Are Lawn Mower Gas Caps Universal?

Gas caps get lost all the time, the most common causes are vibrating mowers shaking the cap loose and either getting mowed or lost in long grass.

Mower gas caps are not universal, however many caps may be interchangeable. A mower gas cap allows the mower gas tank to breath and as such a proper fitting cap is important.

In this post you’ll learn about the most common type gas caps and why fitting the correct one is important.

Mower gas cap

Vented Gas Cap

Types Of Mower Gas Caps

No matter which cap type your mower has, it will be a vented cap. A vented cap allows your mower gas tank to breath, and that’s important. A blocked gas cap is a common cause of engine stall. More on this later.

Two types of mower gas caps are common:

  • Winged cap
  • Threaded cap

The winged cap is typically metal and is fitted to metal gas tanks, it’s less common than the plastic threaded cap.

Mower gas tank

Winged & Threaded caps

Plastic threaded caps are unfortunately not universal and although you may find a cap that seems to fit, it may in fact cause a more serious issue.

A poor sealing gas cap may cause fuel to seep out around the filler neck. A gas spill is obviously a fire risk but also an environmental hazard.

Mower gas caps incorporate a rubber seal. If the seal doesn’t meet and tighten snugly around the filler neck, gas will leak.

Mower gas tank

Gas Tank Needs to Breath

Vented gas Caps

A is a cap right? No, it’s a little more sophisticated than that. All mower caps need to be vented. A sealed cap will cause the engine to stall and will prevent restarting.

I’m a mechanic for over twenty years and I’ve seen this scenario play out a ton, goes like this – gas cap gets lost and the enterprising owner repurposes an old oil cap, which actually fits quite well.

Happy with their repair, they proceed to cut the grass only to have the engine stall ten minutes later. Several attempts to start the engine failed. Owner tops up the gas tank and the mower starts again without issue but stops again in another ten minutes.

Vented mower gas caps

Vented gas caps

The fuel system is air locking, the old oil cap seals the tank tight causing the flow of gas to the carburetor to slow down and eventually stop. Piercing a hole in the makeshift oil cap will fix the stalling issue, but fuel may seep from around the vent holes.

Covering the gas tank filler neck with plastic is another common MacGyver fix that will cause the same problems. Proper gas caps are designed to breath while preventing leaks

Gas caps

Oil Can cap

Common symptoms of a bad gas cap includes:

  • Engine surging
  • Engine starts then stalls
  • Engine stops after ten minutes cutting and won’t restart
Auto Technician and Writer at | Website

John Cunningham is an Automotive Technician and writer on Lawnmowerfixed.com. I've been a mechanic for over twenty years, I use my knowledge and experience to write "How to" articles that help fellow gear-heads with all aspects of mechanical repairs, from lawn mowers to classic cars.