I ran the following torque calculations for Grade 8 bolts in aluminum bolt holes.  While perhaps slightly conservative, these values are consistent with industry recommendations and in line with recommendations from bolt manufacturers such as Diamond Engineering.
 
 

7/16" - 14 Pitch Bolt Torque Specs

 

For reference, a 7/16-14, Grade 8 bolt commonly has a maximum torque spec of 78 ft-lbs.[1],[2]

Alternately, the maximum torque values for 7/16-14 threaded holes in Aluminum depends on the yield strength of the particular type of aluminum alloy, the bolt size and pitch of the threads, and the depth of engagement of the threads in the bolt holes.  The following are some typical calculations for recommended and maximum torque specs for Grade 8, 7/16-14 bolts in various aluminum alloys.

A356-T6 aluminum is typical of aluminum used for Cast Aluminum wheels, whereas 2024-T6 and 6061-T6 aluminum are typical of that used for Forged Aluminum wheels.

 

As mentioned above, A356-T6 aluminum is typical of the type of aluminum used for Cast Aluminum wheels.  --  Harley Davidson Customer Service confirms that the XR1200's wheels are made from A356 aluminum.  --  Therefore the first calculation below is typical of the torque values that might be associated with the stock 1.725" bolts and stock drive pulley in the stock XR1200 wheel.  The following, "Shorter" and "Longer", calculations show the effect of getting less or more thread engagement due to perhaps a different bolt choice or pulley hub thickness.

 

7/16-14 Bolt threaded into A356-T6 Cast Aluminum (Yield strength of 25,000 psi)

For a 7/16-14, Grade 8 bolt, threaded 0.8” deep into A356-T6 aluminum, the recommended torque value is indicated at 404.2 in-lbs (or about 33.7 ft-lbs).  The maximum Torque value is 539 in-lbs (or about 44.9 ft-lbs). [3]   Therefore, while the bolt is rated up to 78 ft-lbs of torque, the maximum torque for the aluminum threads based on the length of the stock bolts is only about 45 ft-lbs.   (This is typical of the stock 68 tooth drive pulley mounted with stock hardware on the stock XR1200 rear wheel.)

Shorter

For a 7/16-14, Grade 8 bolt, threaded only 0.5” deep into A356-T6 aluminum, the recommended torque value is reduced to 252.93 in-lbs (or about 21.1 ft-lbs).  The maximum Torque value is 337.2 in-lbs (or about 28.1 ft-lbs). [3]   Tighten this to the HD specified 80 ft-lbs of torque for the pulley, and there is a severe risk of bolt hole thread failure.  (This is typical of the V&H 66 tooth drive pulley mounted with stock hardware on the stock XR1200 rear wheel.)

Longer

For a longer 7/16-14, Grade 8 bolt, threaded 1.3” deep into A356-T6 aluminum, the recommended torque is increased to 655.9 in-lbs (or about 54.66 ft-lbs).  The maximum Torque is 874.5 in-lbs (or about 72.9 ft-lbs). [3]   This would support theHD specified 80 ft-lbs of torque without risk.   (This is typical of the V&H 66 tooth drive pulley mounted with 2.25" long Grade 8 bolts on the stock XR1200 rear wheel.)

 

The following calculations for 2024-T6 and 6061-T6 aluminum show the differing torque values recommended for harder aluminum alloys.  According to Performance Machine, 6061 aluminum is used for their Assault wheel for the XR1200.

Again, the first calculation in each set is typical of the torque values that might be associated with the stock 1.725" bolts, with the stock pulley, in the stock XR1200 wheel.  The "Shorter" and "Longer" calculations show the effect of getting less or more thread engagement due to perhaps a different bolt choice or pulley hub thickness.

 

Threaded into 6061-T6 aluminum (Yield strength of 40,000 psi)

For a 7/16-14, Grade 8 bolt, threaded 0.8” deep into 6061-T6 aluminum, the recommended torque value is 646.8 in-lbs (or about 53.9 ft-lbs).  The maximum Torque value is 862.3 in-lbs (or about 71.86 ft-lbs). [3]  (This is typical of the stock 68 tooth drive pulley mounted with stock hardware on the Performance Machine Assault forged aluminum rear wheel.)

Shorter

For a 7/16-14, Grade 8 bolt, threaded 0.5” deep into 6061-T6 aluminum, the recommended torque value is 404.7 in-lbs (or about 33.72 ft-lbs).  The maximum Torque value is 620.5 in-lbs (or about 51.71 ft-lbs). [3]  (This is typical of the V&H 66 tooth drive pulley mounted with stock hardware on the Performance Machine Assault forged aluminum rear wheel.)

