Best sealant?

So, working on my sons old (1942) flat head Harley. The intake manifolds are a threaded nipple that is then held in place by rivets that are put in holes that are radial to the bore. The rivets are installed from the inside and then peened over from the outside. These need to be air tight after peeing or you have a vacuum leak. I have tried twice now, but always end up with a small leak when we pressurize it and test it.

So, what would be the best sealant to use on the rivet? The river and the nipple are steel. Heat, gasoline and pressure are present.

Thanks in advance.

 

Well shit......Ducati ThreeBond it is then.........and I take my thanks in advance back

 

I was going to say Yamabond 4 (AKA Gray Matter)
Same or very similar stuff probably

 

put the sealant on the outside since its going to be under vacuum not pressure??

 

I would think this has been solved in the past 70 years...

 

You would think so but the Harleys in my shop would tell you other wise.

 

I would use sealant like Dirko sealant.
We use it on regular basis when we are putting Stihl two stroke crank cases together, they don't use gaskets. As you know for those little engines to operate properly they need to be air tight and Dirko delivered that to us so far at same time being exposed to high temperature, gas, oil...