Plugging fuel/fluid/vacuum/vent bungs

I can't help but notice that the pre-fab rubber caps I've found in the Dorman/HELP/etc aisle of the auto parts store are just rubber, usually cheap, no reinforcement. Fuel lines are generally made of sterner, more chemical-resistant stuff, in addition to being fiber-reinforced.

As a result, it seems like the caps are doomed to eventually die, resulting either in vacuum leak or fluid spillage.

I hate the bolt-in-the-hose-with-clamp cap, no matter how effective it is.

What's the right way to do this (short of removing the bung and welding in a patch to whatever manifold/tank/cover you're working on )? Are there better-quality caps?

I've used the Dorman caps stuffed with RTV on a bunch of vacuum lines in past projects with good success.

Interesting... Guess the RTV should make a more resistant interior surface, sparing the rubber?

I have at least one bung on the bottom of my motorcycle's tank I need to cap, and in this case, breakage means dumping fuel on the exhaust...

I wouldn't trust anything that wasn't made out of the proper material for fuel for something like that. Neons (at least the first year) have a cap like that where the fuel pump goes into the tank, and the one on my dads cracked and emptied the fuel tank down the driveway, so even ones that are made for that probably need replacing sometimes.

Just tried googling for various versions of viton, bung, plug, etc to no avail...

ransom wrote: Interesting... Guess the RTV should make a more resistant interior surface, sparing the rubber? I have at least one bung on the bottom of my motorcycle's tank I need to cap, and in this case, breakage means dumping fuel on the exhaust...

Yeah, I completely understand what you are getting at. A bung cracking on a vac line = crappy idle. On a fuel line, like you said the consequences are much higher.

I do know that you can buy viton cord (think of an o-ring, but just never spliced to form a loop) from a few places and the cross sections get pretty big. If you could find a diameter that you know would work, I'd imagine you could make a plug out of it... Just throwing it out there.

In reply to golfduke:

That's not a bad idea...

I could either poke a hole down the middle and put a screw in to expand it, or just get a snug fit and use a normal rubber cap with clamp to capture the viton plug so it can't back out. Or go crazy and do both

overkill is the best kill, right?

I use www.dichtomatik.us a lot at work, and I know they sell cord of all shapes, sizes, and colors. I am pretty sure anyone can order with a credit card, but there may be a price minimum. It's probably best to call and ask if you do decide to go this direction.

In reply to golfduke:

Awesome, thanks!

Okay, while I think golfduke's suggestion is the best answer I've come across so far, I'm replying to myself once more just to bump.

It's hard to believe there isn't an easily available better product that people use regularly considering how often this comes up...

Maybe I should have a peek over at mcmaster-carr...

http://www.mcmaster.com/#rubber-end-caps/=lhb0s6
http://www.stockcap.com/store/caps.html
http://www.034motorsport.com/hardware-fittings-hoses-an-rubber-and-vac-hose-silicone-vacuum-nipple-caps-p-359.html
http://www.034motorsport.com/hardware-fittings-hoses-an-rubber-and-vac-hose-silicone-high-pressure-nipple-block-off-caps-p-22485.html

For something on the fuel system, I'd use something that is rated for pressure. Actually, that's not true, I wouldn't use a cap at all, I'd figure out a way to avoid using a cap on a fuel line (like braze it closed)...but if I absolutely had to use a cap I'd use something rated for pressure and chemical resistance (like the last link above).

Bryce

Braze it closed and be done.

golfduke wrote: I've used the Dorman caps stuffed with RTV on a bunch of vacuum lines in past projects with good success.

golf tees work great for vacuum lines

If it were part of a pressurized fuel system I'd be even more concerned. What I have is two bungs which are part of the original vapor recovery system which happen to get splashed with fuel under braking.

And since it's a fuel tank, the rigmarole surrounding brazing on it is significantly involved...

The scary thing is that it's made it I have no idea how long with effectively nothing at all, bungs wide open...

If there are 2 and its just old vapor lines is looping a line between the 2 a possibility?

With caps and plugs, I rather doubt cordage will do much. It certainly doesn't prevent low grade rubber from cracking.

I have found low grade rubber to be a problem. Not always, but sometimes.

When they crack, they seem to invariably do it around cut end, never at the actual capping portion.

Effective fix/prevention I've found is to use heatshrink tubing as a clamp/seal. For those that are cracked up and I'm determined to reuse (weird propretary shapes for example) I'll smear some sort of RTV in the cracks before heatshrinking the outside.

I have a port on my intake manifold on the Mini that I keep sealed with one of those cheap caps from the HELP! section. Due to heat coming from the header just below, it get hot and cracks. It seems to last a year or so before it starts to look like it needs replaced due to cracking, then it will hold for another year before I have a vacuum leak. Then I repeat.

In reply to Travis_K:

Unfortunately I need one of them as a vent; As foxtrapper pointed out in the Sprockets thread about this particular application, I could drill a small hole in the cap, but it rains a lot here, so I'd rather vent it to someplace less likely to take on water...

ransom wrote: If it were part of a pressurized fuel system I'd be even more concerned.

It's good practice to assume that every part of a fuel system will see some pressure at some time, even if it's not after the fuel pump, due to heat expansion. Some do it purposefully to control emissions (on newer stuff), but in many cases all it takes is a mud dauber, faulty fuel cap, or a squished vent line screwing with you to end up with pressure in a system that was supposed to be vented to atmosphere. Sure, we're not talking about 50 psi, I'm just saying if I HAD to use a cap I'd find one that is supposed to be ok with some pressure.

Bryce

Is there enough metal in the part(s) with the fittings needing to be plugged to cut the fitting off, then tap the hole for a NPT pipe plug?

In reply to Nashco:

I think that's a good approach. Really, that's the reason I started this whole thread; I didn't want to trust a Dorman cap with the task. It comes up outside of this particular instance, and I've never been happy with any of the caps I've found thus far.

While it is certainly conceivable that the vent could become clogged and the tank could develop pressure from being heated by the engine and sun, I think the level of pressure there and the likelihood of that happening make something like one of the 034motorsport caps a reasonable compromise (though I do want to double check with them, since they specify compatibility with oil and coolant but don't mention fuel).

A quick run with an online ideal gas calculator suggests that a 3-gal volume raised from 70 degrees to 120 degrees will rise about 1.3 psi from atmospheric.

Attach two hoses that go up neear the steering stem then put a "T" in with a 3rd section of tubing poking back under the tank. If it is steel frame pop the seat it the end of the main tube right under the seat and not pluged? if so run you vent line up a few inches you'll never get water of crud up there.