Guys,
What is a good sealant for the NVA to case area? I have had some issues with the seal where the NVA goes through the case/venture on my Fox 35. I am using a Randy PA Fox NVA. This is an excellent needle valve with good needle seal and fine adjustability. On the needle side there is a boss that has two flats cut in to allow a wench to hold it for tightening. The area of the boss by cuts is very small. If not sealed as the case heats any amount of leakage changes and the setting can go erratic. What sealants work for this?

Best,    DennisT

Hi all.

Tom Dixon told me about 20 years ago (throgh letter, as today) the recommendation to use a drop of epoxy in the área between the case and NVA in order to seal the joint in Merco 61. I´ve used that without a fail. Removing it is simple heating a Little.

Hope it helps.
Regards.
Manuel.

Regular old dow Silicone II solicone sealant appears to work fine, although clear is a problem because you can never see where it goes for sure. I use Ultra Copper since I have it anyway, and, you can tell when you get it in the wrong place, which can be disastrous.

     BTW the value of sealing the venturi to the case for the OS/Fox/McCoy "through the middle" spraybar is minimal. Even if it leaks, it has no real effect unless the leak is intermittent. It can have a drastic effect on the ST/PA/RO-Jett style sprinkler-type venturi, it is exactly identical to having a leak in the fuel line. You just have to be very careful to only get it where it needs to go, and not into the manifold or spraybar itself.

 Interestingly. most of the difference is detectable on the ground when needling it, or just sitting there while you are walking out to the handle - it will just surge up and down. If you aren't changing something, it should just sit there at a constant RPM, or *slowly* speed up or slowing down. If not, something is leaking somewhere.

      Brett

The issue I have been fighting is very close to what Brett described. The engine (Fox 35) will start set in a good 4 cycle then as it continues to run starts going leaner. If you richen it is will go back to the deep 4 cycle, if you try to fly it will either be sloppy rich or if launched a bit closer will fly great for a few minute then lean out.

I have changed tanks three times, change the engine out, changed all the tubing. Only thing that is common that was not changed (in both engines) was the PA NVA. Looking at the NVA, needle side where the wrench flats are cut, there is no real shoulder to seal along the case boss. The sealant (Permatex gasket sealant) I used seems to be dissolvable by fuel (finally tested that today). I think some got washed out on the bottom edge (I had the venture fill during fueling). I will get the Ultra Blue or Copper and try again. 

Best,    DennisT

Ara,
Great idea with the Teflon tape. My problem is that with the PA NVA there is really no area to seal against the case boss where the flats are cut. If I make the shim washer and solder it to the NVA then the Teflon tape will work.

Best,    DennisT

Hi.
Sorry but I don't understand what kind off stuff you are talking about if there is an issue with sealant dissolving in fuel. That kind of things should be avoided.
Also, the pressure difference is not so massive, it's usually enough just to smear some sealant on outside of engine/valve connection, just to make a kind of a fillet. I like the red Plastik Padding "Gasket" silicone sealant. Any similar silicone will work but red is nice as you can see where it is. If you don't see, get a magnifying glass.
Also, it's important to apply the sealant to clean and dry surfaces.
Other reason why you don't want sealant inside the joint is that it's very slippery, and you need to tighten the valve much tighter to avoid rotating. Some engines, like Retro, are sensitive to this because over-tightening will deform the venturi hole and engine front end, and screw up the bearing play. In this case also a snug fitting aluminium venturi helps. L

The issue I have been fighting is very close to what Brett described. The engine (Fox 35) will start set in a good 4 cycle then as it continues to run starts going leaner. If you richen it is will go back to the deep 4 cycle, if you try to fly it will either be sloppy rich or if launched a bit closer will fly great for a few minute then lean out.

      That's overheating or worn out, not a leak. The surges from leaks take place over the period of a few seconds, up and down. What is happening here is something heating up over the period of 10+ seconds, if I understand correctly. When you launch it, it cools back down, making it go richer (usually).

    My first suggestion is to put in more oil, to cool off the engine, or to alter the cowl (full-fuselage) or a cylinder baffle (profile).

      It would be very unusual for a case/spraybar leak on this type of spraybar to be a problem.

      Brett

OK, changed the NVA to a ST 51 NVA with an added washer to center the hole, sealed with black silicon. NVA was positioned with the hole facing rearward same as with the other NVA. Beside the NVA itself the other change was the sealant. All the Fox's I seen have a very small boss on the case to seal against and it is not always perfect and in my option needs sealant. I had used a material from Permatex called gasket sealant. This material never hardens, later I did a test that showed fuel could easily wash it away. With the black silicon it cures and although it is not nitro proof it doesn't dissolve the way the gasket sealant did. I believe this is the real culprit, not the PA NVA but I already changed it so I decided to fly it to see if anything would change.

I went to the field today and flew. This is my El Diablo, open front upright engine, uniflow tank, muffler pressure, 7% N;28% oil (76C/25S), 9x6 Top Flite spoon blade prop. Engine started 1st flip, initial setting was very rich, l leaned to 9500 rpm, kind of just breaking 4/2 for launch. In the air went to a rich 2/4, lap time was 5.0 (60' C to C). The run was totally solid, not a 4-2-4, more of a 2/4 -2- 2/4 but the motor never ran away was almost like flying electric. I got though the whole OTS pattern, totally solid. Upright/inverted laps the same. For me the big test was the 5 inside and 5 outside loops. This is were it would go off before around the 4th loop. Not today just solid. At the end of the flight by the time I walked the ship back to the pit you could hold the cylinder, it was warm but not real hot. Oil was nice clean castor color, no signs of excess heat.

Now that it seems to be flyable I would like to try and get the lap time down to around 4.9 sec. To get a few more rpm to make up for the wider ST NVA I will try to bump the nitro to 10% or play with the prop, try a 10x6 or up the pitch up the 9" dia to 6 1/2 pitch. Last will go back to the PA NVA with the silicon sealer.

Best,    DennisT

I use Red Hi-Temp Permatex silicone for headers and mufflers.  So I use that also to seal spray bars.  I figured out the cap cracking issue, solved it by squeezing out some sealant, screw cap on lightly, let the sealant harden an form complete seal to not let air get to rest of tube.

The needle is set to the total airflow through the venture. A air leak around the spray bar makes no difference as long as it is consistent. The aluminum crank case will expand more than the steel or brass spray bar. The spray bar will tighten up.

Phil Bare

Ive seen some tough redish rubbery stuff on a few S/H Engines I have , just ' clicked ' seeing this thread I was going to keep my nose out of ,
probably PLIOBOND , which you cant get in Aus. As a glue any coments I have would be libelous .

As A Gasket Cement , in the time of NOAH , the top British Bike Overhaul outfit used it , guess wot. No Oil Leaks . 

Bit of a swine to remove from casings , but heat proof on a case . Semi Permanent Semi Elastic . So worth a try .
While on the old torrey cannyons , Copper Coat - copper graphite - pressure sealing compount , and copper head gaskets ,
If youve got a leak with that , somethings seriously wrong . Its also Anto Sieze Paste ! , so comes apart easy & clean
if used on all other gaskets . and good for anything ( bolts, nuts , gaskets ) at the Exhaust Ports . For these tooned pipe flange individuals .   

take an old N/V body .put it in the engine worth some fine valve lapping compound and hook up a variable speed drill to the other end .start drill and pull the N/V head against the body and it should face it off .be sure to clean all the compound off.