Trailing Arm Repair

 

 

 

 

 

 

 

 

 

 

 

 

Trailing Arm Strength Test

OK...so this isn't really a test of an actual Trailing Arm (TA) but more of a simulation using a 1/2" block of 6061 billeted aluminum and Grade 5 bolts. The idea here was to get an indication of at what point a bolt will strip out of aluminum. Ideally it would have been nice to have an old TA to use, as the composition of cast aluminum is quite different from billeted aluminum. Just the fact that cast aluminum is porous would lead you believe that bolts will strip more easily then bolts in billeted aluminum. But everything is relative and we know that the factory torque recommendation for the 5/16"-24 TA studs is 14-16 lbs. Here's a couple of industry standard charts dealing with torque and shear limits.

This table shows the shear point of bolts but
 I don't know what material they test in

So here's how my totally unscientific test was done:

As you can see from the picture below, a 1/2" thick block of 6061 billeted aluminum was drilled and/or tapped at few different sizes.

  • 3/8-24 is for people thinking of upgrading the stud size to get more thread in the TA.
  • 5/16-24 is the stock thread size that the factory rates at 14-16 lbs of torque.
  • "X" is the hole size recommended for the 7/16-14 tap that KeenSerts use.
  • "X" tapped @ 7/16-14 is ready for the KeenSert size used with the TA Repair kit.
  • "U" is the industry recommended hole size for a 7/16-14 tap. This is a slightly smaller hole size which should yield a deeper thread cut versus what KeenSerts recommend.
  • "U" tapped @ 7/16-14 to see if the KeenSerts will work with this size hole.

Six options to choose from

So now I clamped the aluminum block in my vise, grabbed my torque wrench went to work.

For both 3/8 & 5/16, multiple washers were used to set the depth
 and make sure I  was testing thread strength and not insert grip.

The bolt threaded in the full 1/2" in all tests

Here's how the first three tests went using Grade 5 bolts:

  • The 3/8-24 bolt stripped the aluminum threads at 55 lbs
  • The 5/16-24 bolt stripped the aluminum threads at 45 lbs
  • The 5/16-24 bolt in the KeenSerts "X" tapped hole had the bolt head snap off at 60 lbs. The KeenSert threads were undamaged

Next I tried to screw a KeenSert into the "U" tapped hole. We did a "U" sized hole because this is the industry recommended hole size for a 7/16-14 tap size.................well...... KeenSert knows what they're doing when they specify the larger "X" drill............you can't thread a KeenSert into a "U" hole that's been tapped to 7/16-14. It just won't fit.

The last test was to try and rip the KeenSert out of the aluminum. For this test I put used a socket larger then the KeenSert diameter so I could draw it up into the socket. I slipped the bolt through a grade 8 washer and into the socket and started tightening in 5 pounds increments. When I hit 60 lbs of torque, the only thing I had accomplished was to suck the washer into the head of the socket to the point were I had very little bolt head left to grip. Plan 2 involved using some scrap pieces of aluminum that Rick had sent me.

This was used in place of the socket. Plenty of room to draw the KeenSert into
Then I used some 1/4" plate as the cover piece. In hindsight, I should have put a
 grade 8 washer under the bolt head because..........
At 60 lbs, the head was digging deep enough into the plate that
 I had nothing left to grip
After getting the bolt out, here's what the top of the plate looked like
And the plate bottom looked equally as bad.

Now I'm getting ticked off as I really wanted to see if I could wreck the KeenSert. The next step was to put it in my vise and see what it takes to force it out.

Here it is in my vise where I cranked it tight by hand and then started whacking the
vise handle with a framing hammer to tighten it down until it failed. I was making so
much noise that my wife heard me two flights up and came into the cellar to see what
all the racket was about

Success! I finally forced the KeenSert to rip out the aluminum threads.

What does this all prove? Probably nothing because it wasn't done in the same cast aluminum as the TAs are made from but it did show that those KeenSerts are pretty tough. I liked the fact that a Grade 5 bolt head snapped off without damaging the KeenSerts and it was fun to do.