Correct Exhaust Nuts?

[N729LS]

I am doing some work on my IO-360-A1B6 and I noted when I took off the exhaust that the nuts were non-standard. That is, they were much taller (maybe 3/8") than regular 5/16-18 nuts and appeared to be made of some gold-ish colored material. They look kind of like the brass Continental exhaust nuts, but those are 5/16-24.

When I checked the parts manual, I found the exhaust nuts listed as STD-1410, NUT, 5/16-18 plain.

Anyone have any idea what the existing nuts might be, and why they are there? And whether I should re-use them or replace with the standard nuts from Aircraft Spruce ($1.65 each), or maybe ARP flanged header nuts (10/$10)?

Thanks,
Andy

[chaskuss]

Andy,
What you describe may well be automotive exhaust nuts, made of brass. The previous owner may have had problems with the "proper" nuts seizing to the studs. The nuts you describe are often used on older GM automotive products to prevent the nuts seizing to the studs. The extra length is to allow for the reduced strength of brass, versus steel.

Below is a link to a 7/16" version of what you describe

https://www.dormanproducts.com/p-245...origin=keyword

Charlie

[JDeanda]

I?ve seen brass exhaust nuts pretty often in my 30-odd years of fooling with light airplanes. I love ?em. When you need to remove them, they just break loose and unscrew without bringing the stud along for the ride. I changed my engine (Lyc. O-320) over to them. If the threads are correct, the Continental nuts are fine. If?n I recollect rightly, mine came from the local hardware store. I?ve even gone so far as to drill out and tap smaller nuts so I could get a socket on them.

[snopercod]

On my #2 cylinder, it was too tight to get a socket wrench on the "approved" nuts. So I'm flying with 12 pt. nuts from Summit "Aviation". I used Nickel anti-seize on the threads. No problem getting a Snap-On 12 pt. swivel socket in there. https://www.summitracing.com/parts/ARP-301-8343

[AK4x4]

I believe the current Lycoming exhaust nut is p/n STD-2233. It is a all stainless, low profile, self locking nut. It is installed along with a NAS1149C0532R thin stainless flat washer. They work great and , when removed, don't pull out properly installed studs. They also maintain torque on the exhaust gaskets reducing or eliminating exhaust leaks, something the old STD-1410 nut never did. The down side is they are expensive. I paid $5.80 each at my local engine shop but I have seen them listed as high as $15.00 each. Good Luck, Russ

[BillL]

Every OEM must specify a different nut, there are apparently many Lyc has used. I asked Jim (formally Lyc training guy, now retired) about that - - the high end nuts are a super alloy, and some are the same, just silver plated.

Jim said the lower alloy will just corrode and come loose ( no specific order) but won't stick to the studs. The super alloy may or may not stick but hold in place longer (no high temperature creep)

$5.80 is a good price for the correct alloy in a standard Mil-Spec nut (MS20500) and IIRC, the same super alloy as the Lycoming, plated.

BTW, my exhaust flange fasteners (got two sets) did not have the self locking feature.

By knowing Lycoming makes lots of different nuts, I would say ( guess) they have not found the perfect one yet. The airframe OEM chooses which failure mode is acceptable. I am just experimenting and inspecting.

EDIT: McMaster has some A286 high temperature SS alloy (which matches the Lyc nuts) a flanged 12 point or standard hex 5/16-18. The hex are high priced, but the flanged are not so bad. Not silver plated, so they will need antiseize and torque modification.

[gmcjetpilot]

Dis-similar metal alert....

Carbon Steel with Stainless will cause the stainless to corrode.

Stainless with Aluminum will cause the aluminum to corrode. (some charts say NO some say OK, andl likely a factor or the actual aluminium alloy)

Carbon Steel and Aluminum are OK from a dis-similar metal potential but corrosion is not eliminated in an corrosive environment.

I like the tall brass nut idea.... on steel stud...