In common with many of the continental manufacturers Siebe Gorman produced a 3 bolt helmet. The helmet was bolted to the breastplate by 3 bolts, 2 at the front and one at the back. The stud was captivated to the breastplate and the suit, with a large rubber flange was clamped between the flange of the breastplate and bonnet. The seal was almost always 100% watertight and it was a very robust system. The main drawback in common with the Russian and European helmets of the same design was that the diver had to squeeze in through the neck seal which was only about 14 inches in diameter. This design was not a very popular one with British divers and not many of this design of helmet were made.

This is what 3 bolt helmets looked like when they left the factory. 
One of viewers mailed us these pictures. 
That's what we call MINT condition...

Siebe Gorman realised the potential for the 3 bolt helmet but as sales were not very high they designed a 3 bolt lightweight helmet and suit known
by divers as the 'Harbour Hat' the helmet was based on the design of the Mine Recovery Diving Apparatus. It had an oval front window and a top window.
The window was hinged downwards so always connected to the helmet and not easily lost. The helmet could be equipped with a communications system
and a spitcock. The helmet was still connected to the breastplate with 3 bolts but in this design they too were hinged and the nuts did not need to be removed
from the breastplate to secure the bonnet. The rubber neck seal was different too. It was made from a soft flexible rubber material similar to latex which is used
now on dry suits . This rubber was much heavier and more robust than the latex of today. It had a ring of rubber which was located in a groove on the breastplate
This was trapped between the breastplate and the bonnet thus creating a watertight seal. The main advantage of this was that it was much easier to get into.
The whole Harbour outfit was lighter and often the suits had Wellington style boots on the feet so were ideally suited to shallow water or harbour use.

The bonnet could be ordered with a range of different port configurations. Seen above is a 2 port helmet.
Above we see the 3 port version with the hinged front window. The side ports are the conventional oval shape with the cast brass grills.
The left side pictures shows the front port open, the hinge being on the bottom. The side view shows the exhaust valve, telephone elbow and the air inlet gooseneck.
This style of exhaust valve is for regulating by hand. The style of this valve has been designed for use with helmets fitted with a telephone.
When receiving a message, the diver presses the spindle which passes through the cap of the valve.
This holds the valve on its seat and stops the noise caused by vibration and by the air escaping into the water.
Generally this is the valve pattern supplied to the British Admiralty.

Photo courtesy of the Clint Green Collection

The new Harbour hat could also be supplied with a conventional screw in face plate and side windows and a top light.
Here we have a tinned Siebe Gorman lightweight helmet with the latest style nametag.

Photo courtesy of Candice Waters