Repairing rear exhaust test ports

The test ports on my rear exhaust header are stripped out (2 of them).

I had my system jet-hot coated and it looks great, but during the process the plugs on two of the rear ports got stripped out (not their fault).

On the front bank, I had to cut everything out to get it appart so I welded those closed. For some dumb reason, I did not check the fronts carefully enough so now I've got two ports where the small screw/plug will not stay in the threaded hole.

I cannot drill them out and weld b/c the heat will damage the coating.

Anyone have a suggestion?

Would it be possible to re-tap the hole to clean up the threads and put in new plugs?

Where could I find new plugs?

 

I had some factory plugs in the rear bank that rattled out...

I dropped by Vintage Motorcars and Mrs. Ferda matched up the proper hex bolt, I greased them up and threaded them in....

Come by and pull one out, if you need to buy a tap....

 

I'm thinking the tap and bolt thing may be my only choice.

I should have just welded them closed to begin with. Problem with a tap is the hole is so shallow that I'm not sure I'll be able to get to many threads turned.

 

Yes, you'll need what is called a bottoming tap (which you can make out of a regular tap by grinding most of the point off).

Can you explain this a little more? The stock thread size of the sampling port is M12 x 1.25P -- is that the thread size of the plug that you say will not stay in (i.e., is what you have actually a "close", but incorrect, Imperial and/or Pipe threaded plug)? Or do you still have the tube fitting in the sampling port and your "small screw" is just blocking where the tube was cut off? Anyway, just a little surprised that a fairly large M12 x 1.25P thread could be easily stripped out.

 

I have some J.B. Weld.....

Seriously we could tack weld it in and touch up your coating....

 

Steve,

On the sample ports for the front (near the fire wall) the OEM had some brass fittings and tubes already threaded into the header system and the sample tubes extened up the fire wall and were clustered near the back distributor. On mine, those were corroded to the point that removing them caused the fittings to break off. I had to drill out the brass fittings which were corroded into the sample port and then weld closed.

On the sample ports for the rear, they just had a blank plug/bolt threaded into them and the extension tubes were included in the OEM tool kit.

If the port is just a simple M12x1.25P thread then a bolt solution will most likely work. The plugs that go into those holes are not very robust and I doubt they've been removed in the last 20-years (my car was sitting for ~10-years before I bought it and I've done a lot of maintenace fixes/upgrades to mend the old stuff like fuel/cooling hoses, etc.).

I'll just need to verify the thread pattern and find a M12x1.25P tap to recut the threads and put in a new bolt and washer to blank it off.

I've got some photos and home and I'll try to remember to post them later.

 

One downside to using a M12 x 1.25P bolt is that the head will be quite massive (and overhang the diameter of the female port) -- PN 110860 looks a lot cleaner IMO (especially on the very visible 1-4 sampling ports of a 308):
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My original plugs, that rattled out and away, were actually topped with a knurled knob????

 

Probably you are correct, but maybe not for $18 each!

 

You can do a little better on price at www.allferrariparts.com, and www.eurospares.co.uk still has the "sane" price (if you can get around the shipping/duty cost) -- to each his own (in this case, I vote with Shamile that a little extra cost is warranted as you aren't going to find an M12 bolt that's 8~10 mm long ).

 

Use a bottom tap - these taps are designed to cut threads almost all the way to the bottom of a hole. My have to go to a specialty tool company to find what you need.

 

Anyone know where i can find an M12x1.25P tap?

My FIL looked at his place of work (does large diesel engine rebuilds) but they did not have that size.

the "P" is metric "pipe" thread, is it not? I doubt this ports will ever be used again so I'm thinking of just using a standard size tap and a headless set screw to fill the hole.

 

Did you find some bolts??

Vintage Motorcars had some in the Bolts Bin.......we can cut them and grind them down, if you don't want them sticking out....

 

No, the "P" stands for "pitch" -- the more correct designation (IMO) is M12 x 1.25P, but people often shorten it to M12 x 1.25

M12 x 1.25P isn't an "oddball" metric size -- www.mcmaster.com has them.

 

I'm trying to tell him, Steve....LOL!

It's just a bolt...they had some pretty short ones...

 

Steve, thanks for the info,

Alan, no, I've not found bolts. Does Vintage just have some regular bolts in a bin?

Here's another question, are the injection ports for the air pump system on the head the same size? If so, I've got 8 of those plugs sitting on my dresser so that would solve that problem.

 

Yep, Mrs. Ferda just handed them over!

Not sure, on the comparison to the injector plugs, they were machine though with a 'special end" I don't think I'd burn them when a regular bolt would do....