For the true gurus...what is this o-ring.

I've attached a Vans Aircraft photo of inlet and outlet fitting they sell for the Lycoming mechanical fuel pump. These fittings screw into the fuel pump with a straight thread that incorporates an o-ring. I believe the same o-ring is used on each fitting.

I want to replace my o-rings. Does anybody here know authoritatively (not guessing) what spec these o-rings are.

I suspect they are either an MS29513-112 or they could be an MS29512-06. Or these could be interchangeable.

Vans doesn't seem to list these o-rings as spares. Feedback would be appreciated.

According to Spruce, it’s a MS 29513-111.

Maybe check with Tom at TS Flightlines.

I replaced mine recently. They aren’t actually milspec. Although it references multiple MS part numbers on aircraft spruce website, an inquiry directly with spruce resulted in the part number 3-906 NBR


You can purchase these from you local bear / seal shop.

Once again, not a milspec seal.

Hope this helps

A variety box of viton o rings should get you the size you need. Summit, jegs, Russel, etc. all sell these straight thread to AN adapters and they all use generic viton o rings.

Lycoming lists them as MS29512-06, but that was an old number. Most certainly from a different material now, since the fuel additives probably have changed. Industrial fittings use an elastomer/viton/buna neoprene---or did.
I dont recall hearing of any fuel leakage with a stock o'ring, and I'll bet most arent changed at conditional inspections.

Tom

Why do you want to change yours? Is it leaking? As noted, I don't think anybody (or most anybody) messes with these at annual CIs, other than to ensure they're still tight.

If it ain't broke, don't fix it...

RV7A Flyer said:

Why do you want to change yours? Is it leaking? As noted, I don't think anybody (or most anybody) messes with these at annual CIs, other than to ensure they're still tight.

If it ain't broke, don't fix it...

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I'm getting fuel pressure fluctuations and trying to track down the culprit. Next suspect is the o-ring on the mechanical fuel pump inlet. Do I think that's really my problem....probably not...but the next culprit is the fuel pump itself. Obviously cheaper and more sensible to replace the o-ring and then the fuel pump in that order rather than the other way around.

O-rings

I use the MS29512-06 o-rings on those. Check for loose connections between the tanks and the fuel pump. A loose connection on the suction side can sometimes leak air but not fuel. Look at all your tubing flares and make sure they are not cracked or not flared properly, or even over tightened and damaged. Fuel pressure fluctuations are not that uncommon and sometimes its a sender or indication problem. Also an old fuel pump (5-6 yrs) can cause that problem because the diaphragms get stiff and the check valves and springs get weak.

The GAHCO catalog lists the MS29513-XX as "Aircraft fuels/Standard sizes" and the MS29512-XX as "Aircraft fuels/Boss sizes".

For the fuel pump fittings you are showing, these are "boss sizes" in that the o-ring is trapped between the flat end of the fitting (or nut and washer for adjustable angle types) and a chamfer in the straight thread female component (MS33649).

So the MS29512-XX is the correct specification for these fittings. Incidentally any AN fitting has an unthreaded shank next to the nut and can also be used with these o-rings in a "boss type" straight thread female component.
Selected sizes are:
O-ring Tubing Nominal OD O-ring ID
MS29512-03 3/16 0.301
MS29512-04 1/4" 0.351
MS29512-05 5/16 0.414
MS29512-06 3/8 0.468
MS29512-07 7/16 0.530
MS29512-08 1/2 0.644
MS29512-09 9/16 0.706
MS29512-10 5/8 0.755

Updated specs, for example AS5202 use a 900 series o-ring number. Parker's numbering for this series goes from 3-901 through 3-932. These o-rings are also used in the SAE J1926 industrial straight thread tube fittings. In the typical sizes used in our airplanes the o-rings and fitting thread sizes are:
O-ring Thread Size O-ring ID
3-901 .250"-28 0.185
3-902 .312"-24 0.239
3-903 .375"-24 0.301
3-904 .437-20 0.351
3-905 .500-20 0.414
3-906 .562-18 0.468
3-907 .625-18 0.530
3-908 .750-16 0.644
3-909 .812-16 0.706
3-910 .875-14 0.755

The elastomer compounds are suitable for aircraft fuels (avgas, jet-a etc.) so trying to substitute a viton compound is not necessary.

Do you have an inverted fuel system / flop tube on either, or both, tanks. Could be sucking air due to a old perished hose?

Dragonfly said:

Do you have an inverted fuel system / flop tube on either, or both, tanks. Could be sucking air due to a old perished hose?

Click to expand...

Very interesting suggestion...but no, I do not have flop tubes.

I think fuel fluctuations are common on rv's. Mine reads from 2 psi to 8 psi when electric pump is off and running on mechanical pump only. I believe lycoming's minimum is .5 psi. After researching some old threads it seems to be normal for some of these carburetor equipped rv's to have variable fuel pressure readings. Don't have a clue why they would do that. if you find out it would be interesting to hear what you find.

nedricher said:

I think fuel fluctuations are common on rv's. Mine reads from 2 psi to 8 psi when electric pump is off and running on mechanical pump only. I believe lycoming's minimum is .5 psi. After researching some old threads it seems to be normal for some of these carburetor equipped rv's to have variable fuel pressure readings. Don't have a clue why they would do that. if you find out it would be interesting to hear what you find.

Click to expand...

Mine is Precision fuel injection.

In Conclusion

Initially I went to Vans Aircraft to get this O-ring information because they sell the inlet and outlet fittings for the mechanical fuel pump. But they do not sell spare O-rings and they did not know the part number.

Next I went to Aircraft Spruce and on their website they nominated the O-ring as an MS 29513-112. But I had reason to believe that information was not correct (and I’m super fussy about fuel supply systems).

So I turned to VansAirforce and I thank everyone who posted a response. The people who knew that the correct O-ring was an MS 29512-06 were Randy Richmond, Terry Edwards (who submitted a wealth of data) and of course Tom Swearengen of Flightlines ( who kindly emailed me a page from the relevant Lycoming Parts Catalog). It would be remiss of me if I did not thank them all.

VansAirforce at its best.