Tomos spark plug guide: heat range, resistance and reach

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Tomos spark plugs explained: heat range, resistance, short and long reach

The right spark plug for your Tomos depends on your cylinder, speed, ignition, spark plug cap and engine setup. A standard 50cc moped often needs a different spark plug than a fast 50cc, 65cc, 70cc or 74cc setup.

In this guide, we explain which NGK, Bosch and Champion spark plugs are commonly used for Tomos setups, what short and long reach means, how heat range works, when to use a resistor spark plug and how to read spark plug color.

Quick answer: always choose the correct thread reach for your cylinder head first. Then choose the heat range based on your setup. The harder and hotter the engine is used, the colder the spark plug usually needs to be. Also check whether your spark plug, spark plug cap and ignition match in terms of resistance.

Spark plug table for Tomos setups

NGK, Bosch and Champion spark plugs can be used for many Tomos setups. This ranges from a standard 38 mm cylinder and 25 km/h moped to faster 50cc, 65cc, 70cc and 74cc setups. For heavy use, such as racing, motocross or sprint riding, an iridium spark plug is often chosen.

NGK spark plugs for Tomos

Spark plug brand	Type	Heat range	Reach	Application
NGK	B5HS	5	Short	25 km/h moped / 50cc
NGK	B6HS	6	Short	40 km/h moped / 50cc
NGK	B7HS	7	Short	Fast 50cc and 65cc moped setups
NGK	B8HS	8	Short	65cc and 70cc moped setups
NGK	B9HS	9	Short	70cc and tuned 70cc moped setups
NGK	B6ES	6	Long	40 km/h moped / 50cc
NGK	B7ES	7	Long	50cc / 6-port moped setups
NGK	B8ES	8	Long	65cc and 70cc moped setups
NGK	B9ES	9	Long	70cc and tuned 70cc moped setups
NGK	B10ES	10	Long	70cc and 74cc moped setups
NGK	BR8ES	8	Long	65cc and 70cc moped setups
NGK	CR7HSA	7	Short	50cc / 6-port moped setups
NGK	BR8HIX	8	Short	65cc and 70cc moped setups
NGK	BR9HIX	9	Short	70cc and tuned 70cc moped setups
NGK	BR10HIX	10	Short	70cc and 74cc moped setups
NGK	BR8EIX	8	Long	65cc and 70cc moped setups

Bosch spark plugs for Tomos

Spark plug brand	Type	Heat range	Reach	Application
Bosch	W4AC	4	Short	65cc and 70cc moped setups
Bosch	W5AC	5	Short	50cc / 6-port moped setups
Bosch	W6AC	6	Short	40 km/h moped / 50cc
Bosch	W7AC	7	Short	40 km/h moped / 50cc
Bosch	W8AC	8	Short	25 km/h moped / 50cc

Champion spark plugs for Tomos

Spark plug brand	Type	Heat range	Reach	Application
Champion	L86C	7	Short	40 km/h moped / 50cc
Champion	P82M	8	Short	65cc and 70cc moped setups

Short or long reach spark plug explained

The difference between short and long reach spark plugs is the length of the threaded section on the spark plug. This matters because the spark plug must fit correctly in the cylinder head.

Reach type	Thread length	What to check
Short reach	Often shown as a short thread type, for example NGK H / HS	Common on many standard moped setups. Always check the old spark plug or cylinder head first.
Long reach	Usually 19 mm, for example NGK E / ES	Only use this if your cylinder head is designed for a long reach spark plug.

Important: do not choose a short or long reach spark plug based only on engine size. First check the old spark plug or measure the depth of the spark plug hole in the cylinder head. The wrong reach can cause damage or poor running.

Spark plug heat range explained

The heat range shows how well a spark plug can remove heat from the firing tip. In practice, a harder-used or more heavily tuned engine usually needs a different spark plug than a standard 50cc setup.

With NGK spark plugs, a higher number usually means a colder spark plug. A colder spark plug can remove more heat. Bosch numbering works differently from NGK numbering, so do not compare heat ranges one-to-one between brands.

Situation	What can happen?	What to watch for
Spark plug too hot for your setup	The spark plug can overheat and cause pre-ignition or pinging.	Watch for a white spark plug color, overheating, pinging or the engine running too hot.
Spark plug too cold for your setup	The spark plug may not reach the right operating temperature and can foul more easily.	Watch for fouling, misfiring, poor idle and increased fuel use.
Correct spark plug	The spark plug removes heat properly and stays clean enough during normal use.	The engine runs well and the spark plug color is usually light brown to brown after checking.

Spark plug electrode gap explained

The electrode gap is the small distance between the centre electrode and the ground electrode of the spark plug. This is the gap the spark has to jump across.

An incorrect electrode gap can affect starting, idle quality, misfiring and combustion quality.

Electrode gap	Possible result
Too small	Can cause misfiring, rough idle and poor combustion.
Too large	Can cause the engine to misfire, especially under load or at higher rpm.
Correct	The spark jumps consistently and the engine runs more smoothly.

