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Post  Marleysky on Wed Aug 14, 2019 8:01 pm

Bob A wrote:It seems to me that if an accurate size measurement and/or correct stock number could be found for the Cox venturi O rings, that a lifetime supply of a box of 100 or more could be purchased at a reasonable price from a commercial O ring supplier. Has anyone ever done this? Bob A

I can’t put my finger on it now, but there is a “o” ring used in darts that is a perfect fit. I think this is the one:

https://www.ebay.com/itm/20-Dart-0-rings-2ba-3-16-Rubber-O-Rings-for-Darts-flight-Shafts/140731004118?hash=item20c438b4d6:g:VWsAAOxyjNlR0Mek
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Post  RknRusty on Wed Aug 14, 2019 9:10 pm

Those little venturi gaskets are actually flat washers, aren't they? Maybe they are the same size as the short pieces of black fuel tube that Bernie sells. Like for the newer PT-19 snap-in tank. They could be sliced off to fit.
https://coxengines.ca/cox-.049-neoprene-fuel-tank-line-10.html
I've never tried it, but it's worth a look.

I looked at Matt's site too(exmodelengines.com), but may have missed it.

Bob, you'll see us refer to Bernie and Matt often. They are two of our top vendors that work closely with us.
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Post  Admin on Wed Aug 14, 2019 9:23 pm

Little round o-rings will work. I've used ones I pulled from empty butane grill lighters before. Original venturi gaskets are flat squared off. I have some here I got from Matt (Ex Model Engines) that are like the originals.

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Post  dckrsn on Wed Aug 14, 2019 9:40 pm

Every time this comes up, I order 25 more. Sad
I probably have........... a lot by now.
https://www.amazon.com/25-Orings-2BA-inch-Washers/dp/B000JEQ4CK
These work well in PeeWee .020s.
Mark and I take turns posting the link.
Bob


Last edited by dckrsn on Tue Aug 20, 2019 3:43 pm; edited 1 time in total
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Post  Bob A on Tue Aug 20, 2019 12:34 am

I ordered a pack of 25 of the recommended "dart shaft" 3/16" rubber O rings through Amazon last week and received them promptly from O-Ring Depot in Tomball TX. I fit tested them VS a couple of original cox rings I had left. The cox rings aren't true O ring shape, but are actually square or rectangular in cross section, just like the ones we make. The dart rings are a true O shaped cross section and a bit thicker than the Cox. The dart rings fit the tank recess nicely but end up protruding a tad more than the Cox.

So slightly more compression is needed to pull the tank against the backplate for a seal. This probably results in a better seal of the ring itself, but requires a little more tension on the four screws to seal the backplate. I've seen lots of caved in back-plates from over-tightening those screws, so a delicate touch is needed. So far it appears that this could be the better solution than making them, but testing and time will tell.

A minor disappointment was that there are 20 in the standard pack, not the 25 advertised. But at just $1.66 total shipped cost they were certainly affordable. Bob A
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Post  Levent Suberk on Tue Aug 20, 2019 8:25 am

I think that o-rings can be a little flattened with wet&dry sandpaper on a hard surface, like a glass plate.
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Post  rsv1cox on Tue Aug 20, 2019 10:27 am

I don't know guys, but I don't mind paying a bit more to Matt and Bernie ($1.95 for 3) if for no other reason but to keep them in business.  And those that read my posts know that I'm not adverse to saving a buck or two.  The money involved isn't going to send anybody to the poor house.  That said, it is somewhat painful to know there are other less expensive options, however...…….and

I'm not telling anybody here anything, but there is a recess that the venturi gasket fits into, it just doesn't sit flat on top like some imitations that I see.  It is my most frequently replaced part and always is an add to of any order that I make. The "factory" part fits.  But I am curious to know their supplier or perhaps it's new old stock.

Just saying

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Post  KariFS on Tue Aug 20, 2019 1:28 pm

You’re right Bob, the ”commercial” ring fits properly, that is, it sits in the recess of the tank venturi, allowing it and the backplate to be tightened ”metal to metal” and the ring gets squeezed just the right amount.

Nothing flexible is left between the venturi and the back plate, just like it is supposed to be with any o-ring sealing arrangement. Gasket would be a different story, as gaskets are obviously meant to sit between the two surfaces, sort of isolating them from one another. Different construction for different application. It does work, though.

I have also tried to make those from fuel hose. I used small enough hose that stretched around the thin part of the venturi and fit in the recess. They might work, but I also like to use the ready-made ones. They are not THAT expensive, although when I had a lot of engines to fix, the $20 for a bag of 30 little tiny-weeny rubber rings did feel a bit steep Money Toss
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Post  rsv1cox on Tue Aug 20, 2019 1:51 pm

KariFS wrote:You’re right Bob, the ”commercial” ring fits properly, that is, it sits in the recess of the tank venturi, allowing it and the backplate to be tightened ”metal to metal” and the ring gets squeezed just the right amount.

Nothing flexible is left between the venturi and the back plate, just like it is supposed to be with any o-ring sealing arrangement. Gasket would be a different story, as gaskets are obviously meant to sit between the two surfaces, sort of isolating them from one another. Different construction for different application. It does work, though.

I have also tried to make those from fuel hose. I used small enough hose that stretched around the thin part of the venturi and fit in the recess. They might work, but I also like to use the ready-made ones. They are not THAT expensive, although when I had a lot of engines to fix, the $20 for a bag of 30 little tiny-weeny rubber rings did feel a bit steep Money Toss

Maybe Bernie and Matt are listening.  Maybe 5 to a bag vice 3.  

But they (maybe them) have to pay someone to stick the little critters in a bag, a most aggravating task with many getting lost in the process.  

I would have liked to be the floor sweeper at the Cox factory back in the good old days.  I heard of dumpster divers there too.

Bob
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Post  Cribbs74 on Tue Aug 20, 2019 10:12 pm

The dart shaft O-rings are an old remedy and a very good one. They provide a superb seal and last nearly forever. The price is super cheap as well. I learned of this several years back from Mark Boesen and never looked back.
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Post  coxaddict on Wed Aug 21, 2019 1:11 am

Quick and easy way to make venturi gaskets. - Page 2 Dscf1310
Where the seal sits is very shallow as shown in the cutaway.  The original style fits the best.
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Post  KariFS on Wed Aug 21, 2019 1:47 am

rsv1cox wrote:

Maybe Bernie and Matt are listening.  Maybe 5 to a bag vice 3.  

But they (maybe them) have to pay someone to stick the little critters in a bag, a most aggravating task with many getting lost in the process.  

I would have liked to be the floor sweeper at the Cox factory back in the good old days.  I heard of dumpster divers there too.

Bob

Yeah, I get aggravated even when I try to fish one out of the bag… And in the bigger scheme of things, the cost is not a huge deal. After all, I may sometimes drop 80€ for a slightly dinged RR-1 that I have no intention to fly. An ornament of sorts. So an 80 cent O-ring as a part of overhauling the said ornament… Oh well Wink

Besides, I think they have a decent deal for larger amounts ($5.85 for 10). Mostly the issue for me, if there even is one, is shipping and customs. I try to save on shipping which leads to ordering large amounts of stuff at a time, I have quite a backlog and try to anticipate future needs, so the little bubble-wrap envelope costs me easily $200 or so delivered. So I get a little freaked out occasionally.

But yes, I still use the "proper" rings, but might look into the darts rings, especially if I really get into flying these things and not just collecting and polishing Smile
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