Are Lawn Mower Gas Caps Universal? – don’t make this mistake

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.

Types Of Mower Gas Caps

No matter which gas cap type your mower has, it needs to do three important jobs, prevent gas spill, keep crap out and vent the tank. The first two are obvious, but the venting may be news to some. Air must replace gas as it leaves the tank. 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 below).

Two types of mower gas caps are common:

  • Winged cap
  • Threaded cap

No matter which type your mower has, it needs to be vented.

Winged Cap

The winged gas cap is typically metal and fitted to metal gas tank, it’s less common than the plastic threaded cap. they are commonly fitted to stationary engines but you may find them on some mowers. These caps are not universal, that said it’s not uncommon for one engines gas cap to fit another.

Winged gas cap

Winged Gas Cap

Plastic Threaded Caps

Plastic threaded caps are more common especially on lawn mowers and lawn tractors. They are not universal and although you may find a cap from one mower model does in fact fit another. That said, 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.

Threaded gas cap

Threaded Gas Cap

Gas Tank Needs to Breath

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 resourceful owner repurposes an old oil can cap, which actually fits quite well.

Mower gas caps

Vented Gas Cap

Happy with their repair, they proceed to cut grass only to have the engine stall ten minutes later. Several attempts to start the engine fail. Owner tops up the gas tank and the mower starts again without issue but stops again in another ten minutes. 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 would fix the stalling issue, but fuel would soon seep from around the pierced hole and that’s both an environmental and a fire hazard.

If you’re interested as to how much gas is spilt every year, check out this post “How to reduce mower pollution”, the answer genuinely surprised me.

Mower oil gas cap

MacGyver Type Gas Cap

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 and keeping crap out.

Common symptoms of a bad gas cap include:

  • Engine surging
  • Engine starts then stalls
  • Engine stops after thirty minutes cutting and won’t restart

If you are that MacGyver, you might like the video repair section. It covers common mower repairs the homeowner can easily fix, video page includes relevant links to parts and tools you’ll need nail the repair.