Stainless steel vs. alloy steel: is it worth the money?

Exhaust Nuts and Studs

Written by: Dave McFarlane, Founder

Alloy and plain steel have been used on aircraft exhaust studs and nuts for the last 100 years. Although they have proven strong enough for the job and can last quite a few years, there are downsides to standard steel hardware. The exhaust heat is quick to destroy the cadmium plating and rust beings to erode where not protected by the cylinder head. Cadmium melts and vapors away at 610° F, which is well below the exhaust flange temperatures. You may have commonly seen studs rusted away to a spindly little stem when pulling the exhaust system on a mid-life engine. Rusty studs need replaced while there is enough metal left to prevent twisting them off during removal. Drilling out and removing broken studs is time consuming and tricky.

The thermal expansion rate of alloy steel is much lower than the thermal expansion rate of the stainless-steel flanges and the aluminum cylinder heads. These different expansion rates create tension stress in the studs because they’re growing and shrinking differently than the aluminum and stainless parts fastened to the studs. These continuous heat fluctuations promote exhaust gasket failure and fastener tension that challenges the fastener locking devices. Additionally, these different expansion rates can warp the exhaust pipe flanges because the hot flanges get thicker than the alloy steel stud grows.

Alternatively, stainless steel is a much closer match to the aluminum head temperatures and matches the exhaust system in thermal expansion. This provides a much more stable attachment with consistent exhaust gasket tension. Plus, the aluminum head threading for the studs experience a lot less stress; the threads also live longer before growing to the point of needing higher-priced oversized studs. You may be thinking this seems fairly micro in the big picture of things, but small movements and changing stresses over many cycles for extended periods have big effects on reliability. Exhaust systems are exposed to high inertia loading as a reaction to each piston firing (engine vibration).

The stainless steel that McFarlane uses is heat-treated to the same (or more) strength as the original alloy steel to handle exhaust system loads and vibrations. Other stainless steel exhaust hardware exists within the market, but it’s made from low strength 300-series stainless steel, which is great for resisting rust, but it does not have nearly as much strength and the threads easily gall. Galling happens when there is a molecular adhesion between two parts under tension, like threads under tensions. Some metals have a propensity to stick to themselves under pressure, causing the two parts to lock together.

McFarlane stainless studs are hard and Black-Max™ coated, which is a hard nitro-carbide process that aids stud installation and prevents dissimilar metal corrosion in the aluminum cylinder head. This coating makes the nuts come apart easily after many heat/cool cycles. Black Max™ is also durable for many exhaust dis-assembly and re-assembly cycles. Stainless steel nuts will not rust and can be re-used several times. McFarlane also cadmium plates the stainless steel nuts to protect the stud threads when alloy steel studs are used.

Although stainless is more expensive than alloy steel, the labor savings from replacing rusty studs, plus fighting rust lock-up when removing the exhaust system makes stainless the least expensive option in the long run. The re-useable stainless nuts are obvious money savers. Follow the manufacture’s requirements for nut re-use; sometimes there are restrictions.

McFarlane Stainless Steel Exhaust Nuts and Studs

FAA-PMA approved for many Continental and Lycoming engines

• Heat-treated, high-strength stainless alloy
• Studs are nitrocarburized Black Max coated to resist galling and corrosion
• Black Max prevents galvanic corrosion between stainless steel and aluminum cylinder
• Stainless steel nuts are CAD plated - better on galvanized and stainless studs
• No more rust!
• Reusable