Quick Question for the Engine Rebuilders.

A. No.

Torque is not an indicator of correct installation. The correct tension will be achieved by how tight the nut is when clamping the head on.


B. You can tell by eye if the studs are stretched so no measurement required. I don't even bother anymore though. Ferrari studs are pretty predictable so when the heads come off they go in the same place as the old gaskets and new ones go in. Too many fail during reassembly. Not the time to find out new ones are needed.

 

stretch is the proper way to measure head tightness but is only measurable with a special torque wrench which is very costly.

Old timers, like myself, just re-torque the studs numerous times to allow for relaxing of the stretch. By re-torque I mean, let bolt or nut rest several hours or over night and apply torque again until nut or bolt does not take any more. Some even apply some heat to the engine before final torque test. Do not loosen and re-torque as some head bolts require.

More modern bolts and studs, especially torque to yield bolts require new bolts. That is generally determined by the factory.

 

Apples and oranges.

You asked the wrong question. During installation of the heads we are torquing the nuts.....not the studs.

The studs are installed and tightened long before a head is installed.


Very different.




Yes in some cases fastener stretch is a valid way to determine correct tension.

 

Glad you mentioned that, I was just going to clarify that difference.

Also the installation of studs of weather or not they are torqued in place before they are stretched by the nut when the head is installed will be dependent upon the design of the fastener.

in other words some are merely finger tight and the thread pitch and class of cut will hold it properly as the fastener design dictates.

class 3B is a looser fit that requires attention of not running into the bottom or shoulder and introducing radial stresses that will cause shear.
Class NC5 is an interference fit that requires the proper torque I.e. a 7/16-14 in a ferrous material would have an approx 15~45 Ft-lbs at full engagement.

 

Ferrari actually uses a calulated stretch of the studs. You apply an initial torque to seat the gaskets and seals and get everything even...

and then you turn the head nut a specific amount to stretch the stud a set amount.


For my 328, its 33 ftlbs initial torque then 120 degrees rotation.


I dont know what the thread size of the Ferrari stud is but you can calculate the stretch from the thread pitch.

120 degrees ( 1/3rd of a turn) on a M14 x 2 thread would be 1/3rd of 2mm per turn or .67mm of stretch. (.026")

120 degrees of turn on a 1/2" x 20 nut (1/20 inch per turn at 1/3rd of a turn) would be .015"

it's a function of thread pitch not diameter. A coarse thread 1/4x20 bolt would stretch the same amount at 1/3rd of turn as a 1/2x20 fine thread bolt but the torque to get there could be very different as well as the stress load in the bolt.

 

Usually the only bolts you can actually measure the stretch on are the rod bolts. You can get both sides of the caliper on the bolt at the same time. Tighten until the desired stretch is measured.

 

The old fashioned way. One long groan while applying pressure to the wrench followed by two short grunts, and ending with a quick wipe to the brow before moving on to another fastener.

A man doesn't need anything more than a shade tree, two pair vice grips and cinder blocks to fix a car up good and proper.

 

how do you figure stretch amount?

deflection = (P*L) / (A*E) where
P = force
L = length
A = cross sectional area
E = modulus of elasticity

 

I can see this getting real techy and nerdy now

Not wanting to type a bunch here's a good read for the engineer types...

http://www.roymech.co.uk/Useful_Tables/Screws/Preloading.html

I've got loads more for those interested

 

back in the days of my structural engineering we just used "crush" washers that deformed under a specified load, thus ensuring appropriate tension (and clamping force) was applied.

 

ahh yes, i found it

"Direct Tension Indicator Washers
A number of direct tension indicator systems are available (see links below) the most popular being the DTI washers. ..The notes below relate to DTI washers..
DTI washers measure the bolt tension developed during tightening, regardless of the torque resistance of the bolt. By far the simplest method, a DTI washer is put on the head or nut end of the bolt. The bolts are then slightly tensioned snugging the joint by partially (but not fully) compressing the DTI (tightening the nut).
Then all the bolts are tightened such that the DTI's are "crushed" to the point where a feeler gage cannot be inserted half way around (tightening the nut). DTI's are completely independent of the torque resistance of the bolt assembly.
If the DTI is put on the nut end of the bolt, tightening can be done by one person because it is not necessary access the bolt head side"

 

For 'turn of nut', from what I recall dia and length are function ratios of the calculated Rad turns. I.e. the length of the bolt in number of divisions by it's dia. the chart div's I think are 1/3 turn 1/2 and 2/3 for faces normal to axis of bolt. from short to long bolts.

 

Are you experiencing this problem on the 355?

We just did a 355 engine rebuild in which the heads had never been removed....the nuts were pre-torqued to the factory specs and during the torque angle procedure we experienced a major loss in final torque reading.

The studs were removed and replaced and everything went very well with the head reinstallation.

Even though the nuts are pre-torqued to the factory specs and then rotated to the correct torque angle(which is independant of torque) I do not want to see the final torque reading drop below the pre-torque measurement.

 

Uro, look me up if you need the engine in your Dino rebuilt.

I've got a JC Whitney catalog, a full set of Costco spanners and enough permatex to make a 1:1 scale copy of Mt Rushmore, so this job should be cake.

I can do the whole thing for $1,783.12 +tax.

 

BTW, you'll note the +tax bit which speaks to the legitimacy of my operation.

 

355 studs are crap too.

I made some changes in how I put those together though.

 

This is a confusing thread. Half of you are talking about installing head studs in the block and half of you are talking about torquing (in some cases torque-to-yield) of the actual head nuts. As Rifledriver has already pointed out, no need for either technique when installing studs. Also, I don't reuse torque-to-yield type fasteners ever. Stretch is almost always a better way to torque fasteners, but very few of us have the equipment to do this accurately on a head stud.

 

You are right. Some here don't know when to stop. Others need to impress us with everything they know.

 

amen to that, do not overthink the thin.g

 

Im redoing my heads on mondi 3.2. Heads came off nicely and head nuts seemed pretty consistent as far as torque. Do I need to replace the head studs? Or just torque 33 lbs. plus 120 degrees on old studs. They look good as far as threads arent fouled......Thanks!

 

Do I Re-torque the heads on my 87 TR that has no problems? Or do I just leave it alone . While doing a cam belt tension er etc. Would like rifle drivers' response.




Ago

 

I think Euro trash has slit his wrists or wishes the question was never asked, too much info. Maybe we should start another "when do I change my timing belts" thread.