2013 Prius Two, trying to pre-order parts for a head gasket DIY. In the past, when I have done exhaust manifold, I have tended to replace about 50% of the nuts (due to corrosion). So, I don't understand why this Exhaust Manifold Nut 90178-A0048 is listed as "Not For Sale"? (with no superseded part number)
90178-A0048 is (or perhaps was) a locally-available substitute for 94151-80841 in the U.S. and certain other markets, but for a 2013 model, this wouldn’t be the correct nut, anyway. For vehicles made in September 2012 and later, in catalog Figure 17-01, Manifold, and Figure 25-01, Exhaust Gas Recirculation System, the ※1 callouts in the illustrations specify a different nut, 90177-08003. Toyota says you should replace them all: the ⚫︎ symbol in the illustrations indicates a non-reusable part.
The nut is basically an M8x1.25 or M10x1.25 flange nut meeting the J.I.S. (Japan Industrial Standard). You can get them in stainless steel or even titanium which is anti-corrosive and not rust out like the steel ones.
Wow, titanium and stainless steel, I never dared to hope. Would you have an online, trustworthy source for these?
If the studs are steel, or zinc-plated steel, I think titanium nuts will make a corrosion battery of nearly a volt. No, the titanium nuts won't corrode, but oh, those studs ....
I’d respectfully disagree, both in general—fasteners for Toyota vehicles are made to proprietary Toyota Engineering Standards, which may or may not align with JIS, ISO, or other standards for similar parts—and specifically in this case, given the history of design changes revealed in the catalog. The original flange nut was changed (twice!) to styles with added annuli and slots, and the torque specification, per the Repair Manual (more info), was changed from 21 N·m (15 ft.-lbf.) for the original to 37 N·m (27 ft.-lbf.) for the later styles. If it were my car, I’d spend the $8.67 (at list prices) to get seven nuts of exactly the type selected by Toyota’s engineers, rather than trying to redesign the car without benefit of technical data. (Actually, I’d splurge and buy a whole package of ten for $12.40.)
I was going to ask if these were the slotted exhaust manifold type things yeah you can buy these from all kinds of suppliers there's no need to spend $8.64 I'm almost positive .
I've reused all of mine on one and two Z's in the other cars they go in The normally aspirated ice. No problems long as the stud looks good and you haven't torn it up taking your others off which I'm in a non-rust area so mine usually just spin right off by hand
Many German cars have traditionally used copper nuts to fasten exhaust manifolds to steel studs for corrosion free life. Since exhaust manifolds heat up with each use, moisture that promotes electrolytic corrosion is not an issue. The same is true with stainless steel and titanium. The only reason why copper is no longer used in all but the most expensive cars, as well as stainless steel and titanium, in OE manufacturing is cost. A lot of exhaust components like catalytic converter cases, pipes and mufflers use 409 martensitic stainless steel. The only reason steel is used for the studs and nuts is to defray cost.
I'm not sure which way the regular heating of exhaust manifolds really plays. Yes, they dry themselves out regularly, but they also seem to be sites of some of the heaviest corrosion seen around a car. Copper to steel looks like only around a third of a volt on the galvanic chart I'm eyeballing, way lower than titanium to steel.
