What size is the drain plug bolt?
The drain plug bolt size varies by application; there is no single universal size. In most passenger cars, the engine oil drain plug is typically M14x1.5 or 1/2-20 UNF, with a hex head that often fits a 14 mm or 17 mm socket. Other machines and fixtures use different standards. Below is a practical guide to the most common sizes you’ll encounter and how to identify them.
Automotive engine oil drain plugs
For engine oil pans, sizes are most commonly metric M14x1.5 or imperial 1/2-20 UNF. The hex head size is typically 14 mm or 17 mm, depending on the model.
- M14x1.5 (metric) – common on many modern vehicles; hex heads often 14 mm or 17 mm.
- 1/2-20 UNF (imperial) – found on some older American models; typical socket sizes are 13/16" or 7/8" depending on the plug.
- Other sizes in use include M12x1.25 or M16x1.5 on specific applications.
Always verify using the vehicle's manual, the existing plug, or by measuring the plug head and threads before replacement.
Automatic and manual transmissions
Transmission drain plugs also vary widely. Many manual transmissions use metric threads such as M12x1.25, M14x1.5, or M16x2, while some use imperial sizes like 1/2-20 UNF or 7/16-14 UNF. The socket size can be 12 mm, 14 mm, 17 mm, or even 19 mm depending on the model.
- M12x1.25 and M14x1.5 (metric) – common on several manuals.
- M16x2 (metric) – used on higher-capacity transmissions; imperial equivalents like 1/2-20 UNF may appear on older designs.
- Imperial: 1/2-20 UNF or 7/16-14 UNF – found on some older transmissions.
Because oil pan and transmission configurations vary by make, always verify from the manufacturer or by measuring the plug and socket before removal or replacement.
Sinks, plumbing, and home drains
In household plumbing, the hardware associated with drains is not described by a single “drain plug bolt” size. Kitchen drains typically use a 1-1/2" drain opening with corresponding basket/strainer threads, while many bathroom drains are 1-1/4" to 1-1/2". The stopper mounting hardware is usually a screw or nut rather than a standard bolt with a conventional size.
- Kitchen sink drains – standard drain body is 1-1/2" in most homes; the stopper/basket hardware uses corresponding threaded components rather than a universal bolt.
- Bathroom sinks – commonly 1-1/4" to 1-1/2" drain sizes; mounting hardware varies by fixture.
For any replacement, check the fixture’s manufacturer specs or bring the part to a hardware or plumbing supply shop to confirm the correct size and thread type.
How to determine your specific size
If you’re unsure, you can determine the correct drain plug size using these methods:
- Measure the head across the flats with a caliper or ruler and compare to standard socket sizes (14 mm, 17 mm, 19 mm, etc.).
- Check the vehicle’s service manual, the parts catalog, or the fixture’s documentation for the exact size.
- Take the existing plug to an auto parts store or plumbing supplier to match the thread type and pitch (e.g., M14x1.5 vs. M12x1.25 vs. 1/2-20 UNF).
Knowing the exact size helps ensure you use the correct replacement and apply proper torque without damaging threads or components.
Summary: Drain plug bolt sizes vary by application. For cars, engine oil drains commonly use M14x1.5 or 1/2-20 UNF; transmissions vary widely; home drains use different standards for fixtures. Always verify with the manual or model-specific specs, and measure if needed.
