John Deere 425 Spark Plug Gap (Replacement Guide)

The John Deere 425 uses two spark plugs, one for each cylinder of its Kawasaki FD590V engine. Set each plug gap to 0.030 inch (0.76 mm). NGK BPR5ES is a common specified plug, but confirm the engine label and applicable manual before buying. Install plugs by hand, then tighten to 13 lbf·ft (18 N·m) on clean, dry seats.

A no-start or rough-running 425 can make a small plug problem feel like a major repair. But the gap is straightforward once you confirm the engine and use the right gauge. The key is to service both plugs carefully, avoid damaging the aluminum cylinder-head threads, and stop if the engine still misfires after the work.

Confirm the 425 engine and spark-plug specification

The John Deere 425 covered here has a Kawasaki FD590V liquid-cooled, four-stroke V-twin engine. It takes two spark plugs, one per cylinder, with a specified gap of 0.030 inch (0.76 mm). Check the engine identification label and the correct manual before ordering or fitting plugs.

The common plug specified for this engine is NGK BPR5ES. Treat that as a starting point, not a reason to skip verification: equipment details or service information can vary, and the engine manual takes priority if it lists a different plug or setting.

A spark-plug gap is the space between the center and ground electrodes. The ignition spark jumps across this space to help ignite the air-fuel mixture. If the gap is too wide, too narrow, or uneven, the plug may not work as intended.

This engine is four-stroke. It does not use premixed fuel or two-stroke oil in the fuel. Keep the fuel and oil requirements in the tractor and engine manuals separate from spark-plug service.

Prepare the tractor safely

Safe preparation means preventing an unexpected start and keeping debris out of the engine while you work. Park the tractor on level ground, lower attachments, set the parking brake, switch off the engine, remove the key, and let the engine cool before touching either plug.

Before starting, gather the correct replacement plugs, a wire-style gap gauge, a torque wrench, and suitable hand tools. Wear eye protection. Keep hands, clothing, and tools away from the radiator fan and other moving parts. Disconnect the negative battery cable before servicing.

Brush loose debris from both plug wells before removing the leads. This matters because dirt around a plug can fall into the cylinder when the plug comes out. Remove and service one plug at a time so the leads stay associated with their original cylinders.

If a plug wire or boot is damaged, do not pull on the wire itself. Handle the boot carefully and inspect it as you remove it. A damaged lead can cause a misfire even when the plug gap is correct.

Inspect, measure, and set the gap

Gap each replacement plug before installation, even if it came in a box labeled as pre-gapped. A wire-style gauge measures the space between the electrodes. For the FD590V, set both plugs to 0.030 inch (0.76 mm), unless the applicable engine manual gives another value.

Remove the plug lead, clean the well, and use a suitable plug socket to remove the plug. Check its electrode and insulator for cracks, heavy deposits, or wear. Compare the two plugs. A clear difference in deposits or wear is useful information to record, but it does not by itself prove what caused an engine problem.

To check a new or reusable plug, slide the 0.030-inch wire on the gauge through the electrode gap. It should pass with light contact. If the gauge is loose or will not fit, adjust the gap in small steps by moving the ground electrode only. Do not press or lever against the center electrode or its ceramic insulator.

A plug that is cracked, heavily fouled, or worn should not be reused. If the gap cannot be set without force, or the electrode is damaged, replace the plug rather than trying to make it serviceable.

Install both plugs without damaging the head

Correct installation protects the aluminum cylinder head and helps the plug seat as intended. Start each plug by hand, turning it gently until it threads freely. Once seated, tighten to 13 lbf·ft (18 N·m) on a clean, dry seat, unless the engine manual specifies a different value.

Guide the plug into its hole without tilting it. Turn it by hand for several rotations before using a wrench. If it will not turn freely, back it out and check alignment. Do not force it: cross-threading a plug can damage the aluminum head and lead to a costly thread repair.

After hand-threading, use a torque wrench to reach the specified value. Do not use an impact tool. Avoid adding lubricant or anti-seize unless the applicable manual directs you to do so, because the stated torque is for a clean, dry seat.

