How many threads for safety?

Paul from Maine

Paul from Maine

Senior Member
While I am waiting for parts to rebuild my steering box, I decided to see if an old jeep rim that was lying around would fit the Model A lug pattern. By golly it did! However, I could only turn the lug nuts five revolutions before they were snug and tight. I loosened a lug nut on one of the 21 inch rims and counted nine revolutions before it came off. Just out of curiosity, how many threads need to make contact to be safe? I've seen A's with later Ford wheels and am curious how they fit on the lugs, too. Probably would never use the jeep rims on my A as I like the look of the wire wheels, but would like to know if it were possible.
 
Appears "possible" & appears very "unsafe".

Loosing a wheel while in motion is most dangerous for the passengers in the vehicle; but also the wheel running loose can be a killer & cause multiple funerals as an example of a noreworthy explanation for safety.

Model A lugs have 20 threads per inch; hence 9-10 revolitions are about 1/2" of threads on a nut or lug; & 5 revolutions are about half or 1/4" of threads.

Good idea not to use your jeep wheels unless one removes the engine & pulls the car with a pair of mules on a farm.
 
Generally four threads are considered full strength on a bolted connection. This a general rule of thumb based on contact, cantilever effect of the thread, area in shear.

But Charlie is correct that your lug nut should "pinch" your wheel. Merely putting an aftermarket wheel on a Ford brake drum may not cut it if the rim is not fully supported and pinched between the lug nut and brake drum.

Another way to check strength is to use the bolt sizing and look at the torque tables. If you can approach anything similar to what a grade 5 bolt may be able to sustain torque wise, then you're probably approaching the bolt strength indicated.

If a bolt - any bolt - doesn't fail during the action of torqueing at rated torque for it's size - then it's unlikely to fail in service ever.

Joe K
 
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