Original Bolts and Nuts
G.K.N. supplied the bulk of the fasteners for the AC and these were mostly BSF. BSW was used for coach bolts and countersunk screws that passed through the wood frame. These BSW bolts/screws were fitted with square full nuts (chamfered on one side only), except for the detachable wing panels. These panels were secured to the rear end of the front wings by 1/4 BSW x 1" coach bolts with hot-formed wing-nuts (hot-formed wing nuts have rounded wings). Curiously, the smaller machine screws and nuts used extensively on the bodywork, were American ANC, 10-24. Presumably left over from AC's engineering work in World War 2? The ANC nuts were thin pressed hex. machine-screw nuts.
High tensile (HT) BSF bolts were grade D (equivalent to the later grade R). High tensile hex. bolts and screws were used to help bolt the wood frame to the chassis (in addition to the coach bolts). Mild steel hex. set screws were used for the front wings and also the wire mesh stone guards under each wing.
The washers used on the bodywork were table 4 light gauge washers. Table 4 washers have a slightly larger outside diameter than the more usual table 3 version.
G.K.N. ceased making fasteners many years ago, so you will be lucky if you can find any old stock. That's assuming that you want perfection so that bolt heads are embossed with GKN and Grade D! Grade R will have to do if old HT bolts need to be replaced. Self-locking nuts were mostly GKN's Autolok all-metal locking nuts. These are no longer available, but may need to be replaced, especially for highly stressed and/or safety critcial joints. Nyloc nuts can be used where temperatures are not high (below 120 deg.C), otherwise Aerotight or Phillidas are possible alternatives. The little 10-24 ANC screws/nuts are equivalent to 10-24 UNC still widely available.
Stainless steel is an attractive option for replacing the mild steel fasteners used extensively on the AC. It may be strictly non-original, but components rusting away are not going to help originality in the long term. It is especially useful for smaller fasteners that would otherwise be quickly destroyed by rust. Wood screws and the little 10-24 ANC (UNC) screws are the most vulnerable to corrosion.
There are a couple of words of caution to give, regarding stainless steel replacements. Firstly, strength: Ultimate strength of A2-70 stainless steel is lower than for the grade S (in the UK), grade 5 (USA) and grade 8.8 (metric) bolts used in later years. It's ultimate strength equals the grade R (old grade D) found on the AC, but stainless should still not be used as a replacement. This is because its effective yield strength is lower than the figure for grade R HT steel.
The second word of caution for stainless steel regards corrosion of other metals it comes into contact with. If stainless steel touches aluminium, the latter metal will corrode due to electrolytic action if it becomes wet. One's choice of material will still depend on various factors, and sometimes it works out best to use stainless steel and then take precautions to protect the aluminium. For example, the rear ends of the front wings are fastened to the plywood wing panels with 10-24 ANC screws and nuts. These are exposed to road spray and slush, and pass through wood as well as 3 layers of aluminium panelling. I decided to use some stainless steel fasteners on my AC, provided that the screws avoid contact with the aluminium. Waxoil is ideal for those fasteners passing through wood, as it does not harm the timber, and should keep out moisture and reduce direct contact between the metals.
One factor affecting electrolytic corrosion is the ratio of area of aluminium to stainless steel. That is, the stainless steel bolts have a small surface area compared to the aluminium panelling which in simple theory means little corrosion. However, it is more complicated than this. If the surfaces go through wet and drying cycles, there will be times that only small areas will be wet. It is the wet area that affects the intensity of electrolytic corrosion rather than the whole area. The fact is that even the mild steel original bolts caused body corrosion on the AC.
1) When making a typical halving joint between two pieces of timber, the wood screws should be angled so the they will tend to pull the joint up tightly (in the longitudinal direction).
2) Ordinary mineral oil and grease promotes rot if it gets onto wood. Therefore, wood screws and any bolts that pass through the wood, should be dipped in Waxoyl for protection.
3) Use either mild steel or stainless steel wood screws for the frame. Aluminium and brass screws are unlikley to have the required strength.
Torque Settings for Engine Bolt/Nut Tightening
Cylinder-head: 40 ft-lbs
Big-end bolts: 25 ft-lbs
Main bearing caps: 40 ft-lbs
Flywheel: 28 ft-lbs.
<< Page 8
Page 1 - Screw Threads
Page 2 - Bolts, Screws and Studs
Page 3 - Nuts
Page 4 - Nuts (Self-Locking)
Page 5 - Washers
Page 6 - Materials and Strength Grades
Page 7 - Tightening
Page 8 - Wood Screws and Nails