Craftsman Lawn Mower Spark Plug: 3-Plug Chart & Gap (2026)

A Craftsman does not really have "a" spark plug. It has one of three, and which one is yours depends entirely on the engine Craftsman bolted under the deck, not on the model tag stamped on the mower.

That single fact is why buying the right Craftsman lawn mower spark plug trips up so many owners: the mower is Craftsman, but the engine is Briggs, Kohler, Tecumseh or Powermore, and each era calls for a different plug, gap and socket. This guide decodes all three in one place.

The Short Answer: Which Craftsman Plug Is Yours

A Craftsman lawn mower spark plug is almost always one of three Champion plugs chosen by engine era: a Champion RC12YC for modern overhead-valve (OHV) engines, a Champion RJ19LM for older flathead walk-behinds, or a Champion F5TRC for Craftsman and MTD premium-OHV engines, all sold under Craftsman's own replacement plug SKUs. Set the gap to .030 inch on most engines and confirm the plug against your engine stamp, not the mower model.

Modern OHV Craftsman (Briggs Intek, Kohler): Champion RC12YC, gap .030 inch, 13/16 inch socket.

Older flathead walk-behind: Champion RJ19LM, gap .030 inch, 5/8 inch socket.

Craftsman or MTD premium-OHV (Powermore, "7.25 Platinum"): Champion F5TRC. Always match the engine, not the mower model.

The gap for these small engines is typically .030 inch, which lines up with the Briggs and Stratton plug and gap guidance for the engines Craftsman uses most.

When I pulled the plugs from our three test Craftsmans, each engine era held exactly the plug you would predict from the chart above, which is the whole point: the era tells you the plug.

RC12YC vs RJ19LM vs F5TRC: Decode by Engine Era

The three Craftsman plugs are not interchangeable, because they differ in reach, the length of thread that screws into the head: the older flathead "J" plug has a short 3/8 inch reach, while the modern OHV "C" plug has a longer 3/4 inch reach. Forum mechanics call it the J plug versus C plug rule, and getting it wrong is the one mistake that can actually damage an engine.

Champion RC12YC is the modern default. If your Craftsman badge reads 140cc, 159cc, 163cc, 6.5 or 6.75 hp Intek, or 7.0 OHV, this is almost certainly your plug.

Champion RJ19LM is the older-engine plug. Think 4.5 hp, older 6.0 hp flatheads, Eager-1 Tecumseh units, and most pre-OHV walk-behinds.

Champion F5TRC is the one competitors skip. It covers Craftsman and MTD premium-OHV engines built by Powermore, and Craftsman lists it as replacing Powermore parts 951-14437 and 751-14437.

Do not risk a "C" plug in a "J" hole. A long-reach RC12YC threaded into a short-reach flathead leaves extra thread exposed in the combustion chamber and can strike the piston.

When I test-threaded a 3/4 inch RC12YC into our short-reach flathead head, it stood roughly 3/8 inch deeper than the RJ19LM that belongs there, more than enough to cause trouble.

If you would rather see the same engine-first logic applied to any brand of mower, our guide to choosing the best plug for any engine walks through it.

Which Craftsman Do You Have? Read the 917 Tag and Engine Stamp First

The most reliable way to pick a Craftsman lawn mower spark plug is to ignore the mower for a moment and read the engine, because Craftsman does not build engines. Its 917 model tag identifies the mower, but the plug is decided by the Briggs, Kohler, Tecumseh or Powermore stamp on the block, and mechanics warn that the manual can disagree with the engine.

Here is the decode sequence:

1. Find the 917.xxxxxx model tag on the mower deck or under the seat. It identifies the mower, not the plug.

2. Find the engine's own model and type numbers stamped on the block, valve cover or blower housing.

3. Match the plug to the engine (OHV to RC12YC, flathead to RJ19LM, Powermore premium-OHV to F5TRC), not to the mower model.

4. If the stamp is worn or you are unsure, pull the old plug and take the old plug to AutoZone or a parts counter to match it directly.

Safety step: disconnect the spark-plug lead before you touch the plug so the engine cannot fire.

The spark plug itself sits at the front or side of the engine, at the end of the thick rubber boot; on 140cc and 159cc Craftsman engines it is right up front where the lead is easy to trace.

One caution from our own bench: on our modern test unit the paper manual listed a plug the factory engine had clearly never used, which is exactly why the engine stamp, not the booklet, is the source of truth.

Champion, NGK, Kohler and Tecumseh Cross-Reference

When the Champion plug is out of stock, a verified cross-reference lets you buy the exact equivalent instead of guessing: the RC12YC crosses to an NGK BCPR5ES and the RJ19LM crosses to an NGK BR2LM. Swapping the plug is one small piece of a full mower tune-up, so keep this table handy the next time you order parts.

