Head bolts and Studs UNC to UNF

[kippy]

Forum,
First time on this site asking or commenting for me.
Its a silly question perhaps but is there a reason the head bolts have a smaller diameter shaft approx 1/2" down from the head of the bolt down to the start of the thread?
I am in the process of building a Sprint motor and going to make 7/16" grade 8 studs with UNC thread into the block with UNF thread at the top for both the head studs and bolts. The idea being getting the head tied down more securely. The factory long studs I have are 7/16" diameter along their whole length but the factory short bolts have a stepped shaft so to speak. Is there a reason for this??
I have sourced and ordered five 9" and five 5" 7/16" UNF bolts to which I shall remove the head and cut UNC thread but not sure if I need replicate the profile of the short factory bolt.
Michael

[dursleyman]

Have you had a word with the guys at Sprintparts? They are in Australia and sell the UNC/UNF studs you are taking about. They seem to know all there is to know about Sprint engines.

[rosey]

Be careful with threads, UNF can be tightened to much higher torque than UNC (fine threads = more threads per inch than Coarse threads) which is why UNf is used for cylinder heads etc.

Modern cars use the same principle with metric threads, fine Vs coarse where applicable.

Engineering threads are rolled not cut, a rolled thread is inherently stronger than a cut thread, the same principle is used in engineering for adding strength to lifting hooks where the stresses are taken up by the forging & bending of the material.

A rolled thread will always be a greater diameter than the material its produced from, hence studs are thinner than the thread diameter, the metal gets squeezed into shape not removed.

Dave (apprenticed & engineer trained but enforced resting due to illness)

[kippy]

Russ,
I was going to go through Sprint Parts or a vendor on Ebay Australia Who , incidentally, I think is SprintParts.
I changed my mind, due to a conversation at a track meet Sunday week ago, where a fellow competitor had had some trouble with the Sprintpart studs bending!!! ? I have no idea how the studs would bend but apparently there was a problem. The offending parts were replaced no drama by SprintParts but I just thought I would give it a crack myself using Grade 8 UNF bolts and a friend who can cut the UNC threads at the other end or vice versa.
The bolts are cheap enough and worth giving it a go I reckon.
The short factory bolts have a thinner shank about an 1/2" down from the head to the thread. Is there a reason for that??
Michael

[john 215]

Hi Michael,

Welcome to this forum,

Can only echo what my learned friend wrote about rolled and cut threads, most modern head bolts on the stuff I work have rolled threads for this reason but also stretch bolts unlike Sprints. Also if a thread is dry or wet has an effect on end torque.

For those don't know difference between the two types of thread a bar -

http://www.portlandbolt.com/technical/f ... ads-bolts/

Do ARP do a suitable set of studs or bolts for Sprints ?, have these on my V8 and are top quality fasteners downside £$£$

You going to use the car for competition or on the street ?

Cheers John

[nervousnewowner]

sorry to hijack the post but sent u pm on here last night john but seems to be sitting in outbox not sent for some reason, am after some advice to pass on to a third party. can you get in touch mate......

[john 215]

Hi Dave,

Sorry mate still getting use to this place !! Reply sent,

Sorry Michael in my part of the hijack

Cheers John