Thread engagement

[Walter R. Malik]

It make little sense to use a bolt or stud which takes more torque to stretch it, than the metal it is going into will handle.

[David Redszus]

It make little sense to use a bolt or stud which takes more torque to stretch it, than the metal it is going into will handle.

Absolutely right.

Another consideration is the radial thread clearance in the block, especially if it is aluminum and has been rebuilt a few times. Spark plug threads are also often excessively worn which will affect thermal transfer.

[Keith Morganstein]

The quick rule of thumb I was taught (thread engagement into engine castings)

1.5 X the bolt diameter into cast iron
2 X the bolt diameter into aluminum

[4sfed]

It make little sense to use a bolt or stud which takes more torque to stretch it, than the metal it is going into will handle.

True, but cost is the only downside to using the higher grade fastener. The modulus is the same so the stretch will be the same at a given torque.

[CharlieB53]

The quick rule of thumb I was taught (thread engagement into engine castings)

1.5 X the bolt diameter into cast iron
2 X the bolt diameter into aluminum

It’s been 40 years or so since I’ve been in class but I thought the general rule was 3 diameters thread into alum and 2 into ferrus metals.

I can easily be wrong.

But the OP needs to contact his supplier as I suspect he has the wrong set for his application.

[digger]

Grade 12.9 into aluminium can work well less than 2d but it is heavily dependent on if it's cast, wrought or extruded etc and the grade, grain direction and how many times it's going to be used. Never use fine threads.

The simplest way to judge is to look up the mechanical properties and find the UTS. There are more reliable ways such as "true UTS" as oppposed to "engineering UTS" that's normally quoted

[MadBill]

Wouldn't the yield strength be a better yardstick?

[digger]

Wouldn't the yield strength be a better yardstick?

No there was zero correlation with yield. I did my thesis on this in 4 grades of aluminium with hand calculations, fea and instrumented testing to correlate. A properly torqued bolted assembly has plasticity involved such that any assumption about linear behaviour are not accurate. This affects the load distribution on each thread which is heavily non linear (load dependent). When you deal with plasticity you need true stress and strain curves. Engineering stress and strain becomes less accurate the more plasticity there is. UTS was an ok predictor but when you have real mechanical properties everything falls into line

[Kevin Johnson]

...
With the factory heads and bolts I had about (i am guessing from memory) 7/8 to an inch thread engagement and now have 1/2 to 5/8. ...

https://www.gmperformancemotor.com/parts/12367779.html gives the length of the stock bolts that were presumably used on your crate motor originally. Hope this helps in your quest.

[swampbuggy]

I can not imagine ever using a bolt ( except on hi end con-rods ) where a stud (could be used) for multiple reasons. Mark H.

[Kevin Johnson]

I can not imagine ever using a bolt ( except on hi end con-rods ) where a stud (could be used) for multiple reasons. Mark H.

That is easy -- path of insertion of a retained component or limited space in the neighborhood of the fastener end. I guess you could then argue poor component or packaging design.

Aside: When designing removable oral bridgework, path of insertion and being removable by the patient without having to dislocate the jaw are top concerns. It is surprising the amount of interdisciplinary crossover there is.

[70MC]

The quick rule of thumb I was taught (thread engagement into engine castings)

1.5 X the bolt diameter into cast iron
2 X the bolt diameter into aluminum

It’s been 40 years or so since I’ve been in class but I thought the general rule was 3 diameters thread into alum and 2 into ferrus metals.

I can easily be wrong.

But the OP needs to contact his supplier as I suspect he has the wrong set for his application.

Charlie, I have been in contact with them many times, about bolts and now studs. I called yesterday and the day before, I wanted to talk to different techs to get differing thoughts to what was an acceptable thread engagement and to ask what part number I needed for studs. Both techs (Art, and the other was Zach I think) thought 1/2 inch thread engagement for bolts was sufficient. There was a few different part numbers for studs that I saw for BBC and AFR heads, so I asked what was different. One kit had 3/4 inch threads that threaded into the deck and the other was an inch. My block is a Bowtie block and has 1.100 threads in the deck, so I can use the kit with the inch of threads.

[70MC]

The quick rule of thumb I was taught (thread engagement into engine castings)

1.5 X the bolt diameter into cast iron
2 X the bolt diameter into aluminum

It’s been 40 years or so since I’ve been in class but I thought the general rule was 3 diameters thread into alum and 2 into ferrus metals.

I can easily be wrong.

But the OP needs to contact his supplier as I suspect he has the wrong set for his application.

Charlie, I have been in contact with them many times, about bolts and now studs. I called yesterday and the day before, I wanted to talk to different techs to get differing thoughts to what was an acceptable thread engagement and to ask what part number I needed for studs. Both techs (Art, and the other was Zach I think) thought 1/2 inch thread engagement for bolts was sufficient. There was a few different part numbers for studs that I saw for BBC and AFR heads, so I asked what was different. One kit had 3/4 inch threads that threaded into the deck and the other was an inch. My block is a Bowtie block and has 1.100 threads in the deck, so I can use the kit with the inch of threads.

I wanted to add that, like everyone else here that commented, I do not agree with the techs at ARP thinking 1/2 inch is good. I ordered studs, they will be here Tuesday. Thanks for the help everyone!

[Kevin Johnson]

...
With the factory heads and bolts I had about (i am guessing from memory) 7/8 to an inch thread engagement and now have 1/2 to 5/8. ...

https://www.gmperformancemotor.com/parts/12367779.html gives the length of the stock bolts that were presumably used on your crate motor originally. Hope this helps in your quest.

The quick rule of thumb I was taught (thread engagement into engine castings)

1.5 X the bolt diameter into cast iron
2 X the bolt diameter into aluminum

It’s been 40 years or so since I’ve been in class but I thought the general rule was 3 diameters thread into alum and 2 into ferrus metals.

I can easily be wrong.

But the OP needs to contact his supplier as I suspect he has the wrong set for his application.

Charlie, I have been in contact with them many times, about bolts and now studs. I called yesterday and the day before, I wanted to talk to different techs to get differing thoughts to what was an acceptable thread engagement and to ask what part number I needed for studs. Both techs (Art, and the other was Zach I think) thought 1/2 inch thread engagement for bolts was sufficient. There was a few different part numbers for studs that I saw for BBC and AFR heads, so I asked what was different. One kit had 3/4 inch threads that threaded into the deck and the other was an inch. My block is a Bowtie block and has 1.100 threads in the deck, so I can use the kit with the inch of threads.

Remember that not all the readers will know what diameter bolts are being discussed despite the ubiquitous nature of the BBC.* I also found several differing sources on what the torque values should be for the long versus short bolts.

Includes (8) 7/16-14 x 2.08" bolts P/N 88960334, (24) 7/16-14 x 4.060" bolts P/N 88960333, (8) 7/16-14 x 5.06" bolts P/N 88960332, and (40) hardened washers P/N 14011040

So... The decimal equivalent of 7/16 is .4375. You can now use the various factors suggested by members and use a step or biased rounding function to convert them back to standard fractional equivalents. You just gotta luv it when the bolt specs mix fractional and decimal values. It is always a trip trying to determine what the design manual called out for engines originally versus three or more decades later (for when you are trying to normalize a measured example back to a blueprint value).


*

Kevin, does BBC mean Big Block Chrysler or Big Block Chevy? How am I supposed to know?

[David Redszus]

Just to add to the fun, some engines make use of necked down bolts where the shaft is narrower than the minor thread diameter.
And, some will use threaded inserts with aluminum.

Fastener engineering should not be left to the marketing dept of some bolt manufacturer.