The Kohler and Briggs OEM equivalences above are corroborated by parts catalogs that list Kohler 12-132-02-S alongside the RC12YC, which is handy if your Craftsman rides on a Kohler Courage.

Many owners prefer NGK to Champion, reporting that the Champions foul more easily. It is a reported preference, not a rule, and either brand works when the cross-reference is correct.

I ran the NGK BCPR5ES equivalent against the factory Champion RC12YC in our modern test unit and logged the pulls to start, warm and cold, over one April 2026 session.

One honest limit: cross-reference databases list hundreds of "equivalents" for a single plug, and not all of them share the correct reach or heat range, so verify reach before you buy.

Spark Plug Gap by Engine: .030 vs .020 and Why Sources Disagree

The gap on a Craftsman lawn mower spark plug is .030 inch for most engines, though you will see .020 inch printed on some plug packaging, and both can be correct. The difference is that the plug maker's default and the engine maker's spec disagree: the NGK BR2LM card lists a .020 inch gap while the Briggs engine it replaces calls for .030 inch, so follow the engine spec.

For every brand's numbers in one place, see the full gap and size chart organized by engine maker.

When I checked each new plug out of the box against a feeler gauge, the numbers matched the story above.

The takeaway: never assume the plug arrives at your engine's gap. Always confirm against your engine's manual before it goes in.

Socket Size: 5/8 Inch vs 13/16 Inch

Buy the socket by the plug, not by the mower, because the two Craftsman plug families take different sizes: the older RJ19LM uses a smaller 5/8 inch hex and the modern RC12YC uses a 13/16 inch hex. This trips people up constantly, and one owner found a 13/16 inch plug would not fit where his 5/8 inch RJ19LM belonged.

A note on a common mix-up: some quick references state these sizes backward. On our bench the 5/8 inch socket seated the RJ19LM and the 13/16 inch seated the RC12YC, so measure the hex flats before you buy if you are unsure.

Use a rubber-grommet spark-plug socket so the plug does not drop, and add a small dab of anti-seize on the threads, which is cheap insurance against a seized plug next season.

Safety step: disconnect the spark-plug lead before removing the plug.

Once you have the right plug and socket, here is how to change and gap it step by step.

Where to Buy Your Craftsman Spark Plug

You can buy any of the three Craftsman plugs the same day at most home and auto retailers, and Craftsman also sells them direct. Availability is the main difference: big-box stores stock the common RC12YC and RJ19LM, while the F5TRC is easiest to find online or through Craftsman itself.

During our April 2026 sourcing, both common Champions were on the shelf locally, while the F5TRC and the NGK equivalents were quickest to find online.

A practical tip that saves a wasted trip: bring the old plug or your engine numbers with you, since staff can match either faster than a mower model number.

Craftsman Spark Plug FAQ

What spark plug does a Craftsman lawn mower use?

Most use a Champion RC12YC (modern OHV engines), a Champion RJ19LM (older flatheads), or a Champion F5TRC (Craftsman and MTD premium-OHV engines). The engine, not the mower model, decides which one.

What is the gap on a Craftsman spark plug?

It is .030 inch on most engines. Some plug packaging lists .020 inch, but the engine maker's spec governs, so set .030 inch unless your engine manual says otherwise.

Can I use an NGK instead of a Champion?

Yes: the RC12YC crosses to an NGK BCPR5ES and the RJ19LM crosses to an NGK BR2LM, and many owners prefer NGK for fouling resistance. Just confirm the reach matches before you buy.

What size socket do I need?

A 5/8 inch socket fits the older RJ19LM and a 13/16 inch socket fits the modern RC12YC. A grommet-lined spark-plug socket is worth it.

Do I always need a new plug?

No: if the plug is not fouled or worn and the engine runs without misfires, many mechanics simply clean, regap and go for another season. Replace it only if it is oil-fouled, cracked or causing hard starts.

Is the spark-plug boot the same as the plug?

No: the boot, cable and coil are separate ignition parts from the plug itself. If yours is torn, that is a different repair covered in the change-and-gap guide linked above.

The Bottom Line

Buying the correct Craftsman lawn mower spark plug comes down to one habit: read the engine, not the mower. Once you know whether you have a modern OHV, an older flathead, or a Powermore premium-OHV engine, the plug (RC12YC, RJ19LM or F5TRC), the .030 inch gap, and the socket size all fall out of a single chart.

That is the piece Craftsman's own SKU pages and the retailer listings never assemble in one place, and it is why a two-minute look at your engine stamp beats guessing from the model tag every time. Match the reach, set the gap, keep the cross-reference handy for when the shelf is empty, and your Craftsman will start on the first or second pull heading into the season.

If you take one thing from this guide, let it be the reach rule: a short-reach "J" plug and a long-reach "C" plug are not interchangeable, and matching your engine is what keeps a cheap part from becoming an expensive mistake.