Exhaust studs .. what sort of steel?

[Albert Ross]

I have to replace the exhaust studs that hold on the mixing elbow on my Yanmar 3YM30.
New ones from Yanmar are very expensive and appear to be made out of mild steel.

Is there are reason why I shouldn't make my own studs from 8 mm A4 grade stainless threaded rod (AKA "studding") ? Or is there some reason why stainless steel isn't usually used ... at least it wouldnt rust like the old ones did.

Thanks.

 

[earlybird]

My custom made high-rise elbow on my Beta25 is held by 4x 8mm set bolts. A2 grade, readily available, as good as A4. Been like that for 5 years , no sign of any problem, not that I make a habit of unscrewing them!

 

[RichardS]

Sounds like a sound idea to me. If you use copper grease on mild steel or stainless either should last a lifetime as the studs should not be getting wet if the gasket is sealing properly. Ideally use stainless nuts as well and copper grease on the new gasket. If the surfaces as in good condition and are cleaned up properly, these nuts do not need to over-tightened.

Measure the depth of the stud drilling and make sure that the studding goes right to the bottom. You might need temporary locknuts to wind them home.

Richard

 

[Albert Ross]

Thanks. That's helpful. Yes, not worried about the exact grade of stainless, more about whether there was some metallurgical reason why the original studs appear to have been made out of mild steel.

 

[Poignard]

My BUKH engine is 35 years old. I have owned it for 20 years. The m/s exhaust studs must be at least 20 years old, probably more. They are not rusty because they are greased and no part of them is exposed to the atmosphere. I use brass nuts on the studs.

Allthread rod is not suitable for studs because it is all thread. A stud has an unthreaded portion for a reason. The stud is screwed into the head until that unthreaded part stops it going any further and bottoming in the tapped hole. If properly tightened it ensures the stud does not unscrew when you unscrew the nuts. And the unthreaded part acts, to some extent, as a dowel.

 

[vyv_cox]

There is no metallurgical reason not to use A4 studs but you would need to be very careful with them. The yield point of A4 is only 50% of the UTS, compared with around 70% for the better grades of mild steel that would be used for studs. It would be only too easy to strip the threads or elongate the replacement stud if a little vigorous with the spanner. I doubt that there is any significant difference between the steel studs used in cars and those on a Yanmar engine. I would take a sample stud to an automotive factor and buy some of theirs.

 

[RichardS]

If properly tightened it ensures the stud does not unscrew when you unscrew the nuts.

I reckon that on the studded fittings I have worked on over 50 years around 25% of the studs have unscrewed rather than the nut. In fact, 100% studs unscrewed on the king pins I did last weekend. :ambivalence:

The exhaust manifold is not a "mission critical" stud alignment like a cylinder head and the holes in the manifold flange are oversize anyway and the gasket has plenty of give in it.

Richard

 

[AntarcticPilot]

QUOTE=vyv_cox;5993171]There is no metallurgical reason not to use A4 studs but you would need to be very careful with them. The yield point of A4 is only 50% of the UTS, compared with around 70% for the better grades of mild steel that would be used for studs. It would be only too easy to strip the threads or elongate the replacement stud if a little vigorous with the spanner. I doubt that there is any significant difference between the steel studs used in cars and those on a Yanmar engine. I would take a sample stud to an automotive factor and buy some of theirs.[/QUOTE]

I have had experience of this. The bolt on which my alternator swings broke, and I could only get a stainless replacement, which was all the chandlers had. The stainless bolt sheared after very few hours running. Replaced it with another bolt from the same packet, and it sheared again. I vaguely recalled some of the wisdom from the forum and realised that a steel bolt would be a) stronger and b) resist fatigue better. Went to a hardware shop and got a suitable bolt, and since then have had no problems!

 

[burgundyben]

If you want the joint to work long term you need to use the material the engineer that designed the joint specified.

How long is the stud? How much expansion do you get? How much expansion is there in the flange? What compression will the gasket withstand? How much margin is there in the proposed stainless material at what temp? How high does the temp need to be before the stainless would yield and let the joint come loose on cooling?

