Which ARP Head Studs

[S2central.net]

I've done a quick search on this here, but cannot find the specific answer to this question regarding ARP head studs on the S2. I see that one US distributors ARP list a couple of different products against the 20V I5 engine...

Two options are 204-4207 with 'standard cut studs'
and there is 204-4704 with 'undercut studs'

Seemingly this 'undercut' trick is about making better / more reliable clamping force. Can I assume that the ARP studs come with correct nuts, washers and instructions about the Audi application. I am planning to get the head off maybe this winter - for the RS2 upgrade - and a general health / internal check (please god I hope not to find anything too horrific/expensive).

I don't plan on doing lots of head swaps but I've not heard any bad things about ARP headstuds (other than some confusion about torqueing the boogers down properly)


Cheers for any insight,


Paul

[diesel des]

I ordered mine from Aelred. This will be the second set i got from him. He is away for a week, but drop him a PM.

[S2central.net]

Aye - but which ones are they - does it matter ?

[diesel des]

Dont know, must read the number from the packet....

[Keeper of The Kitten]

Generally, undercut is just where the thread runs out - there is a nicely radiused groove where the thread turns into plain shank. It's definitely required for rolled threads and preferred for cut threads too, so the tooling can run off the end of the area to be threaded. I think I'd prefer a stud to have undercuts at the end of the threads.

__MDC__

[mcandmar]

The set i have are part number 204-4207 "Audi 5 cyl 20v HSk", I think i bought them from 034, was a long time ago.

They do come with correct nuts, washers and instructions, however the washers are a bit small. I think MarkH replaced them with QSport washers, the info is somewhere in the forum here.

Mark.

http://www.034motorsport.com/product_info.php?products_id=158

[S2central.net]

Generally, undercut is just where the thread runs out - there is a nicely radiused groove where the thread turns into plain shank. It's definitely required for rolled threads and preferred for cut threads too, so the tooling can run off the end of the area to be threaded. I think I'd prefer a stud to have undercuts at the end of the threads.

__MDC__

Spot the proper engineer ! Whats the difference / advantages between rolled threads and cut threads then ?


Cheers,

Paul

[Corey]

The differences between the 2 are manufacturing labour costs, the cut thread takes significantly longer to produce than a rolled thread.
There is no difference in strength between the 2 (if they are manufactured to the same specification), the weak point of any thread is the minor diameter. Under cut thread are beneficial when you need the body of the stud to sit flush.
If the undercut stud minor diameter is below that of the threads minor diameter then this becomes the weak point.
I would need to check but I don't think our ARP's have the undercut?

[Keeper of The Kitten]

Technically the cut thread goes through the grain of the material whereas the rolled thread reshapes the grain of the material, rolled thread in theory is stronger - but as Corey says, the minor diameter of the stud is an important factor (it is the cross section at the smallest point that determines the strength of the stud).

Given the choice I would tend towards choosing rolled thread instead of cut thread if the actual thread strength is a critical factor.

Corey's point about flush seating is also valid, more so where there is a shoulder and a change in diameter for the plain shank area, any radius improves the strength at the corner of a shoulder on a bolt or stud, but that requires a corresponding chamfer on the hole that the bolt/stud is screwed into. To overcome this, the radius can be undercut either into the minor diameter, or if the shoulder is large enough it can be cut into the face of the shoulder. But in all cases where a thread is machined or machine formed, ideally there will be an undercut to the thread root, it doesn't need to be deeper than that.

__MDC__

[S2central.net]

I'm more confused now than when I asked the question in the 1st place...

[5inline]

ARP studs has cold rolled threads,this machining process is tough for the toolings,and more expensive than cutted or warm rolled threads.
As far as i know,undercut studs are ment for "paper" head gaskets,since they seem to compress more than the three layer steel gasket.
Undercuts seems to be more stretchy than regular ones,
I use regular studs in my 3B motor.

[twoqu]

And another explanation!


Undercutting:
Reducing the shank diameter of fasteners reduces head gasket sealing problems. Short fasteners are very stiff and the amount of bolt stretch available is about the same as the amount of compression in the gasket. Should the gasket lose some of it's compression due to set, it can quickly eliminate the pre-load (torque) in the fastener - which will unload the gasket. This results in a blown head gasket. Providing optimum stretch is absolutely critical. Using an undercut stud helps control head to gasket clamping force better than conventional studs.

If ARP offer those for our application and you can afford the difference, might as well do it

[S2central.net]

Whats the smilie for drowning in a sea of confusion ?

Paul

[twoqu]

OK! How about a rational rather than scientific decision.
ARP have developed a new design of stud that they claim improves clamping of the headgasket and there is not much of a price differential!

[volcano rs]

I'm more confused now than when I asked the question in the 1st place...

i'd use arp if they are good enough for american muscle cars they are good enough for s2's .all or most of big power engine builders in america and japanese tuners use them . i think people get carried away sometimes .
how many people on here have had arp's break on them ? i bet not very many.