Distributor Vacuum Lines

[Cartravel]

We started up the motor for our '41 pickup last weekend. It fired right up, with no problems other than needing a little tuning. We are running a crab distributor converted to electronic and an Offy 2 duece manifold. We need to hook up the distributor vacuum brake. I understand there are special threads in the front plate. I have bought a vacuum line with two of these special fittings. However, I'm not sure about the threads in the manifold. Are they the special Ford fitting, which measures about 0.362 with 24 tpi, or are they 1/8" pipe, 0.388 and 27 tpi? The Ford fitting seems a bit loose and the pipe fitting seems too tight. TIA.
Larry Young

[Cartravel]

Since I got no response, I dug into this a bit more. I understand the Ford fitting is designed so the tubing will be squeezed when the fitting is tightened. This occurs because both the fitting and the hole in the timing cover are tapered. The hole in the Offenhauser manifold is listed as 3/8 x 24tpi, which is a normal fine thread bolt. It is larger than the Ford fitting and is not tapered, so a Ford fitting should probably not be used. I'm wondering whether I should retap the hole to a pipe thread. What have the rest of you done?

[JSeery]

Sure an adapter is not intended to be used, something like a lot of master cylinders use? I don't know, but it is fairly common when going into a casting to use an adapter with npt into the casting and a flare female fitting on the other side. Believe this is so if the female flair part is damaged it can be replaced. Also, it is a lot easier to build the flair into an adapter fitting than into a casting.

[Bassman/NZ]

I have an Offy manifold. I tapped mine to 7/16 fine, drilled a bolt down the shank and across it where the little banjo would sit, similar to an original Ford bolt. A problem I had was the Offy manifold is not flat where the hole is, so I got leakage. Rather than pull it all apart again to mill the area flat, I just smeared a tiny amount of JBWeld around the edge of the hole and buttoned it up. Works fine.

[Binx]

As Bassman/NZ described it. Special fitting at the cover and banjo at the manifold. Are you planning on vacuum wipers?

Lonnie

[Cartravel]

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Speedway lists the 3/8 fine thread spec in their catalog. I tried a bolt in it and that seems to be correct. I can see why a banjo would not work, because the manifold is not flat around the hole. I plan to tap it to 1/8 npt and then use a 90 degree compression fitting. I wonder why Offy would choose to use a 3/8 fine thread??

[Ol' Ron]

I like the idea of 1/8 NPT in both. Makes it easy to find fittings.