Repeat the steps for the second cylinder. Refit each lead securely, then reconnect the negative battery cable after both plugs are installed. Keep the plug wells and surrounding area free of loose tools and debris.

Start the tractor and check the result

A post-service test checks whether the engine starts and runs smoothly after the plugs are installed. Test-start outdoors in a ventilated area, with the tractor stationary and attachments disengaged. Keep clear of the fan and other moving parts while the engine runs.

Listen for a steady idle and watch for signs of a misfire or rough running. If the engine still misfires, switch it off and diagnose further rather than running it repeatedly. A damaged plug lead, an ignition fault, or a fuel or air problem can also cause poor running.

After the test, switch off, remove the key, and let the engine cool before inspecting the work. Check that both leads remain seated and note the service date. If you removed plugs for inspection, record which cylinder each came from and any differences in condition.

Practical service scenarios and seasonal care

A useful service record notes the plug gap, parts fitted, date, and any unusual condition. The scenarios below are common situations a 425 owner may encounter; they are examples for applying the procedure, not documented repair cases or proof of a specific fault.

Scenario: The tractor starts poorly after sitting. Confirm the FD590V identification, inspect both plugs, and measure the gaps rather than assuming the old settings are correct. Replace plugs that are cracked, heavily fouled, or worn. If the engine remains difficult to start, check other causes instead of repeatedly changing the gap.

Scenario: One plug looks different from the other. Record which cylinder the plug came from and compare the electrodes and deposits. Install serviceable plugs at the specified gap, then test the engine. A difference between plugs is a reason to investigate if symptoms remain, not enough on its own to identify a failed component.

Store new plugs dry in their packaging until needed. For seasonal storage, follow the tractor and engine manuals for fuel and battery care. Replacing or gapping plugs does not replace those storage steps.

Frequently asked questions

What is the spark-plug gap on a John Deere 425?

The gap is 0.030 inch (0.76 mm) for the Kawasaki FD590V setup covered here. Confirm the engine identification and applicable manual before servicing.

How many spark plugs does the 425 use?

It uses two spark plugs, one for each cylinder of the FD590V V-twin engine. Service and inspect both.

What spark plug fits a John Deere 425?

NGK BPR5ES is a common specified plug. Verify it against the engine manual or existing plug before buying, since the engine-specific information should guide the final choice.

Do both plugs need the same gap?

Yes. Set both plugs to 0.030 inch (0.76 mm), unless the applicable engine manual gives a different specification.

Can I use a coin-style gap tool?

A wire-style gauge is recommended for checking this gap. It gives a direct measurement in inches or millimeters and helps avoid pushing against the center electrode.

What torque should I use for the plugs?

Use 13 lbf·ft (18 N·m) on clean, dry plug seats. If the engine manual gives a different value, follow that instruction.

Should I add two-stroke oil to the fuel?

No. The FD590V is a four-stroke engine. Do not add two-stroke oil to the 425’s fuel or use premixed two-stroke fuel.

What if a plug will not thread by hand?

Stop and back it out. Check that the plug is aligned with the hole and that the threads are clean. Do not force it, since cross-threading can damage the aluminum cylinder head.

What should I do if the engine still misfires?

Switch it off and diagnose the cause instead of repeatedly running it. A damaged lead, ignition fault, or fuel or air problem may be involved; a correct plug gap does not rule out those issues.

Can I reuse a plug that looks dirty?

Do not reuse a plug that is cracked, heavily fouled, or worn. If it appears sound, inspect its condition and gap carefully, and follow the engine manual’s service guidance.

Final check before returning the tractor to work

The main service targets are simple: verify the FD590V, confirm the correct plug, set both gaps to 0.030 inch (0.76 mm), and torque clean, dry seats to 13 lbf·ft (18 N·m), unless the manual says otherwise. Hand-start each plug and stop if it does not thread freely.

Once both leads are secure, test the tractor outdoors with attachments disengaged. Record the service date and plug condition. If rough running continues, move on to diagnosis rather than assuming the gap is still the problem.

(This article was written by one of our staff writers, Harrison Croft. Visit our Meet the Team page.)