Author Topic: Finding the right petcock (Read 2284 times)

Brianskunk · « **on:** February 10, 2020, 06:48:56 PM »

I have now bought two petcocks for my 1976 tank. The first one was too large, so I entered the year and model and bought the next one , which is also too large. The threads were 18 and 15mm, so I measured mine and it’s close to 5mm diameter. Can someone tell me why this is happening and where I can find the correct one? I’m thinking maybe the gas tank I have is from a different year, although it does fit.....I bought it used from a dude I now think was slippery.....thanks, Jim

steeldoncb550 · « **Reply #1 on:** February 10, 2020, 06:55:38 PM »

Everyone is going to want to see a picture of your tank to verify year. Also when I bought my bike the petcock was leaking so I bought a cheap $5. It lasted about 3 months before the shut off arm broke off. I ended up buying oem and that would be my suggestion once people point out what year it’s from.

jaytee-nz · « **Reply #2 on:** February 10, 2020, 07:17:15 PM »

Yep, we will need to know what tank it is but regardless, I've never seen one with a 5 mm thread - that's tiny !

Brianskunk · « **Reply #3 on:** February 10, 2020, 11:41:00 PM »

Is this picture enough.? Or do I need to take it off the bike and turn it over?

Brianskunk · « **Reply #4 on:** February 10, 2020, 11:42:29 PM »

Not my best pic...let me know if you need more

jaytee-nz · « **Reply #5 on:** February 10, 2020, 11:56:59 PM »

Is the petcock fitting on the left or the right of the tank when you are sitting on the bike ? That will determine what F model tank it is.

Brianskunk · « **Reply #6 on:** February 11, 2020, 12:21:41 AM »

It’s on the left as I’m sitting on the bike

steeldoncb550 · « **Reply #7 on:** February 11, 2020, 01:06:08 AM »

Same as mine you need a part #16950-388-015

Brianskunk · « **Reply #8 on:** February 11, 2020, 09:07:37 AM »

Do you know what the thread size is on your tank? Is it closer to 5 than 15?

steeldoncb550 · « **Reply #9 on:** February 11, 2020, 10:32:31 AM »

Quote from: Brianskunk on February 11, 2020, 09:07:37 AM

Do you know what the thread size is on your tank? Is it closer to 5 than 15?

It’s 16mm x 1.5mm

Brianskunk · « **Reply #10 on:** February 11, 2020, 10:56:11 AM »

Maybe I’m asking the wrong question. When you measure the diameter of the bolt that the petcock attaches to, what do you get? Because mine measures 5 mm give or take a mm or two.

seanbarney41 · « **Reply #11 on:** February 11, 2020, 11:35:27 AM »

Brian, you should have two bungs on the bottom of your f tank. First one is on the left side bottom edge and is what the petcock attaches to. It is a threaded bung. If your tank is a 75-76F, it should be 15mm. If your tank is from a 77-78f it should be 18mm.

There is another bung in the middle. It is not for the petcock, it is not threaded, and it is closer to 5mm than 15mm, maybe 8mm? It is a hose nipple for the drain attached to the recessed gas cap area to drain rain water and/or fuel overflow.

If you have something else going on, someone has done some major butchery to your tank.

Brianskunk · « **Reply #12 on:** February 11, 2020, 11:46:31 AM »

Sean, you got me worried. I’ll send you a photo of the bottom of my tank when I get home. I can’t for the life of me think why someone would change the petcock attachment on a gas tank. Especially to a smaller one

seanbarney41 · « **Reply #13 on:** February 11, 2020, 11:57:35 AM »

I suspect there is a communication problem and a lack of correct terminology going on here. The fuel outlet on the petcock is 5.5mm. If someone hacked up your tank, a picture will make that very obvious.

