I need to replace an exhaust manifold to downpipe gasket and the prospect got me wondering why studs threaded into the manifold are used and not bolts and nuts running through both components.

Bolts and nuts could be more easily cut off and replaced if the high heat of the exhaust system causes the fasteners to seize.

Do any of you know why exhaust systems are engineered as they are?

 

LOL. Replace the words 'exhaust systems' with the words 'Land Rovers.'

As the wise owl Tootsie Roll commercial from the '70s says "The world may never know." Just replace the studs, Never-Seez the crap out of them and move on.

 

The bolts will probably snap.
They are just studs. So if you snap them - no problem.
buy a stud remover socket and remove the studs.
Then get a threader - die and rethread it to the same threads.
Put in a new stud.
and new nuts.
You are set.

See (for Discovery I) but close enough to Discovery 2

http://jeffreyfall.com/landrover

 

Since I started using seafoam brand "deep creep" I have not broken an exhaust stud.

 

Deep creep is your friend on exhaust. I just did my p38 which is twice the work, got every bolt out. soak everything on deep cleep for as long as possible.

 

Okay, but if they are just bolts and nuts and they snap they can simply be pulled out.

Interesting, but what if the stud breaks off inside the manifold and there's nothing for the stud remover to grip?

Happily all three nuts came off easily.

Three and a half weeks ago I was slated to leave on vacation in a few days but wanted the replace the manifold-to-downpipe gasket that had failed and made the truck very noisy, especially when accelerating. I was worried that if I tried the job I'd break off a stud and be screwed for the vacation.

So I took the truck to my local indie shop thinking this was one of those "Know when to hold 'em, know when to fold 'em" situations. Adam confirmed that it's not uncommon for a stud to break and when that happens they take off the manifold and send it to a machine shop, which sometimes has to drill out the broken stud and re-tap the hole. Adam said they don't do that work in-house because of how hard the steel of the manifolds is. That would take a few days and jeopardize my vacation where driving the truck on the beach was an integral part.

Adam checked out my truck and assured me that driving it as-is on the vacation would not do any further damage, just be annoying. The noise turned out to be less irritating on the road than around town, especially with the stereo and air conditioning on and at highway speeds.

A few days after we got home I decided to try the repair myself, knowing that if I ran into a problem I could get it to the indie easily and that the vacation was no longer at risk.

I sprayed PB Blaster liberally, top and bottom. (Adam had said that you never know whether the nut will come off or the stud will unscrew.) I was prepared to wait several hours or overnight for the PB Blaster to work, but after a half hour or so I decided it couldn't hurt to try removing the nuts. To my great surprise all three came off easily!

I then had to loosen the motor mounts and jack up the engine to be able to drop the pipe far enough to get the gasket to clear the studs because the right cat hit the crossmember as I lowered it. I'd installed the motor mounts in January so I knew those two nuts would loosen easily, as opposed to the four crossmember bolts that have been in place 17 years. Once that was done the rest of the repair was a piece of cake.

After putting the new gasket in position I repositioned the downpipe and reinstalled the nuts, after applying anti-seize. I found in the RAVE that the torque spec for the nuts is only 22 lb.ft. Maybe that's part of why they came off easily and hadn't seized to the studs. I don't know; I'm not a metallurgist.

 

Everything is engineered for cheapness, why do Ford use 2 studs instead of 3 ............ cheapness. Studs are used on most vehicles, if not all, to attach the exhaust to the manifold, you would have similar problems with nuts and bolts unless they were stainless steel. It's all about engineering to a price, most only keep their vehicle for 2-4 years so they expect these problems from a 15 year old truck. Long gone are the days of copper coated studs and brass nuts, it's all about supplier price to the manufacturuer and consumer. Why are most computer parts made in Taiwan, Malaysia or China, cheapness of parts and labor. It's sad but true.

 

Yeah, I don't think I buy this explanation.

Would a bolt and nut really be that much more expensive? There must be an engineering reason. And perhaps I didn't pose my original question very well.

I still don't understand why a bolt (cap screw) through the flanges with a nut on the other side wouldn't be a better choice. If the nut became seized the bolt could be cut and would just drop out, without requiring a stud remover, a drill or any other extra work. Much more simple I think. Whatever.

 

I changed to bolts on one of my D2's. I can say it was harder to line up the holes and get the bolt in straight, where as the studs allow you to get the y pipe up to the manifolds and thread the nuts on pretty easy. I'm guessing its for ease of assembly.

I think a bolt and nut would be cheaper than tapping threads into an exhaust manifold, a stud and a nut. But what do I know...

 

Bimmer is responsible for the D2 cheapness shortcomings, including the inferior cast blocks. Ford came onboard in 2000 and didn't change much until the year they upgraded to the 4.6.