Improvising a lawnmower gas cap

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I've somehow lost the cap to the gas tank on my Sears Craftsman push lawnmower. I found a container cap that fits, but if I use that on the gas tank the mower quits soon after I start it.
I assume that I should bore a hole in the replacement cap to let some air in, .but I'm not sure how big the hole should be. Should it be as small as possible to avoid gas splashing out? Could it be too small?
And I'm out in the country and weeks go by before I'm anywhere near a lawnmower dealer, but should I eventually get a "real" gas tank cap to replace my improvised one?
Thanks.
Lefty
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NYLefty wrote:

Keep your eyes open for someont throwing away a mower and grap the cap? Or just buy one at any hardware store?
You could drill a very small hole in the cap, and make a cap washer for it with a tiny hole in that too. You could cut the washer out of a clorox jug or similar plastic. That should eliminat any spillage.
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Are all lawnmower gas caps the same size? I was assuming that they're different sizes and that I'd have to get a replacement from Sears -- or at least another lawnmower dealer.
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NYLefty wrote:

I think there are a few very common caps, so they shouldn't be too hard to duplicate.
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Variety of different sizes. I'd try punching a hole with a hammer and small nail, probably end up with a smaller hole than a twist drill.
Yes, size matters. For lawn mowers, too.
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Find out who made the motor, and contact them (or one of their representatives) for parts.
Jon
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wrote:

Very small is good enough. It's for air, which is even better than a mouse at squeezing through small spaces. 1/16". myabe even smaller. Or start very small and see if it works. If you still have the same problem, make it a bit bigger. Maybe it won't splash out. Try it and see. If it does, you can do what BobF says.
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Andy adds: Very good advice, in my opinion. Remember NY, you don't have to get it perfect the first time, and "too small" doesn't hurt anything........ It's not like you were drilling for oil in the Gulp of Texaco :>))))))
Andy in Eureka, Texas
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I think you're in good hands on this one- fix what you've got, scrounge a cap or try a hardware store. . . .
-snip-

I'm in the burbs and within a 1/2 hour of a 1/2 dozen dealers-- but I'd probably get a gas cap online. T'aint worth the time or effort to get in the car and drive when I can order it before my afternoon nap & often have it before lunch the next day. I've got enough projects started that I just move to another until the parts come in.

If you've got some way to make some foam stay up there so your cap isn't sucking dust, it should be ok. OTOH- a generic cap is only a couple bucks. Not worth a whole lot of effort.
Jim
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On 6/18/10 8:55 AM, Jim Elbrecht wrote:

>snip<

...as long as you don't mind adding $20+ to your cost for priority overnight shipping!!
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That's a suckers bet IMO. I've gotten 13 hour service from Amazon, Newegg & McMaster Carr on many occasions. Order at 10pm- arrived UPS at 11AM. 24-48 is the norm. And I never pay for fast shipping. [Amazon is free if the order is over $25- and I usually have something in the cart to bring it up to that-- often Newegg is free, and McMaster is reasonable- usually $4-5 for a box of bolts]
Jim
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Newegg sells lawnmower gas caps?
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I went into the nearest small town today and was surprised to find a cap that fits at the hardware store there.....thanks to those who suggested doing this. I'd been thinking that there wouldn't be much of a demand for such caps and that I've have to get one from Sears or a lawnmower dealer.
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I always find it heartening, when I do something silly like lose a gas cap, that I'm not only *not* the first person to do this--- there have been enough folks down this path before to make it profitable for someone to carry replacements.<g>
Jim
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On Fri, 18 Jun 2010 19:09:32 -0400, Jim Elbrecht wrote:

My Craftsman 917.388853 6.5 HP 21" push mower (Consumer Reports Best Buy in 2005) came with this "fancy" gas cap that supposedly "preserved the gas".
After a couple of seasons, the fancy gas-preserving cap couldn't preserve itself, and it broke into pieces. I just put a gas-can cap on. It fit. But the engine kept quitting after a while. So I loosened it up when I was cutting the lawn.
I'm curious WHAT is the difference between a gas-can cap and a lawn mower gasoline tank cap? (The threads are the same.)
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gas can caps are non vented designed to seal and not let vapors in or out........
mower caps are vented so as gasoline is used air can enter the tank, otherwise the engine will quit after awhile stareved for fuel.
how do i know this? many many years ago my mowers vent cap malfunctioned and quit venting.
engine would run for awhile then quit. i would check gasoline level put back cap mower would run again for awhile then quit.....
what the heck? thats when i learned about gasoline cap venting:)
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On Saturday, July 10, 2010 at 5:52:29 AM UTC-4, snipped-for-privacy@aol.com wrote:

many gasoline can caps and engine caps are identical except for the vent.
i lost my gasoline engine cap and needed to get the grass cut, so i removed the gas can washer and left the lid a tiny bit loose.
its a good emergency patch
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Fellow at work had a mower that would run for a short time and quit. Told him to checkthe vent hole. Next day he admitted he had mixed up the gas can and the engines gas caps.
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On Saturday, July 10, 2010 at 1:38:27 AM UTC-4, James H. wrote:

in

t

Gas caps *should* provide a complete seal. Mowers, particularly the modern carb compliant ones, as well as other small equipment, have a cap that has a negative pressure valve in the top.
This means that as the pump draws fuel it creates a vacuum in the tank that opens that valve to let air in, but the valve closes so no gas fumes escap e when there is no suction, when the engine isn't running - till it wears o ut, then you will see the rubber falling apart and smell gas fumes if it's left sitting in a non-ventilated area. In other cases it just gums up and plastic parts stick shut or open and it can be cleaned out, but usually the re's some rubber that went bad unless your gas was terribly dirty.
You could certainly make a TINY hole in a gas can cap, plug that hole while it's not running and unplug the hole while it is, though you might still e nd up with a little gas splashing out when the tank is full.
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NYLefty wrote:

Just a hole will make it leak and splash out. You need an insert under the cap also with a side hole (not in line with the cap hole) to prevent splash out and let what splashes into the first hole to drain back in the tank. A 1/16 or 1/32 hole is adaquate.
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