Question for anyone with mid '70's 302 log style exhaust manifolds (off the engine)

[Boomyal]

Does anyone happen to have one of these off of the engine? I need someone to take a look and determine if there is enough meat to safely enlarge the drain plug hole for a 1/4 npt plug. If you do and can make that determination, you might also note and report how deep the iron threading surface is in that area.

My brass 1/8 npt brass plug was striped so I lightly ran a 1/8 npt tap up the hole to clean the threads. Without much effort the tap ran in further than will allow a new plug to tighten. From the feel of the tap, I now have good threads in the mani but they are too big. I really wished that someone made an oversized stainless steel 1/8 npt plug or a 3/16 npt plug but alas a google on the subject produced no results.

I do not want to have to take the manifold off to do this. I have sufficient access from underneath to drill and retap a new hole. It's just that it is down too low for me to make a reasonable determination if a 1/4 npt hole is safely feasible.

 

[Boomyal]

Re: Question for anyone with mid '70's 302 log style exhaust manifolds (off the engin

Well being as no one could assist I have taken an intermediate step. Instead of going straight for the bigger 1/4 npt hogging out, I am going to try a 7/16 x 20 straight thread stainless bolt with a heavy nylon washer. This way I will only have to retap the hole a smaller amount. If that does not work then I would still be able to do a 1/4 npt tap, available meat not withstanding..

 

[Don S]

Re: Question for anyone with mid '70's 302 log style exhaust manifolds (off the engin

You are going to have to drill and tap dead straight. If angled, the bolt and gasket will only hit one side.

 

[Boomyal]

Re: Question for anyone with mid '70's 302 log style exhaust manifolds (off the engin

You are going to have to drill and tap dead straight. If angled, the bolt and gasket will only hit one side.

Good reminder Don. I will start with the smallest bit that will take a bite and progressively use larger bits. It will probably only take about 3 fractional or numbered bits to get to the required final 25/64. The theory being that A) they drilled the original NPT hole straight to begin with, and B) that starting with smaller drill bits they will want to follow the existing hole better. Unfortunately I do not want to take the time to remove the manifold and make a squared hole using the drill press.

This would have been a whole lot easier if only they had made 3/16 npt plugs or 1/8 npt 'repair' plugs with a longer (ultimately wider) taper. Grrr!

 

[Boomyal]

Re: Question for anyone with mid '70's 302 log style exhaust manifolds (off the engin

The manifold drain hole repair was a success. I used a 7/16 x 20 stainless bolt with a thick nylon washer and 'rectum' seal slathered on the threads. The re-tapping of the hole was easy and the bolt tightened up nicely and does not leak. Now, if ever necessary at a later date, there will still be room to re-tap for a 1/4 npt plug.

 

[Nivekt]

Re: Question for anyone with mid '70's 302 log style exhaust manifolds (off the engin

The manifold drain hole repair was a success. I used a 7/16 x 20 stainless bolt with a thick nylon washer and 'rectum' seal slathered on the threads. The re-tapping of the hole was easy and the bolt tightened up nicely and does not leak. Now, if ever necessary at a later date, there will still be room to re-tap for a 1/4 npt plug.

Happy to hear of the success. Could you post up some pictures detailing the challenges and solution so the rest of us have a better idea of what happened? Im a little lost in the details as I have never worked on this type of manifold before.

 

[Boomyal]

Re: Question for anyone with mid '70's 302 log style exhaust manifolds (off the engin

Happy to hear of the success. Could you post up some pictures detailing the challenges and solution so the rest of us have a better idea of what happened? Im a little lost in the details as I have never worked on this type of manifold before.

Nivekt, it would be a little hard to take any pictures as the bottom of the exhaust manifolds are too close to the stringers to get a camera aimed at it. Suffice it to say that each of the Log Style OMC Ford mani's have a 1/8 npt drain plug on the bottom, just a little off center from end to end. It is on one of these that the the pipe threaded hole became compromised with age and ??rust.

An attempt to clean up the threads caused the tapered Pipe tap to drive too far into the hole making it too large to grasp the threads on a new 1/8 pipe plug. I had hoped to find an oversized 1/8 pipe plug because my hole threads were now sound, just too large. No oversized plugs were to be found so my only seeming option was to go up to a 1/4 npt pipe plug and drill out and re-tap the hole accordingly.

I did not like this option because the 1/4" pipe plug required a huge jump in hole size. I found that a standard 7/16 x 20 tpi bolt was very close to the enlarged 1/8 pipe plug hole. By feel, I was able to get the 7/16 pipe tap up the enlarged hole and there was just enough meat there to cut new threads. I did not even have to drill the hole larger to use the 7/16 tap. Now the question became, will a straight thread bolt with a heavy nylon washer effectively seal any exhaust cooling water from passing thru the threads. It appears that it was a success. I did use a Stainless Bolt and applied a pipe thread compound on it for added insurance.

I am not sure what kind of exhaust mani's you have on your '79 GM motor but I am sure that they would have a drain plug in a similar position. My Ford log style manifolds are ambidextrous. That means when you remove the fore caps and the rear elbows, you could swap the manifolds to either side of the motor. Yours may be similar or yours may be the more squarish rear risor style that have a dedicated left and right manifold. I do know that they would have a similar drain plug or petcock somewhere around their center.

If you are in a freeze zone, these plus must be removed to drain the water out lest they freeze and break. Like the engine block drains, you need to stick a little pocket screw driver up the holes to make sure that any flakes of rust do not block the water from draining out.