Want to replace fuel petcock with manual, thread size?

[buster7591]

I've already had a no start issues with the vacuum line after pulling carb to install larger main jet. Pinched the line and didn't know it. So I want to install a petcock with manual shut off.   I used my micrometer the check the thread diameter on the fitting coming out of the bottom of the tank and got 15mm. So would that mean I want a 14mm petcock, vs a 16mm?

[90GTVert]

Sounds like you'd want 15mm. Google should bring up quite a few of those. You may want to check thread pitch as well to be sure.

[buster7591]

I won't order anything till I pull this and recheck diameter and pitch. I haven't seen a 15mm petcock on any sight, so I'm guessing it's got to be a 14 or 16. I did go to Rural King and buy 14mm and 16mm nuts so will see which is closest. Their pitch is wrong but hopefully it will give me an idea.

[billwill]

For what it’s worth, I picked one up on Amazon for cheap and it works great. It was like 10 bucks compared to stuff scooter and motocross parts shops sell for $35-40.

[90GTVert]

Jan 19, 2024 19:32:27 GMT -5 buster7591 said:

I haven't seen a 15mm petcock on any sight, so I'm guessing it's got to be a 14 or 16.

Just one of many examples from a google search for "15mm petcock" :
www.ebay.com/itm/293280248202

[buster7591]

Thank you for pointing that out! I'm famous for ordering the wrong thing, at least once...And I'm finding this Jiajue is cut from different cloth than other Chinese bikes. It had a genuine NGK plug, and Gates belt, and made in Japan Keihan carb, and 11 coil stator. So not the cheapest way to make it. But then the phantom air filter, and now a 15mm petcock?

[FrankenMech]

Nominal straight thread size is designated by the outside diameter of the male thread. The outside diameter, as measured, will be a few thousandths or about 0.1mm undersize due to truncated crests of the thread form. Small threads around <10mm, or fine threads, will have a smaller amount of thread truncation than larger threads. I usually look up thread forms in a Machinery's Manual, a costly basic reference book for machinists and engineers. Just remember the male threads will measure a tiny bit smaller than their basic nominal size. Female threads are really difficult to measure for thread size.

I won't even try to explain how to ID tapered pipe thread sizes here.

[AtariGuy]

Jan 20, 2024 11:27:56 GMT -5 FrankenMech said:

I won't even try to explain how to ID tapered pipe thread sizes here.

i will! With comedy! Female end? Grab a few sizes of plugs. Throw em at the connection. The one that sticks, fits. If it dont fit, get a npt tap and ream it til it does! Male end? Grip a pipe wrench all over them threads. Throw the thread bits on the ground too. Mash em up with a brass hammer even! Then huck it in the trash...

[Timmer]

If you're unsure, couldn't you like, order the 16mm one and if it's too large, retaper the hole to like 16mm?
But if you measured 15, order a 15mm of amazon (or similar). It doesn't fit, just send it back

[jackrides]

Two of the most useful & timesaving tools you can have are a not fancy 6" or so vernier caliper and a metric thread gauge. The caliper should have both metric and English (American) markings on it, so it also functions as a metric/English converter.

[milly]

Thread size is usually M1 not M1.25 on petrol cocks

[buster7591]

I got the manual petcock installed today. Red caps are where vacuum lines were capped that went to the old petcock. Had to cut away a bit of the plastic shield so I can easily get my hand under there to operate it. Funny thing was I put it in the off position and started the scooter to see how long it would run. And it ran, and ran, and ran. Never shut off leading me to believe the valve was somehow bad. So, I removed the hose from it and rotated it thru its 3 positions. Fuel, no fuel, fuel so it was working. Reconnected hose and started scoot again. Finally it ran out of gas. Took a good 5 minutes or so. Amazing how little gas this motor uses at idle!

[90GTVert]

Glad you got it! So was it a 15mm?

Vacuum petcocks are a nice safety feature so you can't forget to turn off fuel when it's parked, but they are notorious for failing.