The correct Vespa spark plug

This article focuses on Vespa spark plugs, the markings of spark plugs from the brands Bosch, NGK, and Champion, as well as the characteristics of individual models. Undoubtedly, these manufacturers produce high-quality spark plugs and wires. Therefore, the key is to determine which spark plug from their extensive range is best suited for one's own vehicle. Recommendations for various Vespa models aim to help find the right spark plug.

The Spark Plug Code

Vespa uses spark plugs from Bosch and NGK, which have 4 digits in the spark plug code. The spark plug should always be chosen to match the respective model and engine setup to avoid piston holes, improve engine starting, and increase spark plug durability.

1st Position: Thread Diameter W (for Bosch) and B (for NGK) and L/N for Champion

The first position of the spark plug code describes the thread size of the spark plug. The diameter of the spark plug thread is identical for all Vespa cylinder heads and independent of the Vespa model. All three manufacturers decode the thread diameter using the first position. However, Champion additionally decodes the thread length at the first position.

2nd Position: Heat Value 2, 3, 4, 5 (for Bosch) and 6, 7, 8, 9 (for NGK) and 78-86 / 3-4 for Champion

The second position in the spark plug code denotes the heat value of the spark plug. There are hot and cold spark plugs. In Bosch, a high numerical value represents a hot plug (e.g., W5AC), and a low numerical value represents a cold spark plug (e.g., W2AC).

In NGK, a high numerical value represents a cold spark plug (e.g., B9ES), and a low numerical value represents a hot spark plug (e.g., B6ES).

In Champion, a high numerical value represents a hot spark plug (e.g., L86C or N4C), and a low numerical value represents a cold spark plug (e.g., L78C or N3C). Champion uses a two-digit number for short-thread spark plugs, and for long-thread spark plugs, the heat value is single-digit.

Original Vespa engines do not develop very high temperatures in the combustion chamber and therefore require a hot spark plug. A hot spark plug conducts the heat generated in the combustion chamber through the ceramic insulator less strongly to the cylinder head than a cold spark plug.
If the spark plug is too cold, the self-cleaning temperature is not reached, and the spark plug becomes sooty, causing it to function suboptimally or even quickly deteriorate.

High-performance engines, solely due to their higher nominal speed, have a higher combustion chamber temperature and require colder spark plugs that conduct the generated temperature to the cylinder head, which is cooled by the airflow of the flywheel. A spark plug that is too hot thermally stresses the combustion chamber more and can even lead to piston holes in tuned engines in such cases.

Hot or Cold Spark Plug

In summary, it can be said: the more original the cylinder, the hotter the plug.
The more tuned the engine is or when used for long distances in the summer, the colder the plug should be.

The heat values overlap slightly, so there may be slight variations in different comparison tables on the internet.

If a new spark plug is needed on the go and you want to convert the heat value, you can use a simple formula:
Bosch Heat Value + NGK Heat Value always equals 11

Example Calculation for the Spark Plug Heat Value

In this example, the Bosch spark plug W5AC to be replaced has a heat value of 5. However, if NGK spark plugs are offered in the trade, their (unknown) heat value can be easily determined by the following formula:

5 + x = 11
x = 6

A Bosch W5AC spark plug with a heat value "5" can be replaced by an NGK B6HS spark plug with a heat value "6."

3rd Position: A / C (for Bosch) and H / E (for NGK): Thread Length

Two thread lengths are common in Vespa. The Vespa Smallframe models V50, PK 50 (S, XL, XL2), PK 80, PK 125, and PV/ET3 all have in common that the cylinder head accommodates spark plugs with short threads.
PX 125 cc also has a short spark plug thread.

The PX 200 has a long spark plug thread and requires Bosch spark plugs labeled C (e.g., W3CC) or NGK spark plugs labeled E (e.g., B8ES).

3.1. Alternative Designation for Champion Spark Plugs

Both thread diameter and thread length are coded in a single digit for a Champion spark plug. "L" stands for short thread, "N" stands for long thread.

4th Position: C / S The Electrode Material

The electrodes of spark plugs are available in various materials, such as copper, silver, and platinum.
For simple Vespa engines, we recommend spark plug electrodes made of copper. These spark plugs are durable, function very well, and fortunately, are also very cheap. A branded spark plug for Vespa costs less than 4 €.

With Bosch and Champion, the last digit C denotes the material copper, while with NGK, it is the digit S.

