28 replies to this topic

[Dr Terry]

Hi Guys.

bodallafella gave the ARP specs as being

7/16 Stud = 71 ft/lbs
7/16-UNC = 73 ft/lbs
7/16-UNF = 80 ft/lbs

These figures can't be correct, the UNF torque spec has to be much less than the UNC spec becuase the UNF thread gives a lot more crush or pull on the bolt than does the UNC.

To give an example, if you had a mild steel 5/16" UNC bolt that actually breaks off if you torque it to more than 50Nm, than same bolt would break at much lower than 50Nm if the thread was UNF. There is simply more mechanical advantage, with a finer thread. It's been more than 20 years since I looked this up, I'll see if I can find it again.

I can see that a stud nut's torque should be less than a head bolt's torque because there is less thread contact, so therefore less resistance to overcome.

Dr Terry

[Struggler]

Hi Guys.

Without opening up the old can of worms, I'd like to hear some of you guy's thoughts on a problem I've had with head stud kits. I've used them on several engines, mainly blown or nitrous V8s & have found that if you use the same torque figures as factory bolts they distort the deck & overcompress the gasket material. What I have found is that because the nut thread is UNF, which is a finer pitch than the UNC thread into the block, the same torque figure causes a much higher bolt 'tension'. In other words the bolt (or stud) is 'pulled' more because the pitch is finer.

It took me a while to find any info on this, but there are some engineering standards that allow the difference to a factor of 1.5. In other words a UNF bolt torqued to a 100 Nm will result in the same actual bolt tension as 150 Nm for a thread of the same diameter but with a UNC thread.

Anybody else had experience in this area. Many people don't even think of this & torque the studs to the factory UNC bolt specs.

Dr Terry.

Hi Doc,
FWIW every early Holden V8 (ones with head studs passing into the water jacket) I have fitted the 1/2" studs to has totally distorted the deck, the bores look OK but the whole area around the stud gets pulled up significantly. I never saw this with the 7/16" studs. The late blocks with the blind head bolt holes fare a lot better. I have never thought about the UNC/UNF significance, just followed the manufacturers advice. I have also noticed the nuts on the studs come loose quickly, this is a direct result of the stud pulling up on the deck. The block isn't strong enough to correctly stretch the bolt. I have given up fitting 1/2" head studs now to the early blocks, if someone wants a beast they have to get a late block.

Basically the studs are too good quality wise for the early blocks.