gas cap discussion looking at filler neck I.D.

[Mark Goldberg]

We (the forum) have spent a bit of time looking at gas cap issues. One variable that could be affecting everyone's experiences is the ID of the filler neck. I assumed (incorrectly) that the ID of the 2"x.125 wall tubing would be very close to the 1.750 spec. I thought it would vary a few thousandths. But we measured the current piece of tubing we have been cutting for filler necks and found it has an ID of 1.740. 10 thousandths under the spec (smaller). So that would seem to account for some difficulty in removing the gas caps.

A 20 ft piece of this aluminum tubing lasts for a few years at the factory. So I request a few of you measure the ID of the filler necks and see what you have. Especially if you think your caps are tight or alternately loose. Then we can better come up with guidance to help. Thanks. Mark

[bkyser]

Both of mine measure 1.736. The caps are very difficult to remove. Patrol wings S/N 81P and 82P.

[Bissetg]

Mine measure at 1.732.

[Mark Goldberg]

For you guys whose filler necks are 1.736 & 1.732 - a different oring one size smaller would likely help make the caps more easily removed. Mark

[Ed.Meyer]

I have not measured our openings and it has been nearly 8 years since we got the Patrol kit but, we found the gas cap to be much too tight and solved it with different O rings as Mark suggested. I don't recall where we found them but it was through internet search. I think it was Viton O rings we used and they have held up fine.

[gregc]

Mine are 1.739. Haven’t reached the point of needing caps.

[jaredyates]

If anyone wants to try some smaller o-rings, I ordered a 25 pack and used 4, let me know and I'll send some.
Update: all of the extras that I had are spoken for.

[PLTemple]

Jared, do you still have any extra of the smaller O-rings? I really need to do something as my caps are extremely difficult to remove and to re-insert in the tanks on my Patrol.

[jaredyates]

I do, just send me your address and I'll get some on the way for you.

[Utah-Jay]

Jared

I would be happy to get 4

PM’ing my address.

[Nev]

If anyone wants to try some smaller o-rings, I ordered a 25 pack and used 4, let me know and I'll send some.

Jared did you find any difference with the smaller O rings ? I tried the -323 rings on the existing caps and found that because both sized O rings have the same 5mm ring diameter I couldn't notice any difference. Just slightly tighter to fit on the cap shank, but the caps were still difficult to fit and remove. Maybe you've found an O ring that has a smaller profile ?

[jaredyates]

Hi Nev, I need to test them and report back. One way I was thinking about it is that in the finite space between the cap and the neck, there is some amount of rubber that has to fit. the 323 rings will have less rubber to go in that gap, even if they are the same ring diameter, if that makes sense.

[Dpearson]

On my four "circa 2006" tanks I measured 1.73, 1.74, 1.75, 1.75". I haven't tried fitting caps yet, still a long way off from needing to do that!

[schu]

How are people measuring? Calipers? Or t-bore gauge and micrometer?

This is why I abandoned my alternate fuel cap mod, because I couldn't get decent measurements and they appeared to vary which doesn't work in a design that has static O-rings. In the case of the bearhawk fuel caps, the o-rings are compressed which can take up the variance if it's not too tight to begin with.

[bkyser]

I used a pair of digital calipers. They're not perfect for measuring I.D.s, but with practice one can get close enough for this sort of stuff.

[schu]

Mark,

It might make sense to pick up a reamer for the factory. Something like this:

https://www.mscdirect.com/product/details/02321487

It would be really easy for the guys to run it through tube after welding then rinse out the tank. That would make all of the tanks absolutely uniform even post weld.

schu

[Mark Goldberg]

At the factory we are taking a different approach. A machinist will now cut the filler necks and open up the ID of the tube with his lathe to very close to 1.750 if not dead on. It seems like over the years (and not just recently) that the opening has been a little undersize. No one has yet reported an opening over 1.750. Just under. Mark

[schu]

Sure, but what about weld distortion?