Additional Letter R for Shielded Spark  Plugs

The additional letter "R" indicates that a spark plug is shielded. Normally, with a Vespa ignition, normal spark plugs without shielding (recognizable by the four-digit spark plug code) are paired with a spark plug wire shielded with 5k ohms. If an additionally shielded spark plug is used with such a spark plug wire, there is a risk of weakening the spark.

Unofficial Recommendations for Vespa Smallframe (PK50 S-XL2 / V50) and Vespa Largeframe (PX 80-200 / Sprint / Cosa) Original and Tuning Engines

Unofficial Recommendations for original Vespa Smallframe Engines

• V50:
  Bosch W5AC bzw. NGK B6HS or Champion L86
• PK 50 / PK 50 S:
  Bosch W5AC / NGK B6HS / Champion L86
• PK 50 XL-XL2:
  Bosch W5AC / NGK B6HS / Champion L86
• PK 50 XL2 Kat (Austrian Modell): NGK B9ES
• PV 125 / ET3:
  Bosch W4AC / Bosch W5AC / NGK B7HS / Champion L82
• PK 80-125:
  Bosch W4AC or NGK B7HS / Champion L82

Unofficial Recommendations for Tuned Vespa Smallframe

• DR 50 ccm:
  Bosch W5AC / NGK B6HS / Champion L86C
• Polini 50 ccm:
  Bosch W5AC / NGK B6HS / Champion L86C
• DR 75 ccm:
  Bosch W4AC / NGK B7HS / Champion L82C
• Polini 75 ccm:
  Bosch W4AC / NGK B7HS / Champion L82C
• Malossi 75 ccm:
  Bosch W3AC or NGK B8HS
• DR102 ccm:
  Bosch W4AC / NGK B7HS / Champion L82C
• Polini 102 ccm:
  Bosch W3AC / NGK B8HS / Champion L78C
• Polini 110 ccm:
  Bosch W3AC / NGK B8HS / Champion L78C oder colder e.g. NGK B9HS
• Polini 133 ccm:
  Bosch W3AC / NGK B8HS / Champion L78C
• Malossi 136 ccm:
  Bosch W3AC or NGK B8HS oder
  Bosch W2AC or NGK B9HS

Unofficial Recommendations for original Vespa Largeframe Engines

• PX 80:
  Bosch W5CC / NGK B6ES
• PX 125:
  Bosch W4AC / NGK B7HS / Champion L82C oder
  Bosch W3AC or NGK B8HS / Champion L78C
• Cosa 125:
  Bosch W4AC / NGK B7HS / Champion L82C oder
  Bosch W3AC or NGK B8HS / Champion L78C
• PX 200:
  Bosch W4CC / Champion N4C und NGK B7ES
• Cosa 200:
  Bosch W4CC or NGK B7ES and Champion N4C

Unofficial Recommendations for Engines with Tuning-Cylinderkits Vespa Largeframe

• DR 135 ccm:
  Bosch W4CC or NGK B7ES and Champion N4C or colder
• DR 177 ccm:
  Bosch W4AC or NGK B7HS and Champion L82C or colder
• Olympia 177 ccm:
  Bosch W4AC or NGK B7HS and Champion L82C or colder
• Malossi 166 ccm:
  Bosch W3AC or NGK B8HS and Champion L78C or colder
• Polini 207 ccm:
  Bosch W3CC, Champion N3C or NGK B8ES
• Malossi 210 ccm:
  Bosch W3CC or NGK B8ES and Champion N3C or
  Bosch W2CC or NGK B9ES
• VMC 244 ccm: NGK B9ES

The Spark Plug Condition

In the past, the finely graded differentiation of the spark plug condition for assessing the mixture composition has significantly lost importance due to the abandonment of lead additives in gasoline. While in the past, one would orient oneself on the "reddish-brown" color, nowadays, the spark plug condition serves only as a rough but still considerable indicator for fundamental faults and disturbances. From today's perspective, the spark plug condition is no longer sufficient for assessing the mixture composition. For this reason, there are no image series of spark plug conditions in this article.

The spark plug condition is considered normal when the spark plug: 

1. is not defective, i.e., not bent, broken, or burned out, 
2. the electrode of the spark plug is not black, sooty, or oily, 
3. or conversely, it is not gray or even white.

Service - Cleaning and Maintenance of a Spark Plug

A spark plug has a limited mileage. If a defect is suspected, the spark plug and spark plug connector, as well as the mechanically and electrically stressed spark plug cable, should be replaced immediately.

The condition should be checked at regular service intervals. The following tools are required: 

• Spark plug brush for cleaning before reinstallation
• Spark plug wrench (preferably suitable for on-the-go)
• Feeler gauge for checking or adjusting the electrode gap.