Remaining thread material at the thread throat area is the important dimension. The overall dia of parent metal thread and overall helicoil although similar leads to different results.
The original trailing arm castings had their patterns for the hub retention blanks positioned by a 3 month in service YTS (remember it's the 70s) spotty youth, ...unfortunately he was drunk ! This has lead to...variable positionings (the photo in No 12 post above looks pretty good to me) and I've found quite a few trailing arms where additional helicoils have breached the side walls.
The helicoiled 5/16" is obviously within an larger outside coarser thread form of the helicoil, but doesn't mirror it's shape exactly.
ie M
....m
This requires a tapping drill dia of .328 thou in the trailing arm to be used to tap and then draw the expanded helicoil down to a finished required interior thread size of 5/16th.
However a 3/8th UNC thread in parent metal (the trailing arm directly) only requires drilling at .312 thou dia (which happily is the size of the hole in the trailing arm when the 5/16th UNF threads are stripped or corroded out of it). This paltry 16 thou extra retained material in the hole dimension adds extra strength to the fasteners employed. That allows the 3/8th tap to achieve it's UNC form which has a thicker thread throat dimension (the material measured across the thread at it's base as it transfers into the core dia of the stud), with more retained material than can be achieved at the same dimension in the helicoil. ie it's achieved torque strength is stronger.
However strong the helicoil it's limited by the strength of the material it's fitted in.
Helicoils are excellent, on large engines subjected to repeated removal of the fastenings (ie Rolls Royce truck engines) the cylinder head threads are all helicoiled from new. This allows the helicoils to absorb wear and be replaced if a problem is found in their thread form.
If you examine the comparison photo (post No 8 above) of the standard stud against the hybrid 3/8th UNC-5/16 UNF stud, you will see the hybrid stud is longer on it's thread form by about 3 thread sections. This is useful for it allows you to continue drilling the original 5/16 dia hole deeper into virgin material and use this useful addition for extra threads and strength in the stud fixing.
Micky
PS: Whether the above influences your choice of retention method for enhanced rear hub fixings, many of the TR racers using their IRS/Stag Innsbruk trailing arm systems, drill and tap the trailing arms 3/8th UNC and use 3/8th UNC cap head bolts through the enlarged hub bolt dias to suit...apparently they don't come off.
The original trailing arm castings had their patterns for the hub retention blanks positioned by a 3 month in service YTS (remember it's the 70s) spotty youth, ...unfortunately he was drunk ! This has lead to...variable positionings (the photo in No 12 post above looks pretty good to me) and I've found quite a few trailing arms where additional helicoils have breached the side walls.
The helicoiled 5/16" is obviously within an larger outside coarser thread form of the helicoil, but doesn't mirror it's shape exactly.
ie M
....m
This requires a tapping drill dia of .328 thou in the trailing arm to be used to tap and then draw the expanded helicoil down to a finished required interior thread size of 5/16th.
However a 3/8th UNC thread in parent metal (the trailing arm directly) only requires drilling at .312 thou dia (which happily is the size of the hole in the trailing arm when the 5/16th UNF threads are stripped or corroded out of it). This paltry 16 thou extra retained material in the hole dimension adds extra strength to the fasteners employed. That allows the 3/8th tap to achieve it's UNC form which has a thicker thread throat dimension (the material measured across the thread at it's base as it transfers into the core dia of the stud), with more retained material than can be achieved at the same dimension in the helicoil. ie it's achieved torque strength is stronger.
However strong the helicoil it's limited by the strength of the material it's fitted in.
Helicoils are excellent, on large engines subjected to repeated removal of the fastenings (ie Rolls Royce truck engines) the cylinder head threads are all helicoiled from new. This allows the helicoils to absorb wear and be replaced if a problem is found in their thread form.
If you examine the comparison photo (post No 8 above) of the standard stud against the hybrid 3/8th UNC-5/16 UNF stud, you will see the hybrid stud is longer on it's thread form by about 3 thread sections. This is useful for it allows you to continue drilling the original 5/16 dia hole deeper into virgin material and use this useful addition for extra threads and strength in the stud fixing.
Micky
PS: Whether the above influences your choice of retention method for enhanced rear hub fixings, many of the TR racers using their IRS/Stag Innsbruk trailing arm systems, drill and tap the trailing arms 3/8th UNC and use 3/8th UNC cap head bolts through the enlarged hub bolt dias to suit...apparently they don't come off.
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