Straight for 560 Millimetres: 
Building a Custom Square Head Bolt 
That Held
How EFS coordinated forging, heat treatment, precision machining and thread rolling to 
produce a 560 mm structural bolt within tolerance.
A structural steelwork contractor 
needed something no catalogue could 
offer: a square head bolt in M20 × 560, 
property class 8.8, with a wide 45 × 29 
mm head and a fully machined shank 
running for most of its length. Achieving 
that geometry required a carefully 
coordinated manufacturing process 
rather than adapting a standard product.
When the first attempt fell short
The first attempt failed before the bolts 
even reached site. The supplier forged 
the head correctly, then heat-treated 
the bolts lying down, which is standard 
practice for shorter fasteners. At 560 
mm long, however, that approach created 
a problem. Several shanks emerged 
from quenching and tempering with a 
visible bow, placing them outside the 
straightness tolerance specified on the 
drawing. The client was left with bolts that 
could not be relied upon to sit correctly 
within a structural connection.
That is when the project came to EFS.
How EFS got the sequence right
EFS’s solution began with the heat-
treatment process. The bolts were 
quenched and tempered while standing 
vertically rather than lying down, 
specifically to control distortion across 
the full 560 mm length. This addressed 
the same defect that had caused the 
original production attempt to fail.
Straightness, however, was only the 
starting point. The drawing specified very 
tight tolerances on the shank because 
the bolt needed to seat accurately in its 
housing. The surface left by hot forging, 
with its characteristic rough finish, was 
not suitable for that level of fit.
To achieve the required dimensions and 
surface quality, the entire sub-head shank 
was machined by turning. This process 
restored the precise dimensions needed 
after forging and heat treatment while 
producing the surface finish necessary 
for correct positioning within the seat. 
Once this stage was complete, the 
bolts moved on to thread rolling, which 
formed the 115 mm M20 thread through 
plastic deformation rather than by 
removing material.
Because thread rolling preserves 
continuous grain flow instead of cutting 
through it, rolled threads offer improved 
mechanical properties compared with cut 
threads, provided the diameter has been 
prepared accurately beforehand to ensure 
the profile forms correctly.
Four processes, one coordinated solution
None of these four stages, hot forging, 
quenching and tempering, turning, and 
thread rolling, is particularly difficult in 
isolation. What makes a component like 
this successful is ensuring that every 
stage works together in the correct 
sequence on the same part.
Coordination was EFS’s role throughout 
the project. Rather than assigning the 
entire job to a single workshop, EFS 
selected and managed each specialist 
At 560 mm long, a new 
approach was needed
The treated bolt alongside  
Let’s Fasten.it magazine at EFS
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