Packard Forum Index » Post-War (1946-54)

Dear all,
i have a new brake light switch from kanter and it is sweating brake fluid a little bit at the thread. the old switch was assembled without any seal inside the hollow screw number 1.0100 . there was no leak.
my question: must there be normaly a cupper ring or something else? in the "master parts book" is no picture.
I have tighten the switch firmly. i am afraid to destroy the switch if i tight it more tight or to have a leaking break. do you have got any thoughts?
best regards Josef

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Unless it's different than the prewar or '50s era Packards, the brake light switch has a taper pipe thread so no need for any seal. Just be sure it's very tight but if it still leaks, then maybe the threads in the screw have elongated or worn out. Also, the brake light switch is a common part that can be purchased at Napa or nearly any parts house.

For a new replacement switch, NAPA Echlin SL 134. Thread is NPT, no sealant needed.

I had the same issue on my 47 with the SL 134. Even replaced the banjo bolt and still had a leak. Though it is not recommended I finally used a teflon wrap on the threads. You could also use a thread sealant as long as it is compatible with brake fluid.

What I believe may be happening is there is an incompatibility in the threads between the banjo bolt and the switch threads. I imagine Kanter might use a common modern switch replacement and Napa does not specify on their SL134 switch (aftermarket replacement) if it has NPT or NPS threads. O_D thinks it is tapered and it may be. I don't know what Packard used on their banjo bolt but halfway suspect it might be straight threads.

Here is a bit on threads from the Engineering Toolbox.engineeringtoolbox.com The first line is all that Napa says on the SL 134. The remainder is info from the Engineering site. Note the thread dimensions are nearly identical but the telling factor is the taper takes an R drill for the thread tap which is .339 and the straight takes an S drill at .348 so slightly larger. If Packard used straight that slight difference in hole size along with the thread differences could be the source of the leak.

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Determining if it is straight threads is first principles. Go to the junk drawer and see what nut spins onto the thing.

Using a pipe thread on a high pressure system with a banjo bolt doesn't seem ideal because if it bottoms before achieving clamping force the thing will leak and if it clamps before sufficient interference it will strip.

However, anything is possible as the V-12 used a banjo bolt on the anti-flood and a screwball Packard specific pipe thread to boot. But that was a low pressure situation.

As well, who knows what kind of crap is being sold these days. I had to order a part for a piece of equipment and there are programs like Junkazon that will take your search number and plug it into any thing they have in a storage unit somewhere. Management didn't like the price of the OEM specification part I found from a reputable vender so they went with a Wal-Mart drop shipped item. I told them they could get one even cheaper on Junkazon but who knows what piece of garbage they will get. My neighbor bought a new Toyota after having all kinds of problems with bad starters on her Jeep. I didn't have the heart to tell her you have to cough up top dollar for parts from the dealer because the aftermarket is full of junk. And Chrysler has the worst markup over the aftermarket.

The world in in a race to the bottom. The quality control engineers have all been fired and industry is in liquidation mode.

yes, thank you very much. all these thoughts are very helpful. my cylinder is overhauled, so the thread can be worn out. as well as the thread of kanter may not fit exactly. i will test the idea with the teflon tape quickly. it is easy and the resistance of teflon is very good. thank you very much for your help ! best regards Josef