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

Puch spark plug application table

The table below gives a practical overview of common spark plugs for Puch mopeds. Use it as a starting point for choosing a spark plug for your setup. Final choice can still depend on ignition timing, carburetor setup, compression, fuel, exhaust and riding conditions.

NGK spark plugs for Puch

Bosch spark plugs for Puch

Champion spark plugs for Puch

Short thread vs long thread spark plugs

The difference between a short-thread and long-thread spark plug is the length of the threaded part that screws into the cylinder head. This is also often called short shaft or long shaft in moped language.

• Short thread: commonly used on many Puch cylinder heads. NGK plugs with the letter H, such as B6HS or B8HS, are short-reach spark plugs.
• Long thread: used on cylinder heads that are made for a longer spark plug thread. NGK plugs with the letter E, such as B6ES or B8ES, are long-reach spark plugs.

Spark plug heat range explained

The heat range tells you how well the spark plug can transfer heat away from the firing tip into the cylinder head. In simple terms, a higher NGK heat number is a colder spark plug. A tuned engine that makes more heat usually needs a colder spark plug.

A spark plug that is too hot can overheat and cause pre-ignition. A spark plug that is too cold may not clean itself properly, especially at low rpm. That can cause misfiring, poor running, higher fuel consumption and a dirty plug.

Spark plug electrode gap

The electrode gap is the small distance between the center electrode and the ground electrode of the spark plug. This is where the ignition spark jumps across.

On an original Puch Maxi, the standard spark plug gap is commonly set around 0.45 mm. This also works as a practical starting point for many faster setups, but the best gap can depend on the ignition system and engine specification.

• If the electrode gap is too large, the engine may misfire, stall or become difficult to start.
• If the electrode gap is incorrect, idle quality and throttle response can suffer.
• The gap can be checked and adjusted with a feeler gauge.

NGK and Bosch spark plug codes explained

Spark plug names are not random. The letters and numbers tell you the plug diameter, heat range, thread length, resistor type and electrode design. Below are practical examples for common Puch spark plugs.

NGK B5HS

• B = 14 mm thread diameter
• 5 = heat range 5
• H = short thread reach
• S = standard electrode type without internal resistor

NGK B6ES

• B = 14 mm thread diameter
• 6 = heat range 6
• E = long thread reach
• S = standard electrode type without internal resistor

NGK CR7HSA

• C = smaller 10 mm thread diameter
• R = internal resistor
• 7 = heat range 7
• H = short thread reach
• S = standard electrode type
• A = special design or dimension version

NGK BR10HIX

• B = 14 mm thread diameter
• R = internal resistor
• 10 = heat range 10
• H = short thread reach
• I = iridium spark plug
• X = electrode gap and special plug specification

Bosch W4AC

• W = M14 x 1.25 thread, common moped spark plug size
• 4 = Bosch heat range 4
• A = short thread reach
• C = copper electrode material

Spark plug caps and resistance explained

Spark plug caps are available with different resistance values, often ranging from 0 kΩ to 5 kΩ. The right choice depends on whether the spark plug itself has internal resistance and which ignition system you are using.

• A spark plug with the letter R, such as BR8ES or BR9HIX, has internal resistance.
• A spark plug without the letter R, such as B6HS or B8HS, does not have internal resistance.
• A high-resistance spark plug cap is often used together with a spark plug without internal resistance.
• A low-resistance cap is often used together with a resistor spark plug or an ignition system that already has resistance.

For example, the analog PVL ignition uses a 5 kΩ resistor. In that case, the PVL spark plug cap with 0 kΩ resistance is used.

Mounting a spark plug correctly

Always install a spark plug carefully. Start threading it by hand first so you do not cross-thread the cylinder head. Once the plug sits correctly, tighten it to the correct torque.

If the spark plug is overtightened, it can damage the thread in the cylinder head. If the plug is not tightened enough, it may not transfer heat properly into the cylinder head and can also cause compression leakage.

Reading spark plug colors

Reading the spark plug can help you understand how your Puch engine is running. Remove the plug and look at the color of the firing end, especially the electrode and insulator area.

Spark plug color is a useful rule of thumb, but modern fuel types and additives can make the plug look lighter than older tuning guides suggest. E5 and E10 fuel can change the way plug color appears, so use spark plug reading together with engine behavior, jetting, temperature and riding conditions.

FAQ: Puch spark plugs