Cylinder head bolt tension

I read this with interest, especially when the question asked about thread engagement when bolt or stud is in tension. I.e. Studs pulling out of the block.
The calculation for this need to take into account the stretch of the stud where it engages with the female threads (block). Convention says that the the first thread take about ~33% of the tensile load, the 2nd thread takes ~25% of the tensile load. This is due to the stud/bolt stretching between the first and 2nd threads, causing more of a pull on the first thread than the 2nd thread.

distribution of load along the length of a bolt shaft

This means that more than 50% of load is carried by the first two threads, A rule of thumb is that engagement past 1.5xOD is doing nothing more. If recoiling a pulled thread always use the 1.5OD inserts. Also, you can see why a course thread is used in female threads for studs/bolts in tension.

Your idea for empirical data from pulling stud from a scrap block is the best, then helicol and try again. Try both UNF and UNC helicol.....

My current engine had a head stud or two start to pull out of the block. I've recoiled them all (all 20), but not used the engine yet..... hence my interest.

A great reference book is from the great racer Carroll Smith, Nuts, Bolts, Fasteners and Plumbing Handbook.
It doesn't go into the deep dive calculations, but is full of rules of thumb (or is it Rule of thumbs?) and explanations.
 
From
www.assemblymag.com

Analyzing Bolted Joints for Clamp Load and Joint Stress

Many important considerations in bolted joint development are often overlooked. While estimating the bolt tension achieved for a given tightening strategy is certainly a common focus, the effect of load on joint components is less fully discussed and understood.
www.assemblymag.com www.assemblymag.com

Analyzing Bolted Joints for Clamp Load and Joint Stress


Analyzing Bolted Joints for Clamp Load and Joint Stress

This all reminds me of designing aircraft test rigs. We were being asked to fatigue test structures, which have safety factors of ~1.1 or 1.2, but we'd build our rigs with a safety factor of 8 or 10, so we didn't need to go deep into stressing. This was fine for the majority of our rigs, but where we attached to the structures, with the aircraft fasteners, we could never achieve the high safety factors, as we couldn't just use larger/longer bolts etc. We'd stress our parts for fatigue and calculate how often to change them. The aircraft stress engineers hated us, as they would be concerned with the damage to their structures from us replacing our rig parts, I.e. removing and replacing the fasteners.
Very interesting work.
 
One way to preserve a block if the threads are getting a bit iffy is to fit a complete set of Lock n Stitch Full Torque thread inserts.

I imported a kit from the US in order to repair Ford 289 HiPo cylinder heads that had some of the screw in rocker studs fail due to cracking around the holes.

These kits are incredible, they blow Helicoils out of the water, the repair is stronger than original.
The inserts use a special thread that has the outer threads angled upward so they bite into the parent metal. There is also a variant that has a shoulder on the top that has profile such that it actually pulls the sides of the hole inward when tightened down, this repairs cracking around the hole.
Here is an article on the Full Torque inserts. Everything is in the kit, inserts, drill bits, tap, top cutter, locking pins.

 
Hi Mark,

Marginal is quite possible. What they would normally do, certainly these days, is to both factor the bolt tension by increasing it by say 20% and use a capacity reduction factor on the block threads. Then when they are compared provided the shear resistance still exceeds the load imposed by the bolt, all is good. If on the other hand the outcome will see block thread failure, then the design will be changed. It could encompass an increase in thread engagement length or a change in block material properties.

Destructive testing is a very good idea.

Absolutely, it shows Rover required good quality components for them to keep going after more than 50 years. The weak link as you know are the female block threads. Grade 8 imperial bolts are harder than the aluminuim alloy block as a Brinell hardness test would illustrate, so the female threads wear every time the bolts are removed and refitted. That's why studs are preferable if the heads are being removed frequently.

Ron.
 
cobraboy said:
Ron

So if you assumed the worst case scenario on the alloy composition and we have no way of getting to the casting company for any heat treatment specifications then it looks like the stress bearing properties of the block - after 50+ years of working may be marginal if the threads are in any way below par and the blocks did not get heat treated.

What is needed is someone with a scrap block with some decent threads to carry out a destruction test by torquing a bolt in 10 lb/ft increments past 70 lb/ft until it pulls out.

If the Rover bods did not allow much headroom with both the bolts and blocks it is a wonder that 50+ years later we are still able to screw these things together and enjoy them.

Mark
Click to expand...
Mark, it sounds like you've got the scrap block already...with one thread pulled out.

For anyone doing this testing, you would expect a lot of scatter in the results as there are many variables. I'd expect Rover's engineering department would have been all over that, arriving at the recommended 70 foot pounds torque value.

Happy Motoring, Munro.
 
Stuck-in-1972 said:
Mark, it sounds like you've got the scrap block already...with one thread pulled out.

For anyone doing this testing, you would expect a lot of scatter in the results as there are many variables. I'd expect Rover's engineering department would have been all over that, arriving at the recommended 70 foot pounds torque value.

Happy Motoring, Munro.
Click to expand...
Munro
The block was repaired and covered many miles including track days and never gave a problem.
I have SDI and a P6 engines at the moment but am reluctant to test on those as they were both running when removed.

Mark
 
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