brown7373

What effect does the bolt head size have on a torque reading? Case in point, are banjo bolts to attach calipers. The ones I had been using have need a 7/16 socket to tighten. I had to resort to identical banjo bolt that was 1/4 longer and has a 5/8 bolt head. Is 25 lb. ft. the same when tightening both bolt heads? Seems like the larger head would actually be tighter because the larger size would add leverage. I had to use longer bolt because the threads on the rebuilt caliper didn't start until about 1/4 inch down in the hole and the std. bolt thread couldn't catch enough to tighten.

MDchanic

Head size doesn't matter, shaft diameter, depth of thread engagement, and degree of lubrication do.

- Eric

joe_padavano

MUST

NOT

COMMENT.....

oldcutlass

Git yer mind out of the gutter, Joe.

MDchanic

D'oh. Forgot one:

Thread PITCH




- Eric

MDchanic

Hey, it's not like I'm reciting the Story of Joe McClock here.



- Eric

brown7373

I agree that the actual torque would be the same, but wouldn't the size of the nut matter? I can't tighten a water hose connection without the ****. A larger diameter **** makes it easier to turn. You turn your engine by the crank, and a large wheel attached to it make turning it very easy because of the size of the wheel. So doesn't the diameter of the wheel (or bolt head) multiply the force?

TripDeuces

I doubt the bolt size on the head of the bolt matters if you're using a wrench. Of course if you're installing them with your fingers it may be another matter.

You're dealing with a banjo bolt so you can't use the same torque specs. for a solid bolt of the same size. I guarantee that banjo bolt will snap long before you reach that torque spec.

Bolts are sized by there shaft diameter not the size of the bolt head. I don't understand how people have a problem with this concept.

joe_padavano

The only thing the size of the bolt head changes is the friction between the bolt head and the mating surface, or more correctly the torque lost to that friction. Yeah, friction is independent of surface area (μ times normal force), but the larger diameter of the underside of the head increases the lever arm acting on that friction, thus increasing the torque required to overcome it.

I will admit that this is a second-order effect at best and it swamped by the use or non-use of lubricant under the head.

MDchanic

No. The torque specified and measured is with reference to the central axis of the fastener, not to the edge of the head.

- Eric

RandyS

Geez, I feel like I'm back in Physics 101. How do you remember all that stuff!

joe_padavano

I actually to use this stuff day-to-day at work.

MDchanic

It'd be more practical if I could remember the last thing my wife told me to get at the store.

- Eric

joe_padavano

And THAT is why I have the post-it app on my phone!

Octania

What are you, some kind of Engineer?

You guys hiring?

":-)

"I will admit that this is a second-order effect at best and it swamped by the use or non-use of lubricant under the head."

Oh, that's just naughty. As are so many garage comments.

I have no idea how to get that off.
Give me a hand getting this in there will ya?
Oh damn that's heavy
screw it
are you sure this fits in here?
who the hell designed this thing?
etc.

brown7373

The reason I asked this was because I had to use a longer banjo bolt because on the rebuilt caliper, the threads started about 1/4 below the top of the mounting surface. I think after rebuilds and resurfacing, maybe drilling out and installing new seating surface, the threads don't go all the way to the top. If you look at the shorter bolt with the 7/16 head, the lowest thread is stripped. It wasn't long enough to extend far enough into the caliper to grab enough threads to allow it to be torqued. NAPA sells a longer 7/16 20 banjo bolt and it has a 5/8 head.

joe_padavano

Well, the first thing you notice is that while the hex is smaller, the integral flange in the new bolt results in about the same size mating area as the old bolt.

The bottom line here is that the size of the bolt head is irrelevant. I would be far more worried about the longer bolt bottoming in the new caliper and not fully compressing the crush washers, given the spate of leaking caliper threads we've had here lately.

brown7373

believe me Joe, I am very familiar with caliper leaking issues. I have been struggling for 2 months trying to get my car back on the road. NAPA replaced calipers 3 times. I contacted their tech support and they put me in touch with Cardone tech. They sent me calipers twice. The first also leaked, even after they searched their inventory for good cores and tested them, taking over a week. I got a second set last week and they looked good. But the NOS 487293 banjo bolts are too short...because the threads in the caliper do NOT start at the top of the hole. They start 1/4 of an inch down and the bolt isn't long enough to bite. I used the longer NAPA bolt which is 7/16 20 and 1 3/32 long. I measured with a micrometer and all the other dimensions, other than length and size of head (5/8), are the same. I also measured and the longer bolt does NOT bottom out before torque is reached.


That brings up a question. All GM A bodies use the same part number banjo bolt, the 487293. My Olds Parts Book lists it as 7/16 20 but does not list the length. My Pontiac Parts Book does list the length as 1 3/32.


If you look at the pictures I attached you will see the NAPA bolt is 1 3/32 long, but the NOS 487293 is 15/16 long. Did they change the spec at sometime?


I also have a picture of a good sealing surface next to one that is terrible and will not seal. Both of those calipers have threads that go all the way to the top of the hole and the shorter bolt will work fine.


The 3rd picture shows the threads starting 1/4 of an inch down from the top of the hole. The shorter bolt does not extend far enough into the threads to bite and allow tightening. That is why I had to use the longer but correct original length bolt (1 3/32).


I got my brakes bled and the car back on the road yesterday. I started the change over from power drums to power disks in March. I used all factory parts from a doner car, but bought rebuilt calipers and pads. Hopefully the nightmare is over.


Hopefully this may help others when they run into some of these issues.