What Is a Screw Plug?
Screw plugs are used as oil drain hole plugs in automobiles.
Screw plugs are also commonly called “drain plugs” or “drain bolts”. Although it is called a plug, it is not a spark plug for gasoline engine ignition.
The name of the screw plug itself is “Automotive Parts – Screw Plug” as defined in the JIS standard. A plug for a threaded pipe fitting is sometimes called a “Screw Plug” for short. Here, however, we will use the one specified in the aforementioned JIS standard.
Uses of Screw Plugs
Figure 1. Example of screw plug use
As shown in Figure 1, screw plugs are automotive parts used as plugs for oil (lubricating oil) drainage holes in engines and gearboxes, as well as other liquid drainage holes. For example, the sides and bottoms of engines and gearbox housings are threaded with oil drainage holes, and Screw Plugs are screwed into these holes to seal them.
Principle of the Screw Plug
The role of the Screw Plug is the same as that of a plug in a threaded pipe fitting, i.e., to screw and seal a threaded plug into a threaded outlet or the like. However, it is necessary to prevent leakage from the gap between the male and female threads. The method of preventing leakage depends on the type of screw and is explained in the section on Types of Screw Plugs.
The following three types of screws are used in Screw Plugs:
| Type of screw | Applicable standard |
| Tapered pipe threads | JIS B0203 |
| Metric coarse thread | JIS B0205 |
| Fine metric threads | JIS B0207 |
Types of Screw Plugs
Figure 2. Types and shapes of screw plugs
1.Classification by Screw Type and Head Shape
Screw Plugs are classified by screw type and head shape as follows:
| Type | Type of screw | Type by head shape | |
| Type 1 | Type A | Tapered pipe threads | Square head |
| B1 type | Hexagon socket | ||
| B2 type | With square hole | ||
| C type | Hexagonal head | ||
| 2 types | Type A | Metric coarse thread, metric fine thread | Hexagon head |
| Type B | Hexagon head with flange | ||
| Type C | Hexagon socket head with flange | ||
Type 1, B1 and B2, do not have a hexagonal head, but have a hexagonal or square hole drilled in the screw shaft to allow mounting without the head protruding from the surface of the mounting area. They can be used in tight spaces with a hexagonal or square L-shaped wrench, etc.
Other types have a square or hexagonal head protruding from the surface of the mounting area.
2. Classification by Screw Tip Shape
Figure 3. Tip shape of screw plug
Screw Plugs are classified into two types: those with a flat screw tip and those with a slightly tapered pilot tip, and those with a straight or tapered tip (only in the case of tapered pipe screws).
3. Classification by Leakage Prevention Method
Figure 4. Types of screw plugs and leakage prevention methods
In the case of Type 1 Tapered Screw Plugs for pipes, sealing tape is wrapped around the male threads of the Screw Plug and screwed into the female threads of the housing, etc., so that the gap between the screws is tightly sealed.
In the case of two types of metric coarse and fine threads, a gasket is inserted between the seat surface of the Screw Plug and the housing surface. The Screw Plug is then screwed into the eye thread side of the housing, etc., so that the seat surfaces are sealed in close contact.
4. Classification by Material
The table below lists the materials of common Screw Plugs:
| Material | Applicable standards | |
| SWCH10A-12A, SWCH10K-12K, SWCH43K-48K | Cold headed carbon steel | JIS G3539 |
| S10C～S20C, S43C～S48C | Carbon steel for machine structural use | JIS G4051 |
| C3604 | Copper and copper alloy bars | JIS H3250 |
If made of steel, Ep-Fe/Zn5c surface treatment is applied for rust prevention, as specified in JIS D0201 Automotive Parts – General Rules for Electroplating.
5. Other
There are other types of Screw Plugs available on the market that are not specified in the JIS standard. For example, some have a magnet attached to the tip of the screw to absorb sludge and iron powder deposited in the oil tank or oil pan and prevent impurities from flowing into the engine and other parts.
Sludge is a sediment of oil and engine fuel, such as burnt scum and rust.
Other Information on Screw Plugs
Screw Plug Sizes
The sizes of Screw Plugs specified in the above JIS standards are as follows
| Type | Screw size | |
| Type 1 | Type A | R1/8 to R1 |
| B1 type, B2 type | R1/16 to R1 | |
| Type C | R1/8 to R1/2 | |
| 2 types | Type A, Type B, Type C | M6 to M36 |