gas cap discussion looking at filler neck I.D.

[PLTemple]

Mark, I do, as you suggest, always check the Patrol caps when I take them out for refueling, by blowing through the hole to make sure they are clear for venting. I just worry that with the hole being so small, they are much more apt to get blocked than the PA-12 caps with the large forward-facing vent tube which has about a 1/4 " inside diameter. Also, with the redundancy of a top crossover tube between the tanks, the PA-12 would be ok if one vent got blocked in flight. Might be worth thinking about as a design modification.

[jaredyates]

I've thought about the vent hole size too but from the other side. Everything in our hangar with a hole in it gets plugged up by the dirt dobbers, but they seem to have a minimum hole size that they disregard. If I didn't use a pitot cover the tube would be plugged in between visits, but I've never had any trouble with the caps and the bugs. I'd say a quarter inch is probably their favorite!

[Bcone1381]

Guilty of procrastination.

One tank was accessible, its opening varies from 1.717 to 1.730. The kit is 2017 vintage.

[Paul Johnston]

On the cap-I cut the groove deeper,a little at a time . They’re easy to use and still tighten up firm. We spun that part of the cap in the drill press and used a mill file.Using the original O-ring

[Nev]

Paul could you give some more details - maybe some pictures ?

[Paul Johnston]

Nev- to chuck in the dril press I removed the center of cap and replaced it with threaded rod 1/2x13 and maybe washers and locknut. Then put it in the chuck of drill press to spin as you file the seat for O-ring to a smaller dia. I just cut some and try it,then cut more if needed.

[Nev]

This is good Paul, as it can be done by anyone. Thanks.

[Bcone1381]

Edit....I might have jumped in ahead of Paul....shucks.

Short story....Bob's design fit between the cap and neck results in a .020 O-ring squeeze when inserting/removeing a fuel cap. My caps worked well after I adjusted my caps to Bob's design fit. I had to do what Paul did plus reduce the O-ring down one step to match Bob's fit. I also spun mine and removed material with a Mill File.

Originally my caps started out with up to 3 times that amount of squeeze that Bob designed. They were a $!{<# to install and required more colorful words and tools to remove until both the O-ring groove dimension was reduced and the smaller O-ring installed. Now I have a fit of about of .017 O-ring squeeze. It wont come out on its own and gently rock it back and forth twice while pulling before its loose.

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Details for the anal retentive builder:
-The OEM O-ring is 1 3/4". Its AKA a Dash 324, has OD of 1.770 (.020 oversize?). It mated to a 1 3/4" ID filler neck. The O-rings squeeze down .020 when inserting the cap.
-The next size smaller dash 323 O-ring OD is 1.707 is a 1 11/16"

Like Paul, I first also Milled the caps O-ring groove down maybe .010". (Its not a groove, but its a good descriptor for where the O-rings reside) I still required profanity to remove it because my OEM Dash 324 1.770 O-ring was slightly loose on the groove acting like a cork in a wine bottle. I the replaced the OEM Dash 324 O-rings with smaller dash 323's. These fit slightly tight on my cap's groove so its like .030" reduction instead of a .063" reduction in size.

The take-away for me is the cap and o-ring fit gets fine tuned to mate to the filler neck.

I praise Mark for his attention to this and Nev for performing and sharing his experimentation that improved the cap. I continue to procrastinate on adding NEV's the engagment pin seen on on post 58 of this thread. https://bearhawkforums.com/forum/bea...cap-lost/page4



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[Nev]

Thanks for the description Brooks. Out of interest, can your fuel caps be done up by hand ? I've been using a tool on mine. We removed slightly more material, so although the caps now drop easily into the filler neck, I do need to hold them with one hand until the O rings have expanded enough to grip the sides. Yours might have hit the sweet spot.

[Bcone1381]

Hey Nev. I think you ask if I can remove my caps out by hand. Yes. I unscrew the top CCW a generous amount, move the cap back and forth it while pulling with only my hand. I have no need for any tool now. It must be wiggled though. It’s diffacult to remove without rocking it back and forth.

[Nev]

Ok that's good to know thanks Brooks. We might use a very light friction fit and try that. The tool does the job well, but being perched up on the wing strut using one hand can be fiddly just to get it started. However so far there's been no issues with the fuel caps coming off unintentionally, and they're much easier to install and remove.

[BravoGolf]

Using a T-gauge and a micrometer mine are 1.742 L tank and 1.740 R tank.

[Battson]

With calipers I have at least 1.75" on both tanks. A model kit, 2012 vintage

[spinningwrench]

I machined up a plug today to check the inside of my filler neck and a 1.725" aluminum plug just fits in. That would seem to be the smallest I have read about on the forum. Both tanks are the same. It would seem there is a lot of variation in filler neck sizes. Some of that could be because a bushing is never going to be round after being welded in place. I played around with a simple idea of creating a gas cap lid that seals in a more conventional manner along the top of the tank filler and will be machining up a couple samples shortly to play with. I think this will be a no fuss approach. Two turns of the locking screw dog the cap into place using the bottom edge of the neck filler to secure the gas cap.