Longer

For a 7/16-14, Grade 8 bolt, threaded 1.0” deep into 6061-T6 aluminum, the recommended torque value is 697.7 in-lbs (or about 58.14 ft-lbs).  The maximum Torque value is 930.2 in-lbs (or about 77.52 ft-lbs). [3]   (This is typical of the V&H 66 tooth drive pulley mounted with 2.0" long bolts on the Performance Machine Assault forged aluminum rear wheel.  Due to the desing of the pulley mounting flange, this is the maximum depth of thread engagement for this wheel.)

Threaded into 2024-T6 aluminum (Yield strength of 60,000 psi) - (included for comparison, since some forged wheels are made from this material.)

For a 7/16-14, Grade 8 bolt, threaded 0.8” deep into 2024-T6 aluminum, the recommended torque value is indicated at 697.7 in-lbs (or about 58.14 ft-lbs).  The maximum Torque value is 930.2 in-lbs (or about 77.52 ft-lbs). [3]    (This maximum is the maximum torque spec recommended for a Grade 8, 7/16-14 bolt.)

Shorter

For a 7/16-14, Grade 8 bolt, threaded only 0.5” deep into 2024-T6 aluminum, the recommended torque value is decreased to 607 in-lbs (or about 50.59 ft-lbs).  The maximum Torque value is dropped to 809.4 in-lbs (or about 67.45 ft-lbs). [3]

Longer

For a longer 7/16-14, Grade 8 bolt, threaded 1.0” deep into 2024-T6 aluminum, the recommended torque value remains 697.7 in-lbs (or about 58.14 ft-lbs).  The maximum Torque value remains 930.2 in-lbs (or about 77.52 ft-lbs). [3]    (Once again, This maximum is the maximum torque spec recommended for a Grade 8, 7/16-14 bolt.)

 

In essence, the recommended torque value in forged aluminum is based on the aluminum bolt hole material, and the maximum torque value is limited by the maximum spec for the Grade 8, 7/16-14 bolt.

 

Comments

 

In reviewing the tightening sequence specified for the Sportster rear sprocket mounting bolts, I noticed that:

 

     In earlier years (e.g. 2007 Sportster Service Manual) the torque specification for these bolts on a Sportster had been 55-65 ft-lbs.

 

     In the same timeframe (e.g. 2003 Softail Service Manual) the big twins also had a rear wheel sprocket mounting bolt specification of 55-65 ft-lbs. 

 

     Additionally, I noted that 80 ft-lbs of toque is in excess of the commonly referenced maximum torque specification for a Grade 8, 7/16-14 bolt such as is used to mount the sprocket.  These bolts nominally have a maximum torque specification of 78 ft-lbs.

 

     And, while a Grade 8, 7/16-14 bolt might have a maximum torque spec of 78 ft-lbs, the recommended torque setting for such a bolt with a nominal 0.8” of thread engagement (the depth of engagement using the stock mounting bolt) in cast aluminum with a yield strength of 25,000 psi (typical of a wheel cast out of A356-T6 aluminum), is about 33.7 ft-lbs.  The maximum torque setting for this bolt seated to this depth in this type of aluminum is about 44.9 ft-lbs.

 

     Plus for an A366-T6 wheel, even if the mounting bolt length is increased to 2.25” to achieve approximately 1.3” of thread engagement (the maximum thread depth possible with the rear wheel design),the recommended torque specification would be about 54.7 ft-lbs with a maximum value of about 72.9 ft-lbs.

 

     Alternately, even if the rear wheel were forged from 2024-T6 aluminum (vice its actual cast construction), the recommended torque specification with the Grade 8, 7/16-14 bolts engaged to a depth of 0.8” would only be about 58.1 ft-lbs, with a maximum of 77.5.

 

These calculations would indicate that for the 1.725" bolt length in the cast aluminum rear wheel used on the XR1200, that the 80 ft-lb torque specification currently used by Harley Davidson for the rear sprocket mounting bolts is excessive.  And, even to reliably support a torque specification of 55 - 65 ft-lbs in the cast aluminum rear wheel, the sprocket mounting bolt length should be increased to about 2.25”. 

 

Although these torque calculations may be conservative, the Harley specified final torque value of 80 ft-lbs for the stock 1.725” mounting bolts in the cast rear wheel with the nominal thread engagement length of 0.845”, is well in excess of the maximum torque setting of 45 ft-lbs commonly calculated for such an application.

 

In applying 80 ft-lbs of torque to the rear wheel drive sprocket mounting bolts during re-installation of the sprocket, I did strip the threads in two of the five bolt holes in the rear wheel.  (Click here for a look at repairing the bolt threads.)  While it would seem to be possible to initially install these bolts in the factory, the likelihood of installing a V&H 66t pulley using the stock Grade 8, 7/16-14, 1.725” mounting bolts and Harley specified final torque value of 80 ft-lbs would appear to be about 50%.


 

 
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