On approach to a headland, the wind picks up more than you expected, you decide to put a reef in, the reefing process suffers a jammed halyard, you become delayed making no headway while trying to sort it, the tide turns against you, you decide to motor sail while trying to get the main down, its pretty stressful and the main is flogging itself to death, you decide to motor harder than you usually would, the exhaust manifold gets hotter than previous, the stainless steel studs you saved £2.50 by fitting fracture, the cabin is filled with exhaust fumes, its a big swell, the exhaust outlet becomes submerged, raw water from the cooling circuit backs up into the cabin, quite a lot of water is coming into the boat through the exhaust.....you can't send anyone below to deal with it due to the fumes....

Use the correct parts.

 

[Poignard]

I reckon that on the studded fittings I have worked on over 50 years around 25% of the studs have unscrewed rather than the nut. In fact, 100% studs unscrewed on the king pins I did last weekend. :ambivalence:

The exhaust manifold is not a "mission critical" stud alignment like a cylinder head and the holes in the manifold flange are oversize anyway and the gasket has plenty of give in it.

Richard

Yes! Bloody annoying isn't it? Even more so when there's something in the way preventing the stud from coming all the way out! Still the joy of engineering, to my mind anyway, is the problem solving element.

What were the king pins from? Some interesting vintage car perhaps? If so, pics please. :encouragement:

 

[Albert Ross]

Thanks for your collective wisdom.
Vyv of course had the answer
I have now purchased the studs from my local car parts shop ... 75 pence each ! £3.00 the lot!)
(I had been quoted about 50 Euros for 4 last summer in Spain. Got home to UK using set screws).

 

[RichardS]

What were the king pins from? Some interesting vintage car perhaps? If so, pics please. :encouragement:

A Series 3 Land Rover. A really odd design in that the upper kingpin runs in a bearing called a Railko bush which I'd never heard of before. The lower kingpin runs in a tapered roller bearing. The upper kingpin is secured with 4 bolts. The lower kingpin is secured with 4 studs and nuts. It's as if the whole car was assembled out of the BL parts bin rather than actually being "designed".

Richard

 

[rogerthebodger]

A Series 3 Land Rover. A really odd design in that the upper kingpin runs in a bearing called a Railko bush which I'd never heard of before. The lower kingpin runs in a tapered roller bearing. The upper kingpin is secured with 4 bolts. The lower kingpin is secured with 4 studs and nuts. It's as if the whole car was assembled out of the BL parts bin rather than actually being "designed".

Richard

Talking about Classic cars my MGB and most BMC A and B series engines had extra long brass nuts holding the exhaust manifold to the head> This prevented the nuts seasing on to the studs so the nut came off the stud and not the stud out of the engine head.

The stud thread in the head was always a UNC thread and the nut was a UNF.

 

[Poignard]

Talking about Classic cars my MGB and most BMC A and B series engines had extra long brass nuts holding the exhaust manifold to the head> This prevented the nuts seasing on to the studs so the nut came off the stud and not the stud out of the engine head.

The stud thread in the head was always a UNC thread and the nut was a UNF.

I have always used brass nuts on exhaust manifolds since owning an MGB. I don't know why the practice isn't more widespread.

 

[RichardS]

I have always used brass nuts on exhaust manifolds since owning an MGB. I don't know why the practice isn't more widespread.

I seem to remember that all the cars I worked on in the 60s and 70s had brass exhaust manifold nuts. I don't know when or why the practice died out but I don't see them any more which means that having to drill out sheared studs/bolts is a lot more common than I remember.

Richard

 

[vyv_cox]

I seem to remember that all the cars I worked on in the 60s and 70s had brass exhaust manifold nuts. I don't know when or why the practice died out but I don't see them any more which means that having to drill out sheared studs/bolts is a lot more common than I remember.

Richard

My elderly VW vans have had a slight variation on that. The exhaust nuts seem to have an insert rather like a Helicoil in them. I assume this is made from some different metal for the same reason, although I don't know what it was.