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Old 05-04-2001, 03:21 PM   #1
Jay_UK
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Post Studs Versus Bolts

Hi guys...

I have recently been confused by a few things... one being the use of studs instead of bolts for block/head assembly.

When you use bolts.. you are introducing two directions of force/load... in the direction of the bolt... but also twist as the bolt is turned... this will affect the final torqueing of the bolt.. I know use of lube will help.

One solution is to use a stud. This is where I am puzzled.

I always believed that the stud is torqued down first.. so that the stud is tightened onto the block (chamfer). This compresses the material/area around the block. Then the head is installed and the nut is torqued down... at some point this neutralises the compresive forces between the stud and the block..and as you then tighten the head down the remaining force is trying to pull the stud out. Basically what I am trying to say is that by torqueing the stud into the block first.. it minimises the final forces produced by the tightening of the nut onto the stud.

Recently I have read that the stud should not be torqued down first.. it should only be done finger tight.

Which is the correct approach ?

Is it better to use studs than bolts ?

J.

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Old 05-04-2001, 06:02 PM   #2
pottsie350
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I've heard all kinds of things. But the thing that I have stuck with is to use whatever the designer of the engine recommends. The engineers that designed the particular head you are using probably knew what the were doing when they designed it.

Studs are better in some designs and bolts are better in others. Some heads use torque to yield bolts, meaning that you torque them to the specified torque, and then if the bolts are ever removed you will find that the torquing has disformed the bolt rather severely. You can generally torque a stud/nut combo to higher values than you can a bolt. This is due to friction and the torsion flexing the bolt. The nut is at the source of friction and torsion so there is less flexing going on. While you can get a stud/nut combo torqued tighter, it also has a higher chance of losening due to repeated heating cooling cycles, also due to less friction and torsion springing of the fastener.

The engineers have probably taken all this into consideration, so I generally choose to just follow their guidlines, instead of trying to re-engineer their product.

Just my .02

[This message has been edited by pottsie350 (edited May 04, 2001).]
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Old 05-04-2001, 07:41 PM   #3
MPREZYA
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Jay knowing what your planning go with the Studs and nuts as you are a boost junkie!!!
BTW got your PM thanks and keep me posted on the project I am helping to finance .
Rich

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Old 05-05-2001, 02:26 AM   #4
Jay_UK
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pottsie350...

Using what the manufacturers originally specified is fine if you are building the engine back to virtually stock form.

But when you are trying to extract over 3 times the power from it... stock design and components arent always suitable.

I know studding reduces the twist force.. to give a more accurate torque rating for the bolt.

But should the stud be torqued down first into the block (this will then re-introduce the twist problem) ?

Or should it be left loose (finger tight) ?

J.
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Old 05-05-2001, 09:28 AM   #5
Digital_Boy
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The big problem with the Impreza is it's boxer engine has such long head bolts, that there are no manufacturers of studs for it.

ASR has been looking for a source of head studs to do away with the stretch bolts that are used now. If you have a source of studs, please let me know.

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Old 05-05-2001, 11:06 AM   #6
Hashiriya
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Old 05-05-2001, 11:34 AM   #7
Jay_UK
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Howdy...

Obtaining studs.. or special alloy bolts isnt a problem...

I am just trying to weigh up which is best for the subie application.

And... what is the correct process for stud installation.. ARP recommend installing them loose. Where as most other places suggest you torque them into place first.

J.
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Old 05-05-2001, 06:08 PM   #8
MPREZYA
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I would finger tighten rather than torque the bolts down as when you tighten them down with anything else you are definetly adding more torsional force and risking them braking over time. Compared to finger tight and then torquing the nuts down to even out the distribution. They are subjected to trying to hold down a slab of metal trying to maintain proper seals under high pressures after all. Also there is less chance of warping the block with finger tightening.
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Old 05-06-2001, 01:49 PM   #9
Jay_UK
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Hi there,

Thanks for that...

But surely as you tighten the nut down.. you are tightening the stud into the block.. if it is already finger tight then it will become tighter.. and not be "floating" in the block?

either way.. sounds like a chamfer is required...

Bolts are becoming more tempting

J.

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Old 05-07-2001, 10:56 AM   #10
pottsie350
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I didn't know you were building a boosted motor

If the head originally used bolts, I would still stick with them. Go with a higher grade.
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