Motor mount bolts - grade 5, 8, ARP

[jgray]

I am a novice builder building a Type 65. I have a 302 crate motor that I am using. I have very little experience working on motors, or on cars in general. I have a question relating to bolts for the motor and transmission mount, but my question is also an across the board question relating to the type of hardware to use.

I have the complete kit and it comes with motor mounts. It has a bag of six bolts included by FFR. Four are 7/16-14 for the engine and two are 1/2-13 for the transmission. They are grade 5; I learned that by doing a bit of googling and watching a YouTube. Three lines in the heads.

My question is should I replace with Grade 8, which I understand are stronger but less ductile? Or ARP so I get fancier looking bolts that come in cool packaging?

As a kid I took metal shop (back in the UK) and have always been into airplanes; from that interest I know what stress fractures did to the early British jets. So my thinking is higher strength is not always better. Given that the engine block is cast iron it is considerably less strong than steel and the same is true for my Tremec T56.

So I’m thinking that Grade 5 is more than strong enough, and may even be better because of all the vibrations that these bolts absorb. Am I wrong because once the bolts are torqued down friction locks them to the material they are bolting down and absolute strength rules?

Or should I just stop overthinking things?! That is my problem, I have some amount of book knowledge but no real practical experience!

[Namrups]

I have also built a coupe using these supplied fasteners.
IMO:
1: Due to liability risk, I do not think FF would supply fasteners that are not designed for the job.
2: I have not heard of any failures of these fasteners on any of FF cars.

But having said that, it's your car and your build. If you do not think they are proper please by all means replace them. I have upgraded a number of the FF supplied parts, (brakes, push button start, windshield wiper system to name a few) not because I thought they wouldn't do the job but because I went with a better quality.

[Bob Cowan]

Grade 8's supply a superior clamping force, for high torque applications. But they can be brittle in lateral movement. It's a trade off.

Carroll Smith stated, "There is no place on a race car for a grade 8 fastener". Although I don't 100% agree with that statement, I think it's generally true. A grade 8 can safely be used in double shear, and when there is a locating dowel, like a bellhousing. Any other application risks failure.

Motor mount bolts/nuts just need to be tight enough that they don't fall out. They need very little clamping force.

Carroll wrote a series of books that are excellent. His recommendations are a bit dated, but the science is still valid.

[jgray]

Very glad I asked. Thanks. Makes sense to me and aligns with what I was thinking (good for my ego!). Grade 5 it is!

[Jeff Kleiner]

Jim and Jesper are brilliant automotive engineers…you can trust the fasteners they specify throughout the kit.

Jeff

[J R Jones]

Years ago SCCA routinely did examination of racing crashes, cause and effect. The reports of accumulated data indicated fastener failure was the greatest root cause.
In a production environment, purchasing and supply make a contribution that is not as knowledgeable or discriminating as engineering. Then there is the builder.....
Realize that you are the last inspector and assembly determinate. Evaluate every fastener for appropriate grade, length, thread engagement, tool appropriateness and torque.
You will see trends over time that will help assure you that every configuration is correct. There often are alternatives. I am not as complimentary as others, and in the end my judgement prevails.
jim

[jamminj]

[Justin]

I've run both 5 and 8 in the past with zero issue. Solid engine mounts in a 1000HP Mustang. I plan to use Grade 5 in mine without a worry.

[jamminj]

Take everything as information
Properly torqued grade 8 bolts will be way stronger than grade 5 bolts but in the case that a grade 8 bolt is loose then bending forces could break it easier than a grade 5 bolt
so there is truth on bolt sides of the argument.

these guys build frames for the fastest cars in the world.
top fuel funny car pro stock and they use nothing but grade 8 and consider grade 5 farm equipment bolts (it doesn't make them right info only)

Tim McAmis Performance Parts
I won't post his videos here (not family friendly a lot of cussing in his videos)
so, you will have to look him up yourself.

But his stuff is a much watch for those of us building high performance stuff.

[Skuzzy]

Here are the numbers for the two grades being talked about. The following are for bolt sizes from 1/4" to 1".

The SAE j429 minimum tensile strength psi (the load a bolt must stand before fracturing) for Grade 5 is 120,000 and for Grade 8 is 150,000.
The SAE j429 minimum yeild strength psi (the load a bolt must stand without permanent deformation) for Grade 5 is 92,000 and for Grade 8 is 130,000.
The Rockwell hardness for Grade 5 lies between C25 and C34 and for Grade 8 between C33 and C39.

The point of catastrophic failure for a Grade 8 bolt is going to be higher than a Grade 5 bolt, regardless of the direction of force. For bending, Grade 5 bolts will bend sooner than a Grade 8 will, but will also break before a Grade 8 will.

Now, for motor mounts, Grade 5 is going to be fine. If the motor mount is rubber (does not matter what type of rubber) the rubber is going to fail before the bolts will. The strongest urethane you can get today has a tensile strength of about 6500 psi. Of course if the urethane is reinforced with a fiber component, then the tensile strenght is higher. Even so, it probably will never get high enough to overcome a bolt.

That's my two cents on the subject. Personally, I prefer cold rolled threads for any bolt which has substantial torque on it, no matter the grade.

[Inman Lanier]

...

My question is should I replace with Grade 8, which I understand are stronger but less ductile? Or ARP so I get fancier looking bolts that come in cool packaging?

As a kid I took metal shop (back in the UK) and have always been into airplanes; from that interest I know what stress fractures did to the early British jets. So my thinking is higher strength is not always better. Given that the engine block is cast iron it is considerably less strong than steel and the same is true for my Tremec T56.

So I’m thinking that Grade 5 is more than strong enough, and may even be better because of all the vibrations that these bolts absorb. Am I wrong because once the bolts are torqued down friction locks them to the material they are bolting down and absolute strength rules?

Or should I just stop overthinking things?! That is my problem, I have some amount of book knowledge but no real practical experience!

You're on the mark there IMHO on the overthinking. You mentioned ductility - good for you - I assume you have some insight as to what that means (less brittle). Bolt strength is generally related to it's tensile strength. In other words, when you pull the bolt axially, at what tensile strength does it fail. The higher the strength, the more brittle it becomes. This means under load, it takes less of an impact to fail the joint. Regarding vibrations, however your motor mounts are essentially eliminating this load consideration; they dampen them out quite well.

Motor mounts are not in tension, they are in shear. I've never had to run a calc on the shear load, however trust me the OEM material already has a good bit of margin built in. Yes, as you increase the tensile strength of the fastener the shear strength will also increase, but do you really need it? I highly doubt it.