VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1933-1936 36 207 Engine Drain Cock

I got a problem trying to replace a drain cock on the left side of my 36 Engine. In my hast I inadvertently grabbed the wrong tap to clean out the threads. I used a 3/8 x 24 when I should have used a 3/8 x 18. Where do I go from here? Can I re-tap using a 3/8 x 18 knowing that the threads are reduced at best and will it seal? I feel like a real knuckle head because I know better. Just looking for a little advise before I go any further. Thanks for any assistance.

Tom H.

It might be best to install a heli-coil to get it back to the size you want.

The taps you mntioned are for bolts. I would think the drain coc threaded into the block with 1/8" pipe threads and is tapered.

I agree!

Gentlemen,

I do appreciate and I am grateful for all the comments but my question is where do I go from here? Worst case is that since it is threaded for a bolt I could just put a bolt in it but I admit I would much rather have a drain cock. What in you opinion's do I do to put a drain cock back in ???
I have to admit that this is kind of embarrassing.

Thanks,

Tom H.

Here is a possibility.Try to re-tap the hole with a 1/4 npt tap(pipe thread tap.Install a 1/4 npt bushing which is reduced to 1/8 female npt.Then install the drain cock into the bushing.You will probably need thread tape on the bushing where it goes into the radiator,but it should work.The bushing will be 1/4 npt male and 1/8 npt female.

A bolt is straight thread and would need a sealing washer. If the original size drain cock will not thread in and seal then about the only thing I can think of is to go to the next largest pipe thread size and then use a bushing to adapt to the correct size or possibly a larger drain cock.

In about 1948 they increased the drain cock pipe thread to 1/4" .You can determine the proper drill size and pipe tap and get a drain cock with 1/4" pipe thread.....for a later model Chevrolet.

Load up your original 1/8 NPT drain cock with teflon tape and see what happens when you try to screw it in. It might seal up and you have nothing to lose. The brass drain cock will take the damage if there is any.
You could also re-tap with the 1/8 NPT and then do the massive teflon tape wrap. Good luck.

Mike

I think the cleanest fix is to retap the hole to 1/4 NPT (National Pipe Thread) and install a 1/4 to 1/8 reducer bushing. Then, you are back to an original 1/8 NPT threaded hole for the drain valve.

Even though these threads are tapered, I suggest using some pipe thread seal compound (like RectorSeal) on the threads instead of teflon tape. In my experience, the compound seals better.

Cheers, Dean

If the hole will clean up with a 3/8 drill bit, you can install a 1/8 NPT heliCoil to bring it back to standard.
I’ve had good results repairing worn out carb inlets with them.
Helicoil kit P/N 5407-2
Bill