with the car’s paperwork. The next time it stumbles, you’ll have a baseline.

If pressure is now on the high side and the car runs rich or floods after shutdown, the pump may be pushing more than the needle and seat can hold. On a Quadrajet that also throws off idle, so recheck the idle mixture once the fuel side is sorted. Our guide on setting Quadrajet idle mixture screws and the choke covers the procedure.

Check the oil after the first few drives, too. It should look and smell like oil, and the level shouldn’t climb.

Big-Block Chevy Differences

If you’re a small-block guy working on a 396, 427, or 454 for the first time, the job is mostly the same but the pushrod is handled differently, and it catches people. There’s no front-of-block bolt trick. On a big-block, the pushrod comes out through a brass pipe plug underneath the fuel pump mounting pad. There’s a Team Chevelle thread where a longtime small-block guy tried to slide a new rod in from the top, small-block style, and it stopped about an inch in. Another member who’d made the same mistake pointed him to the plug, and a third explained that the bore runs at a different angle on a big-block, so the rod has to go in through that lower hole.

The usual method is to remove the plug, install the rod through it, and put bearing grease or assembly lube on the rod so it stays put while you mount the pump. Then reinstall the plug. A ’71 454 Corvette owner on the forum used grease plus a hacksaw blade bent into an L to hold the rod. Whatever works, as long as the plug goes back in.

The big-block rod has the same basic dimensions as the small-block rod. ARP lists its big-block rod as 135-8701, and it’s also 1/2 inch by 5.75 inches. Retailers list the Chevrolet Performance 3704817 rod for Chevy V8s in general, and Team Chevelle members say Summit lists it for 265 through 454. If you’re running an aftermarket cam, ask the cam maker before you buy, same as on a small-block.

Mistakes I See Over and Over

Forgetting the long bolt. I’ve already said it twice. I’ll say it a third time because it’s the expensive one.

Skipping the tests. If the real problem was a plugged filter or a cracked suction hose, the new pump will act just like the old one, and you’ll be back under the car next weekend.

Too much pump for the carb. A big race pump on a street Quadrajet or AFB style carb can push the needle off its seat and flood the engine. Match the pump to the carb, or add a regulator and a gauge.

Teflon tape on fittings. It shreds, and the shreds end up in the carb’s needle and seat. Use a liquid sealant on pipe threads and nothing at all on flares.

Ignoring a worn pushrod. If the old pump tested weak and the new one does too, pull the rod and measure it.

Not changing the oil after a diaphragm failure. If the dipstick smells like gas, the oil goes. Fuel-thinned oil doesn’t protect bearings the way it should.

Hoses routed near the exhaust. A brand new pump can still vapor lock if the line runs against the exhaust. Reroute or shield it.

A lot of these are the same kind of shortcut that hurts bigger jobs too. We put together a list of common DIY mistakes on V8 engines that’s worth a read before your next weekend project.

A Few Common Questions

Do I have to set the engine to TDC? No. It helps on some engines because it puts the eccentric on its low side, but the bolt trick and a half turn of the crank if the pump fights you will get it done either way.

Can I just rebuild my original pump? Most stock-style pumps are crimped and are serviced by replacement only. Some performance pumps, like the Holley 12-327-11, have repair kits. If you have an original date-coded pump on a numbers-matching car, keep it on the shelf and run a replacement.

Should I switch to an electric pump? That’s a bigger job with its own wiring and safety requirements. For a stock or mild engine, a good mechanical pump is simple, reliable, and doesn’t need a relay, an inertia switch, or anybody wondering why it’s still running after a wreck.

How long should a new pump last? There’s no set number. Keeping the vent hole clear helps, and Edelbrock even recommends a dust shield on off-road and race cars so dirt doesn’t plug it. Good hoses and a clean filter help the most.

Final Thoughts

Replacing a mechanical fuel pump on a small-block Chevy is a job most people can do in an afternoon, and most of that afternoon is testing and cleaning. The actual swap is short once you know the bolt trick. Test pressure and volume at the carb. Hold the pushrod with a long 3/8-16 bolt or some grease. Make sure the pump sits flat by hand before you tighten anything. Torque it to 25 to 35 ft-lb. Put the short plug bolt back. Then test again and write the numbers down.

I still think about that buddy who walked over, swapped one bolt for another, and walked away. He saved me an hour that day, and he’s saved me plenty more since, every time I’ve done this job. Now you’ve got the same trick.

Sources

• 73-87 Chevy truck garage, mechanical fuel pump testing and removal (Mike Ervin)
• CorvetteForum, replacing a small-block mechanical fuel pump (pushrod bolt, grease, and TDC tips)
• Team Chevelle, big-block fuel pump pushrod installed through the plug under the pump pad
• Team Chevelle, small-block vs big-block fuel pump pushrod dimensions
• Carter M6624 mechanical fuel pump specifications and applications
• Edelbrock 1721 Performer RPM fuel pump product page
• Edelbrock fuel pump installation instructions (1711-1726)
• Speedway Motors, Holley 12-327-11 pump and ARP 134-8701 pushrod kit specifications
• CNC-Motorsports, Chevy fuel pump pushrod listings (ARP, Manley, bronze tip rods for steel billet cams)
• Chevytalk, torque specs for small-block Chevy engines
• camaro1.de, Camaro tightening torque table
• National Parts Depot, fuel pump and plate fastener kit C-3901-100AK
• Carburetor-parts.com, carburetor fuel pump pressure chart

For more maintenance and repair guides, see the Car Maintenance Hub.

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