Exhaust Stud How-to

This write-up on replacing V6 SHO y-pipe-to-manifold studs was posted to the V6SHO list by Alex Winbow on 22 Nov 2008:

Factory Hardware

My stock exhaust studs were plain steel, tapered at both ends and with a integral hex nut "shoulder" fairly far down the stud. The short end screws into the exhaust manifold up to the shoulder, and the y-pipe fits over the long end up to the shoulder and is secured by a nut. The exhaust manifold holes for the studs are tapped all the way through, and accessible from the upper end as well (though with little clearance). Thread is M10x1.5.

Supplies Used

[from McMaster-Carr:]

• (4) M10x1.5 70mm 18-8 stainless steel studs (p/n 93805A434) @ $3.61/ea
• (4-oz can) nickel anti-seize lubricant (p/n 1028K63) @ $10.83

[from my hardware/fastener store:]

• (8) stainless steel M10x1.5 nuts
• (4) stainless steel SAE flat washers (to fit M10)
• (4) stainless steel SAE split-lock washers (to fit M10)
• plain motor oil and plastic bowl

Tools

• deep socket to fit Ford hex studs, preferably strong 1/2" drive
• long 1/2" drive extensions
• 1/2" breaker bar
• M10x1.5 tap, decent quality (carbon steel adequate)
• tap wrench
• 1/4" extensions to mount tap (angle extensions preferable)
• deep and shallow sockets to fit replacement nuts

Removal

The studs and nuts were soaked in penetrating oil overnight. The nuts came off with a breaker bar without too much trouble, but it was clear that the threads on the studs were corroded with rust and metal deformation, despite my car being only in Texas and California and far away from road salt. Removing the studs was harder. A deep socket will fit over the hex shoulder "nut" and clear the end of the stud, but the studs were a beast to turn. The upper threads on the studs were completely deformed, and the threads in the exhaust manifold also looked trashed. For many folks, it's clear that the studs break off before they can be removed. Mine were a near thing, and relatively easy at that.

Replacement Hardware

SHONut carries Ford exhaust studs (N811-284-S101) at $20.44 for four, but these are purportedly the same plain steel as the factory units and for that money I wanted to put in much better hardware that would not fail me in future. Metric studs can be found at auto parts stores, but I wanted stainless. In the US it is difficult to find metric fasteners, and even at a true fastener specialty store I found only a limited selection of stainless metric hardware, few studs and not in M10 at the length required. I found it much simpler to mail order.

From [1] I ordered four M10x1.5 studs in 18-8 stainless steel (p/n 93805A434) for $3.61/ea. They also carry a complete selection of nuts and washers, but only in bulk quantities. From my local fastener store I bought M10 stainless nuts and also 7/16" stainless steel flat washers and lock washers (not metric, but the closest SAE size that would fit over M10). Ideally, one would buy brass metric nuts to insure that the outer nut would fail before the stud is damaged, but again MMC has these only in bulk;, and I judged this overkill.

Tapping

Before installing the replacement studs I wanted to clean up the exhaust manifold threads. I have a cheap Harbor Freight tap/die kit, so I bought a good quality M10x1.5 carbon steel tap. The tap is ordinarily clamped into a tap wrench, but there is insufficient clearance near the manifold to swing the wrench. I found the tap would fit in the end of a 1/4" drive socket extension, and the extension would clamp into the wrench, although both can work loose.

Tapping can be gone with proper tap oil, but I used motor oil. I poured a pool of oil into a plastic carton, liberally coated the tap and shook off the excess, and worked the tap into the exhaust threads by finger (to insure no cross-threading!), only using the wrench when firmly in place. When resistance is encountered, back the tap in and out to recondition the threads. Periodically remove the tap and wipe off the metal particles, including those embedded in the threads, and re-oil. Eventually, work the tap as far into the manifold as desired. I re-tapped the entire distance of the stud passage, placing my finger at the top to tell when the tap had run the entire length and to avoid accidentally running the tap into the manifold proper above!

Afterward I cleaned the threaded passages using a narrow metal brush to scrape out any metal particles. I wrapped paper towels around the brush and worked the paper into the passages to soak up oil, and later the brush alone to remove any shreds of paper. By close flashlight inspection the threads were neat and clean.

Installing the replacement studs

Studs can be screwed in using the "two nut trick". Thread two nuts onto the stud, such that a socket will cover the lower nut but not the upper. When the socket is turned, the lower nut will try to rotate but will be stopped by the upper nut, and so the stud as a whole will rotate. However, with good hardware the upper nut may rotate from friction alone, so it may be necessary to hold the upper nut in place at first. Under load, it is preferable to use several nuts, doing either job, to distribute the load among a much longer length of threads, and a shallow socket may be easier to work with than a deep one. To my surprise and pleasure, my replacement studs threaded into the block by fingertips alone, and the two-nut-trick was hardly needed.

These studs from MMC (70mm) were slightly longer than the stock studs, but of course without the integral "nut" shoulder. I threaded a stainless nut at approximately the same place on these studs as the factory units had, but the factory units did not thread all the way into the exhaust manifold passages. For easy consistency, I threaded the studs to the end of the passages, placing my finger on the far side to check. The "shoulder" nut was then threaded down the stud with a stainless lock washer between the nut and the manifold, and tightened strongly (about 45 lbs torque). The y-pipe fits over the stud, and I used a flat washer and another nut to secure the y-pipe.

Diagram

<---- up into engine / down to floor ---->

E /\ [*** exhaust manifold passage ***]
X || ----(stud)-----------------------(l)(n)(y-pipe)(n)-------
A ||
U ||
S ||
T ||(my finger)

Anti-seize

Exhaust hardware should be heavily anti-seized. Ordinary anti-seize is aluminum based and rated to 1600F. This is close to the exhaust temperatures, and so over time the anti-seize could degrade. Copper anti-seize is usually rated to 1800F, and higher yet is nickel rated to over 2000F. I could not find these at my auto parts stores, so I bought some nickel anti-seize from McMaster-Carr (4oz can, $10.83, p/n 1028K63). Nickel anti-seize is also recommended for use with stainless steel (high in nickel), and it's probably the same kind that comes applied already to oxygen sensors.

In case I should need to remove these exhaust studs ever again, I used a lot of anti-seize on the stud itself, but /not/ where the upper nit sits -- ideally, that nut will weld itself to the stud over time becoming an integral shoulder like the factory units. I wiped off excess anti-seize squeezed out when the nuts were torqued down, since the excess can smoke unpleasantly when the manifold heats up. I also used it when reinstalling my oxygen sensors. When torquing, keep in mind that lubrication (such as good anti-seize or even plain oil) will reduce torque values substantially (the figure 30% is often mentioned).

Conclusion

Now I have beautiful exhaust hardware, probably massive overkill. The all-stainless hardware should resist corrosion, the high-temperature nickel anti-seize may also help and will make removing the hardware if necessary immensely easier. The threads in the iron exhaust manifold are in excellent condition. Everything should be strong as an ox and easy to work with.

Most importantly, I do not think there is any danger of breaking exhaust hardware in the future, no need for easy outs or cobalt drill bits or other awkward, expensive complexities. I consider the few bucks more than the factory units to be well worth that insurance. Mail order is much easier than driving store-to-store.