Brianskunk · « **Reply #14 on:** February 11, 2020, 01:27:24 PM »

I suspect I may be using the wrong terminology. How does your 16 mm petcock fit. on the 5 .5 mm petcock threads?

seanbarney41 · « **Reply #15 on:** February 11, 2020, 01:55:05 PM »

No. Petcock screws on to the 16mm(or 15 or 18mm whatever it's NOT 5mm or anything close to that) threaded bung on the lh bottom edge of the tank. The petcock has a lever to select fuel flow...on, reserve, or off. 5.5mm fuel line attaches to the fuel outlet on the back or bottom of the petcock. The fuel outlet is NOT threaded.

seanbarney41 · « **Reply #16 on:** February 11, 2020, 02:03:35 PM »

Here are some pictures...a cb 550 petcock. This is very similar to what would be on a 75 to 76 750f, same size and shape and 15mm thread but slightly different fuel outlets. Also a picture of it mounted on the tank, draining.

Brianskunk · « **Reply #17 on:** February 11, 2020, 02:42:28 PM »

Here is where the petcock attaches Sean. I must be describing something incorrectly.

Brianskunk · « **Reply #18 on:** February 11, 2020, 02:46:42 PM »

You’re right it’s about 15mm, I think I ordered a petcock that is closer to 20, though it says it’s supposed to fit a 76.

Brianskunk · « **Reply #19 on:** February 11, 2020, 02:47:58 PM »

But I realize I was assuming the opposite side of my tape was measuring mm, it’s actually in fractions of an inch. I feel rather stupid. Sorry

Brianskunk · « **Reply #20 on:** February 11, 2020, 02:55:01 PM »

Is your bike a 75 Sean? And Thankyou for trying to help a moron who gets mm and inches confused. I ordered the part number you suggested earlier, that should do it.

BenelliSEI · « **Reply #21 on:** February 11, 2020, 03:10:32 PM »

Brian....... just because no one has mentioned it: you know your fuel filter/ screen is still in the hole? At the very least, you should back it out and make sure it’s not torn, and very clean.

Brianskunk · « **Reply #22 on:** February 11, 2020, 03:14:46 PM »

Yeah I know... I’m going to get it out when get the new one. Thanks though

seanbarney41 · « **Reply #23 on:** February 11, 2020, 03:21:11 PM »

ok, it looks like you have a 75-76 tank and the thread is 15mm. The petcock you got must be for a 77-78 Brian, I have owned every year of cb750 except 69-70. I currently own a 75 750f and a 78 750f3 as well as a bunch of k's. Between 75-78, Honda went crazy with petcock changes, no idea why...so that makes it very hard to figure out what you are supposed to have. Plus lots of them are no longer available from Honda, and oem Honda are really the only ones worth buying. I think Honda offers at least one version that fits your tank.

Stev-o · « **Reply #24 on:** February 11, 2020, 04:45:59 PM »

Sean is the petcock expert!

Brianskunk · « **Reply #25 on:** February 11, 2020, 04:49:28 PM »

Thankyou Sean, you were really helpful

seanbarney41 · « **Reply #26 on:** February 11, 2020, 04:58:33 PM »

come on Steve-O, not trying to be a know it all. I just know how hard it is to deal with the petcocks on these later 750's. Even after all that, warning, there are still issues un-discussed...how many outlets? 1 or 2? and what direction do they point? Honda wants $42 for a fuel t?! Fuel line routing? does any of this stuff really matter?...i dunno all of it...blah blah blah

Stev-o · « **Reply #27 on:** February 12, 2020, 06:46:27 AM »

Quote from: seanbarney41 on February 11, 2020, 04:58:33 PM

come on Steve-O, not trying to be a know it all.

Didnt mean it like that at all! It can you very confusing, and luckily for us, you've had many of the tanks and been through it.

Brianskunk · « **Reply #28 on:** February 12, 2020, 04:53:37 PM »

Being a know it all is really ok, especially when you do know it all. It’s the know it alls that don’t that are annoying

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Author Topic: Finding the right petcock (Read 2284 times)