[PLTemple]

Speaking of the gas caps, has anyone had any problems with venting? On one of my flights in the Patrol, lasting almost an hour, one side drew down but the other tank never did. I became concerned and headed back to land before the working tank got too close to being empty. I have been flying a Super Cruiser, PA-12, for 35 years, and never had a problem between the two wing tanks, I believe partly because it has a high crossover tube between the 2 tanks. The Super Cruiser, as part of the STC for converting to an O-320 has large vent tubes that come up out of the middle of each cap and face forward. I always make sure they are clear during preflight, but actually in many decades of the plane being outdoors night and day [on floats], I have never had any rain water or debris get in the tanks. I have a lot more confidence in that set-up than the tiny holes in the Patrol caps. And I like the redundancy of having the crossover tube in case one vent became blocked.

[Mark Goldberg]

If you think that one tank is not venting - blow through the gas cap to make sure the holes connect. If you can blow through and get air out the other side - then your cap itself is venting properly. Mark

[geraldmorrissey]

I have not measured my tanks but I will. A bore guage and a micrometer are the best method to measure using basic measuring tools an average guy might have. It's tough to measure an I.D. with a caliper.I.wonder how cylindrical the bore is after welding. Need measurement at 4 locations around the circumference. That might be the issue. The reamer sounds like a good idea.

[Archer39J]

Speaking of the gas caps, has anyone had any problems with venting? On one of my flights in the Patrol, lasting almost an hour, one side drew down but the other tank never did. I became concerned and headed back to land before the working tank got too close to being empty. I have been flying a Super Cruiser, PA-12, for 35 years, and never had a problem between the two wing tanks, I believe partly because it has a high crossover tube between the 2 tanks. The Super Cruiser, as part of the STC for converting to an O-320 has large vent tubes that come up out of the middle of each cap and face forward. I always make sure they are clear during preflight, but actually in many decades of the plane being outdoors night and day [on floats], I have never had any rain water or debris get in the tanks. I have a lot more confidence in that set-up than the tiny holes in the Patrol caps. And I like the redundancy of having the crossover tube in case one vent became blocked.

I've always wondered with a crossover tube, it has to bend down in the middle for the wing dihedral right? So what happens if some fuel splashes up and into that tube and what affect does that affect the ability to cross vent the tanks when needed? Seems like you'd occasionally want to drain it with a valve or something.

My thought is to run a vent from the outboard top of the main tank back to the front wing root and small valve for each wing. If I notice symptoms of a clogged vent I can open the alt vent for that tank and carry on, any trapped fuel would also drain (should be a preflight check). It also serves as a place to plumb a ferry tank if the need ever arises. Just an idea I had, have to think on it more.

[PLTemple]

Dave, regarding the crossover tube, I believe that if one tank vent were blocked, a pressure difference would be created, with higher pressure in the top of the tank with a working vent, and lower pressure in top of the tank with the blocked vent, and that would drive air through the crossover tube, from high pressure to low, in spite of any fuel within that tube. That has worked in Super Cruisers and Super Cubs for decades.

[svyolo]

The wings only have 1 degree of dihedral. I cross vented from the inboard side of the tank as the height difference was so little between inboard and outboard.

[alaskabearhawk]

I finally got around to measuring my filler necks. All I have is a digital caliper, so maybe not the most accurate method. I measured in two spots 90 degrees to each other in each filler neck. On my two aux tanks they measured 1.747/1.751 and 1.748/1.752. On my main tanks I had to weld 1/2in extensions on each tank so they would be proud of the top skin.. For whatever reason when we fit the mains we could not get them to fit any closer to the top skin to get the filler neck to poke out. They needed the extension, so these measurements are the mains with the extensions. 1.742/1.746 and 1.737/1.742. I did use a hone to clean up the necks after welding. I hope to get a chance soon to get some avgas in there and give the fuel caps a good shakedown.

http://www.mykitlog.com/users/display_log.php?user=yeeeha&project=527&category=8690&log=280651&row=21

http://www.mykitlog.com/users/display_log.php?user=yeeeha&project=527&category=8690&log=280652&row=20