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How to change a lawn mower spark plug

Updated: October 2026

You have the correct plug on the bench, the code already matched, and you want it in the engine without stripping an aluminium head. How to change a lawn mower spark plug is lead off, gap from the chart for that engine, thread started by hand, then a stop if it will not turn. This is not a ranking of the twelve listings. The prices sit on the [spark plug lawnmower comparison](/hub/lawn-and-garden-equipment/spark-plug-lawnmower). Peter F. used Briggs & Stratton UK, Husqvarna UK and the Health and Safety Executive pages, plus fitting notes in the saved reviews. There was no lab test. A qualifying purchase can earn a commission at no extra cost to you.

You have the correct plug on the bench, the code already matched, and you want it in the engine without stripping an aluminium head.

Our top picks

Briggs & Stratton Briggs & Stratton RC12YC pair

Best overall

Briggs & Stratton Briggs & Stratton RC12YC pair
9.6/10about £7.99
View offer
NGK NGK BPMR7A

Best value

NGK NGK BPMR7A
9.4/10about £3.91
View offer
UYUYong UYUYong RJ19LM, 3

Rank 3

UYUYong UYUYong RJ19LM, 3
8.6/10about £4.89
View offer
On this page
  1. 1.Make it safe before the socket comes out
  2. 2.Remove the plug and read it
  3. 3.Set the gap the chart names
  4. 4.Refit without stripping the head
  5. 5.If it still will not start

Make it safe before the socket comes out

If you are still choosing RJ19LM or RC12YC, use RJ19LM vs RC12YC before you fit anything. Work outside, with the engine cool. The Health and Safety Executive says you refill a petrol mower only when it is cool, and outdoors. Do not open the tank, tilt the mower onto the carburettor side, or look for a spark in fuel vapour. Pull the plug lead off and keep it away from the terminal. Briggs & Stratton UK says check or replace the plug if the engine will not start, every 25 hours, or once a season. That does not mean a new plug fixes stale fuel. Wait until the head is cool.

Remove the plug and read it

Brush debris away from the well before you turn anything. Grit in an open plug hole stays in the cylinder. Use the socket that fits the hex in your hand. One F6RTC review used a 21 mm socket. RJ19LM reviews say a 13/16 socket did not fit a nut smaller than a Champion. Read the code before you bin the old plug. Briggs tells you to replace a plug with stubborn deposits, a cracked porcelain or burnt electrodes. A black nose can be a rich mixture or old fuel. An oily nose can mean the mower was tipped, or oil is reaching the cylinder. Set the new plug to the chart, rather than copying a worn gap.
Briggs & Stratton Briggs & Stratton RC12YC pair

Testsieger

Briggs & Stratton Briggs & Stratton RC12YC pair
9.6/10about £7.99
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Set the gap the chart names

New plugs are not always at the gap your engine wants, and a loose delivery can knock them. Check with a feeler gauge. A loose parcel can move the gap. Husqvarna UK says bend only the ground electrode and take the gap from the manual. Many Briggs rows are 0.030 in (0.76 mm). Some Quantum rows are 0.020 in (0.51 mm). NGK states 0.7 mm for BPMR7A. Do not copy one of those figures onto a different family. One UK review of an L7T pair said three electrodes made the gap difficult.

Refit without stripping the head

Start the thread by hand. If it will not turn, stop. You are cross-threaded or the reach is wrong. Forcing a plug into an aluminium head is a dealer job. Briggs & Stratton UK says not to exceed 15 ft lb on their service page. Do not copy the NGK BPMR7A torque band onto a Briggs head. If your manual prints a lower figure, use that. Refit the boot until it clicks. One RC12YC review said the terminal was too large for a new Briggs coil boot that had taken a Champion RC12YC. If the boot will not seat, compare the terminal with the old plug. Do not trim it. A boot that is half on misfires.

If it still will not start

A correct plug that changes nothing is a different fault. Stale fuel, a wet filter, a loose lead, a shorted kill switch or a sheared flywheel key can all leave you pulling a rope. Do not keep fitting plugs. Whether the plug was the part at all is on lawn mower won't start spark plug. Stop for a dealer when oil keeps wetting a new plug, the thread is damaged, or a correct plug still gives no spark.

Common questions

What gap should I set?
Use the row for your engine, not a single lawnmower number. Many Briggs rows are 0.030 in. Some Quantum and similar rows are 0.020 in. NGK states 0.7 mm for BPMR7A. Husqvarna says the gap comes from the manual. Check a new plug with a feeler, because packing and a loose envelope can move the ground electrode. Bend only that electrode.
How tight is tight enough?
Start the thread by hand so you know it is not cross-threaded. Briggs & Stratton UK says not to go past 15 ft lb on their service page. If your engine manual prints a lower torque, follow the manual. Do not borrow a car-plug torque band from an NGK page and apply it to a Briggs aluminium head. If the plug will not start by hand, stop. A stripped thread is more expensive than the plug.
What socket do I need?
The socket that fits the hex you can see. In this set, one F6RTC review used 21 mm, and RJ19LM reviews said 13/16 did not fit a smaller aftermarket nut. A deep socket helps when the plug sits down a well. A universal lawnmower spark plug socket is only right when it matches that nut. Measure before you round the hex.
How often should I change it?
Briggs & Stratton UK says check or replace the plug if the engine will not start, every 25 hours, or once a season. That is their service interval, not a rule that every brand of plug lasts a fixed number of hours. Replace sooner if the porcelain is cracked, the electrodes are burnt, or deposits will not clean off.
Should I check the gap on a new plug?
Yes. The factory setting is not guaranteed to be your chart row, and one review in this set had a plug arrive loose in an envelope. A feeler gauge takes a minute. Set 0.030 in, 0.020 in or 0.7 mm only when that figure is the one your engine chart prints.

Related pages

Which lawn mower spark plug

Spark plug lawnmower

Best lawn mower spark plug for a code you already know

RJ19LM vs RC12YC spark plug codes for Briggs engines

Lawn mower won't start spark plug checks before you buy

Sources

Buying notes from the sources below, checked for this page on 22 October 2026. Prices, stars and review counts are from the amazon.co.uk captures of that date. Two listings had no customer-reviews line.

  1. Briggs & Stratton UK: identifying the correct spark plug and gap (retrieved 22 October 2026)
  2. Briggs & Stratton UK: servicing the spark plug (retrieved 22 October 2026)
  3. Briggs & Stratton UK: engine problem solving (retrieved 22 October 2026)
  4. NGK: BPMR7A nickel spark plug (retrieved 22 October 2026)
  5. Husqvarna UK: spark plug spare (retrieved 22 October 2026)
  6. Husqvarna UK: how to check a spark plug gap (retrieved 22 October 2026)
  7. HSE: staying safe with petrol (retrieved 22 October 2026)
  8. Consumer Rights Act 2015, section 9 (retrieved 22 October 2026)