The electrode gap can be measured with a feeler gauge. Many original moped setups use around 0.45 mm. If you run a different ignition or race setup, always check which gap suits your setup.

Spark plug codes explained

Spark plug codes are made up of letters and numbers. The code tells you about thread diameter, heat range, thread reach, resistance and spark plug type.

NGK B5HS

B: spark plug thread diameter, in this case 14 mm.
5: heat range, in this case 5.
H: thread reach, short reach.
S: standard spark plug type without internal resistance.

NGK B6ES

B: spark plug thread diameter, in this case 14 mm.
6: heat range, in this case 6.
E: thread reach, long reach of 19 mm.
S: standard spark plug type without internal resistance.

NGK CR7HSA

C: spark plug thread diameter, in this case 10 mm.
R: spark plug with internal suppression resistor.
7: heat range, in this case 7.
H: thread reach, short reach.
S: standard spark plug type.
A: spark plug design code, often related to dimensions or construction.

NGK BR10HIX

B: spark plug thread diameter, in this case 14 mm.
R: spark plug with internal suppression resistor.
10: heat range, in this case 10.
H: thread reach, short reach.
I: iridium spark plug with centre electrode.
X: version according to iridium specifications.

Bosch W4AC

W: thread shape and size, usually M14 x 1.25 for mopeds.
4: heat range according to Bosch numbering.
A: short reach.
C: copper electrode.

Difference in spark plug caps and resistance

Several spark plug caps are available in the shop. Before ordering, make sure you choose the right spark plug cap for your Tomos. The main difference is the resistance value of the cap.

Spark plug cap resistance values often range from 0 kΩ to 5 kΩ. A high-resistance spark plug cap is usually used with a spark plug without internal resistance. A low-resistance spark plug cap is often used with a spark plug that already has internal resistance.

Example: with an analogue PVL ignition that has an internal 5 kΩ resistor, a 0 kΩ PVL spark plug cap is often used. Always check the specifications of your ignition, spark plug and spark plug cap before combining parts.

Installing a spark plug and tightening torque

A spark plug must be installed correctly to avoid damaging the thread in the cylinder head. Always start the spark plug carefully by hand first, so you know the thread is seated correctly before tightening it.

Spark plug type	Thread diameter	Cast iron cylinder head	Aluminium cylinder head
Flat seat, standard with washer	18 mm	34-44 Nm	34-44 Nm
Flat seat, standard with washer	14 mm	24-34 Nm	24-29 Nm
Flat seat, standard with washer	12 mm	15-24 Nm	15-20 Nm
Flat seat, standard with washer	10 mm	10-15 Nm	10-12 Nm
Flat seat, standard with washer	8 mm	-	8-10 Nm

Important: if the spark plug is overtightened, the thread in the cylinder head can be damaged. If the spark plug is too loose, it will not make enough contact with the cylinder head and may not be able to transfer heat properly.

Reading spark plug color

Reading the spark plug helps you learn more about combustion and tuning on your Tomos. After removing the spark plug, look mainly at the color of the section that sits inside the cylinder head.

Color	Possible cause	Possible result
Light / whitish	Mixture too lean, false air, wrong jetting, wrong heat range or wrong fuel/oil mix.	Holding back when accelerating, overheating, pinging or the engine running too hot.
Light brown / brown	Correct mixture and good basic tuning.	The engine runs well and combustion is usually healthy.
Dark brown / black	Mixture too rich, main jet too large, worn piston ring or cylinder/piston wear.	Holding back on throttle, fouled spark plug and increased fuel consumption.

Tip: spark plug color is a useful tuning check, but always look at the full picture. Jet size, air filter, exhaust, ignition, oil ratio, fuel type and riding style can all affect how the spark plug looks.

FAQ: Tomos spark plugs

Which spark plug do I need for a standard Tomos?

For a standard Tomos 50cc setup, a short reach spark plug with the correct heat range is often used. The exact spark plug depends on your cylinder head, speed, ignition and setup. Always check the old spark plug or the cylinder head specification first.

What is the difference between short and long reach spark plugs?

The difference is the length of the threaded section. A short reach spark plug has a shorter thread than a long reach spark plug. Always use the reach that matches your cylinder head.

When should I use an iridium spark plug?

An iridium spark plug is often chosen for heavier use, such as racing, motocross, sprint riding or highly tuned setups. For a standard setup, a normal spark plug is often enough.

What does the R mean in an NGK spark plug code?

The R in an NGK spark plug code means the spark plug has an internal suppression resistor. Pay attention to the spark plug cap and ignition you use, so the total resistance is correct.

What spark plug color is good?

A light brown to brown spark plug color usually points to a good basic setup. A whitish spark plug can point to a lean or hot-running setup. A black spark plug can point to a rich mixture or spark plug fouling.

Explaining different spark plugs, resistance, short or long shaft etc.