Studs & Nuts by JMB Services

Stud & Nut kits for Classic Heritage Aston Martins

For many years cylinder head studs have been made from EN16T. This is a popular low alloy high tensile steel with 1.5% manganese and 0.25% molybdenum content. Whilst this material has good high tensile characteristics, its elasticity is not ideal for the situation that it's used in, i.e. Clamping an all aluminium cylinder head, which expands in use by approximately 0.006". The problem is that when the head cools this material will not readily return to its original length. Therefore it has the tendency to either break or leave the cylinder head unsatisfactorily clamped, causing leakage.

Furthermore the studs were made from bar sized material with cut threads and due to the drawing process, the surface of the material, to a depth of approximately 0.030" it was of inferior quality. I first came across this problem back in the early 80's when Richard Zethrin, then Technical Director of Hyde Vale, apprised me of a problem on their beautifully prepared V8 Aston Martin race car and asked for my thoughts on the matter.

My approach was to select a material of excellent HT quality, good ductility and the ability to retain its tensile quality through a variable temperature range. The choice of material was 817M40 heat treated to the upper end of 'V' condition. These studs were machined all over to remove degraded material, wasted to enhance elasticity and then thread rolled to give maximum strength.

Thread rolling is a process of producing a thread by deforming the grain structure of the material, raising the crest of the thread and crushing the root to the correct dimension. This improves the strength of the thread by bending the grain rather than cutting through it (See pictures below).

(BS970 : 8l7M40) This is an excellent material that includes 1.3% Chromium (for hardness), 1.5% Nickel (for toughness) and a splash of Molybdenum 0.3% (this is like baking powder i.e. it gets all the elements pulling together and loves the heat). This material is perhaps a little over the top, but it is after all for an Aston Martin.

In other words... 817M40 is a good material, that is very strong, it will stretch with heat expansion and return to its original length when the engine cools, keeping its tensile strength. Top this off with good quality nuts, proper washers and the job is finished. We have now repeated this manufacturing process across the full range of head studs - for the DB2 right through to the V8. Many studs have now been in use for over 30 years and we feel it's a job well done.