ISO 3895:1986

Overview

ISO 3895:1986 specifies the essential dimensional characteristics of a screened and waterproof spark-plug (Type 2) and its connector for use in spark-ignition engines. The third edition (1986) updates the earlier editions and focuses on thread profiles, dimensional limits and assembly considerations that ensure interchangeability, reliable sealing and electrical contact.

This standard is primarily concerned with plug and connector geometry rather than materials or performance testing. It includes tabulated limits for major, pitch and minor diameters and references relevant ISO thread standards.

Key Topics

• Scope and purpose: Defines dimensional requirements for Type 2 screened, waterproof spark-plugs and their connections so they can be mounted reliably in compatible cylinder heads.
• Thread dimensions: Tabulates limit dimensions for plug threads (M18 x 1.5) and connector threads (3/4-20 UNEF-3). The standard references ISO 261, ISO 965/1 and ISO 965/3 for metric thread principles and tolerances.
• Tolerance classes: Specifies tolerance classes for the M18 x 1.5 plug thread and corresponding tapped holes - e.g. 6e for plugs and 6H for threaded holes in the cylinder head.
• Connector features: The connector for the spark-plug is specified with a 3/4-20 UNEF-3B thread and hexagon flats of 22.2 mm (with angle dimension 24.6 mm min). Requirements emphasize good water sealing, reliable electrical contact and effective radio-frequency shielding.
• Assembly considerations: The document notes joint thickness requirements after tightening (torque example: 48 N·m) and specifies how joint thickness should correspond to the dimensional table value C. It also details usable thread length minima and surface finish notes shown in the accompanying figure.

Applications

• Ensuring interchangeability of spark-plugs across vehicles and engine designs by standardizing thread and connector geometry.
• Guiding engine designers and cylinder-head manufacturers to provide compatible tapped holes and prevent thread seizure or leakage.
• Helping spark-plug and connector manufacturers meet dimensional and sealing requirements to ensure proper electrical contact and radio-frequency shielding in road vehicles.

Related Standards

• ISO 66 - Basic profile for general-purpose ISO threads
• ISO 261 - Metric threads - general plan
• ISO 965/1 and ISO 965/3 - Metric thread tolerances and classes
• ISO 3412 - Road vehicles - screened and waterproof spark-plug - Type 7
• ISO 3896 - Road vehicles - screened and waterproof spark-plug - Type 3

For designers and quality engineers, ISO 3895:1986 provides the dimensional baseline needed to guarantee fit, sealing and reliable ignition performance for Type 2 screened, waterproof spark-plugs.