Leaking 1939 Left Hand Gas Tank Valve

[Bill D.]

I have a 1939 Knucklehead and the left hand gas valve has leaked since I obtained the bike (about 1 year). The valve has a brass body and steel tapered plug which I believe to be a 3601-36. I am not sure if it is stock or aftermarket as I am not a knucklehead person. I lapped the plug to the body with 320 grit clover compound cut with light oil to thin it out (which was the finest grit I had), followed by AutoSol and then Brasso. When I examine both the body and the plug under a magnifier - both finishes look good but it still weeps at the front of the valve (the larger diameter). Does anyone have any ideas or methods for this type of issue.
You can respond to the forum or email to dok@valornet.com
Thanks in advance for any help.

[Chris Haynes]

Your problem is a common one. Some folks do some machine work and install "O" rings. I can't recommend anybody who does it though.

[Bill D.]

Thanks Chris - I am not surprised at your comment. Any debris would score the brass for sure. I am thinking about building a tephlon insert. I went online and it appears that the small vessel marine industry used similar valves but they also used an oring seal. I appreciate your response thanks.

[Eric MATHIEU]

I lapped mine with valve lapping compound and oil and then I pulled the spring to make it longer/stronger and now no more leaks.
Eric

[Bill D.]

left hand gas tank valve

Eric, what grade lapping compound did you use? Thanks I appreciate the insight. I am waiting on 1200 grit compound. When I lapped with the 600 grit I still had very minor seepage.

[Bill D.]

I made mistake on the grit I used it was 320 grit.

[ricmoran]

Was advised by another forum member either here or on CAIMAG by spreading the RTV completely over the cone, insert that into the female side and assemble it like you plan on using it.

Wait 1 day, then take the assembly apart, remove any excess RTV and then re-assmble the male to the female, installing all related parts. Don't remove any of the RTV that looks squished onto the cone becuase that is what will stop the leak. I used pipe cleaners to get the excess RTV off that had not hardened from all the fuel ports and the assembly.

I did this on my 39 that was leaking with the left fuel valve, and I tried a lot of different lapping compounds and fine grit polishing compounds and nothing worked.

This little gem of a tip did the trick. I used black RTV and so far, so good.

The forum member who advised me said he picked that up from an old timer at one of the meets, he had done this to several of his bikes and been going on 10 years with no leaks. I did that a couple weeks back and not so much as a drop has leaked from the valve.

I'll try to find the forum members name so he can garnish the appropriate credit for that tip.

[Bill D.]

Left hand Gas valve leak

That is really interesting. I can see how the RTV would fill the gaps in the body and plug. I am going to try the 1200 grit lap I just recieved. If that does not work - I have black RTV and that will be the next step. Thanks for the tip that was very helpful. I will report on the where I end up by July 4.

[silentgreyfello]

For the long haul, use the red, heat rated RTV. Your petcocks get pretty hot right next to your heads. This trick does work, but first lap the barrel. I put the extra fine lapping compound on the barrel, push it into the body, and grab the shaft with a cordless drill. Hold onto the body with one hand, pull the drill/barrel with the other. Keep doing it until you can see where it's lapping to your satisfaction. Clean with either brake or carb cleaner. Let it dry, then put the RED RTV on the barrel, push into body, install spring and let it sit 24 hours. Remove the barrel and clean out the pathways and re-assemble. No more leaks or weaping.

[villy]

Hi
Very interesting. But sorry, what is red RTV???, i'm located in Denmark, maybe that's why i don't know this stuff.
Thank in advance from here.
Villy

[cdndewey]

Hi
Very interesting. But sorry, what is red RTV???, i'm located in Denmark, maybe that's why i don't know this stuff.
Thank in advance from here.
Villy

It is a heat resistant silicone. If you can't get any over there PM me and I will send you some.

[ricmoran]

Larryridesagain over at the CAIMAG forum provided me with that tip.

Thanks Larry!!

[silentgreyfello]

Yep, same guy told both of us how to do it. I once modified a set with the lathe, cut a groove in each end, then put o-rings in each groove. That works as well, but the RTV method is much easier.

[Bill D.]

I was successful using the black RTV. I have a couple of observations: 1. Lapping is probably most successful removing scoring to the brass female tapered surface but it has a minimal effect on the steel male taper. So lapping is helpful in getting the brass surface in good order. 2. The rtv at least on my valve filled in the scoring on the steel male taper. This eliminated leaking externally as well as seeping into the carb in my case.
I clearly do not have the run time some of you have indicated but I am confident that this will work and it is a lot easier than cutting O ring grooves as Silentgreyfellow stated. Thanks to all of you who have commented on this problem.

[ricmoran]

Rode the 39 a week back and the bike was running really rough. I started looking at the fuel system because the engine was acting like it was starved for fuel. Well, the RTV seal did not hold and the petcock is leaking again. There wa smore going on than this, which the rough running issue has been rectified. The roughness was due to the float bowel loosening as well as the fuel filter assembly coming slightly loose and letting the fuel line rest against the rear cylinder.

Easy fixes, so now the next step is to use red RTV and then see how long that lasts.

Put about 3,000 miles on the bike since use of the black RTV, so perhaps the heat did fatigue the black rtv.

When I took the petcock apart there was zero evidence of any RTV remaing in the assembly.

Am thinking that perhaps a rough lap would be best, as this would provide the RTV with a surface for better bite and a better depth so the dry film thickness would be a better mate to each surface.

Not sure about having the spring tighter and am thinking if more RTV is left behind perhaps a weaker spring tension with just enough pressure to allow the RTV not to be worn with use of the petcock, that this perhaps would allow better durability with use.