more threads or bigger diameter?

The engineering texts will tell you that threaded fasteners are to be used only in tension. If you need to resist shear, you need to design in features to do that. Dowel pins are one way, machined steps, ledges or mating lips are another way. Threaded fasteners have their own stress risers built-in for shear. See Carroll Smith's series on race-car engineering, particularly the fasteners and plumbing book.

Stan

If the thread is in the shear plane, the screw with the largest root diameter under the thread will be strongest. For a given OD, a finer thread will have a larger root diameter. If the shear is carried by the unthreaded portion of the screw, the thread pitch won't matter.

If the fasteners are torqued tightly enough that load is carried by friction in the joint rather than shear, the fine thread again has the advantage because its greater root diameter makes the bolt stronger in tension, allowing it to apply more force to the joint.

Among your choices, the 1/4-20 is strongest; a 1/4-28 would be stronger still.

What exactly does he say? Are those books the origin of this myth? You can't ignore shear, but it's just not so that screws must never be subject to shear. See the recent thread "Are higher grade bolts more brittle?"

Use Helicoils of similar threded inserts.

Carl Boyd

Well said. (I think he stole my answer!!! har har)

Assuming that you have enough depth in the aluminum to fully engage the threads, and that you're using a decent tap.

let me clarify:

the fastener will go through a 1/8" thick steel bracket (unthreaded, just a hole), and then will enter the threaded aluminum intercooler wall, which is 3/16" thick.

the intercooler weighs 30 lbs, which makes all of this somewhat critical i think, especially if the mounting fails, the intercooler hits the freeway at 80mph pulling the rest of the engine with it... :)

is the 3/16" thickness of the aluminum too thin to do this?

i dont want to weld to the aluminum because thats just a little too permanent, i may need to remove the bracket

what about rivets? stainless rivets can handle 1000 lbs shear...the would be semi-permanent but could probably be drilled out if needed

let me clarify:

the fastener will go through a 1/8" thick steel bracket (unthreaded, just a hole), and then will enter the threaded aluminum intercooler wall, which is 3/16" thick.

the intercooler weighs 30 lbs, which makes all of this somewhat critical i think, especially if the mounting fails, the intercooler hits the freeway at 80mph pulling the rest of the engine with it... :)

is the 3/16" thickness of the aluminum too thin to do this?

i dont want to weld to the aluminum because thats just a little too permanent, i may need to remove the bracket

what about rivets? stainless rivets can handle 1000 lbs shear...the would be semi-permanent but could probably be drilled out if needed

Just curious, are there four fasteners to choose from because there are pre-existing tapped holes in the intercooler of those four sizes?

Fred

The oil cooler on the front of our 1989 Ford Mustang GT is held on with 4 1/4"-20 bolts into some 1/8" thick x 1" wide 5052 aluminum strips that span the 18" or so from the top of the radiator mounting bracket to the bottom. The strips are held to the steel radiator mounting assy with 2 1/8" diameter steel pop rivets at each of the 4 connections. It seems to be holding fine at 165 miles per hour. If you were wondering, it's a semi-pro racecar, not a street car...

Just a thought, we had a sailboat in the So. Pacific with an aluminum mast and SS fittings. It seems that the combination created a small battery and we would find a white powder around the joints. Cad plated bolts had less corrosion and I guess, though we never tried it, titanium would have worked better?

stu

Always been my understanding that SS and aluminum were a no-no. Galvanic action hastens corrosion.

Lets see: a 1/4-20 stainless bolt has a max of 1300 lb/f of clamping force. Shear is about 60% of that, and you plan on using 4 bolts. So four bolts will resist a maximum shear of 3120 lb/f if torqued to about

65 inch/lb (note inch not foot). The intercooler produces a shear force of 30 lbs at 1 G, at 10 Gs its 300, at 100 Gs its 3000. The question is whether it will ever experience 100 Gs.

cheers T.Alan

The shear plane should have a non threaded diameter. Quite often shear bolts have under cuts at shear plane to tailor fastener to application.

Most common application is bolts for your snowblower auger. (at least in my part of the USA)

Wes

Follow up.

How does an intercooler produce sheer? I'd be thinking tension forces.

Wes

imagine sliding the intercooler in from the top of the car, like toast into a toaster, and then putting bolts horizontally into the sides..hence..shear

SOOOO... what does everyone think about the rivets? if they are using SS rivets on aluminum on a boat and its still held together, i think my car wont have too bad corrosion

the rivets will be here tomorrow! yay

The 5-year-old homemade guy line pulleys on my antenna have aluminum side plates, brass sheaves and ss axles. I took them apart Sunday for cleaning and greasing and didn't see corrosion on any of the metals. They are exposed to rain and wood smoke but no salt. Perhaps the grease film protects the aluminum.

The rebuilt antenna's terminals are stainless, too, and the connections have remained good enough that from NH I can receive good NTSC TV from Rhode Island and watchable signals from Hartford.

Jim Wilkins

The 5-year-old homemade guy line pulleys on my antenna have aluminum side plates, brass sheaves and ss axles. I took them apart Sunday for cleaning and greasing and didn't see corrosion on any of the metals. They are exposed to rain and wood smoke but no salt. Perhaps the grease film protects the aluminum.

The rebuilt antenna's terminals are stainless, too, and the connections have remained good enough that from NH I can receive good NTSC TV from Rhode Island and watchable signals from Hartford.

Jim Wilkins

Jim: My experience was with a McGregor catamaran in the Marshall Islands. We had an abundance of salt and warm air. It would only take one year for the white powder to show around a SS bolt in the Aluminum Mast. If I recall, there was some greasy like compound when applied to the bolt that would slow the process. Stu Fields