There are many SPH series helicopter flight helmets now available for auction, learning a bit about them just might help you get the best overall helmet for your money!

The SPH helmet series orignates back to the SPH-1 a helmet that dates to the 1950's the helmet was developed to provide a dual purpose function for the wearer, one was to reduce the ambient noise level in and around aircraft and the second was to provide head protection from impact.

SPH means Sound Protection Helmet

This series was developed for rotary wing flight applications, and has become the standard by which helicopter flight helmets are built by, they should not be used for fixed wing flight ops, just as the fixed wing helmet is not suited for rotary wing flight use. For a short while during the late 1970's and early 1980's USAF did use the HGU-55/P in helicopter flight ops, but soon standardized their flightgear to provide proper safety to their helicopter aircrews. Sometimes you will see the oddball HGU-55/P built to helicopter specs offered for auction.

The SPH-4 was the replacement helmet for the APH-5A and AFH-1 both of which saw widespread use during the early 1960's as a helmet for helicopter flight crews in the Vietnam war. These two helmets look very similar to one another but are nothing alike, the APH-5A is a much lighter helmet than the AFH-1 with the later providing ballistic protection from small arm fire and shrapnel.

APH means Aircrew Protective Helmet

AFH means Anti-fragmentation Flight Helmet

The SPH series developed over the years in to the SPH-2, SPH-3, SPH-3B, SPH-4, SPH-4B and currently the SPH-5
The various helmets can be seen at the Gentex SPH Helmet Evolution Exhibit visit:
 http://www.gentexcorp.com/default.aspx?pageid=1141
Visit the above link to see the developments to the series. These early helmets were made by General Textiles a compnay which later became known as Gentex. At times other companies have produced SPH helmets under license, Bell Helmet is the most notable here and the other major maker would be Astrocomms Inc.

Their are several variants to the SPH helmet often seen on eBay, the most common version is the standard SPH-4. The 4 helmet that was built to a US Army requirement from the late 1960's. The shell is made of a glass resin and fiberglass, with a leather and nylon strap webbing suspension system, and most often fitted with a single lens visor system. Although a dual lens system can sometime be installed. These dual lens systems are usually one of two type,  a ramshorn dual lens system (refered to the ramshorn since a riser is located on each side of the helmet that deploys or retracts the lens from the visor housing). This dual lens system is most often seen on the SPH-3 or SPH-3B (The 3 utilizes Resin/Fiberglass shells & 3B utilizes Kevlar shells) The other is the dual lens visor kit that is a standard item is that for the SPH-4B. And easily identified by the the ANVIS/PVS mount in the center of the visor cover.

SPH-4 Vietnam era (identified by the leather on the retention harness)

Now know this, many a SPH-4 under went a mid-life upgrade modification while in US military service, in which the web lining was replaced with a high energy impact styrofoam liner and a TPL liner (Thermo Plastic Liner), a kind of cloth and plastic sock system that has multi-layered bubble like plastic that can be added or removed to allow for the best overall fit on the wearers head. These plastic layers when properly fitted are eventually cooked to allow for the best proper conforming shape for the wearers head. This mid-life upgrade also saw the installing of the dual lens visor system, the standard visor on the SPH-4B and most often noted by the ANVIS/PVS mount on the front center of the visor housing. These SPH-4's are easily identified by the rubber plugs that are inserted in to the shell to fill the open screw points left behind when the webbing suspension system was removed and replaced with the TPL system. These are still SPH-4's and are not 4B's by nature of their just their mid-life upgrade.
 
Now a SPH-4B is made of a Kevlar shell and most often fitted with the dual lens visor system that allows for a ANVIS/PVS device to be fitted to the visor housing. The helmet is fitted with a TPL from the factory and has a nomex retention harness that utilizes a D-Ring fastener for securing the chin strap. The best way to tell a 4B apart from a 4 is this retention harness, as the 4 will have a  pull-the-dot snap fastern type chin strap.
What is the big deal here, between the 4 and 4B? Well it is the overall weight! The 4 in standard garb weighs in between 8 and 9 lbs. The 4B weighs in at half that!

SPH-4B with dual lens visor ANVIS/PVS mount, lip light and battery box

SPH-4B's never come with a pull-the-dot snap type retention harness, they ALWAYS have the nomex D-ring chin strap type retention harness. Also a factory new helmet ships with a smooth surface rubber edge beading along the shells edge. Never do they ship from the factory new with the hammered texture finish rubber edge beading.
These are sure signs that the helmet has been rebuilt and is not factory new.

The SPH-4 and 4B are no longer being manufactured...the current helmets in production are the SPH-5, HGU-56/P and HGU-84/P
Anyone telling you that they have a factory new SPH-4B manufactured in September 2006 is not to be trusted!
They just do not make them anymore!

There is yet another variant that should be mentioned within the SPH-4 series, the SPH-4AF was built to a US Air Force requirement in the late 1980's and early 1990's. The 4AF is fitted with a black retnetion harness and the TPL liner system from the factory and has the dual lens visor system installed that allows for ANVIS/PVS mounting.

Often seen as well here on eBay are the SPH-3 and SPH-3B that were primarly built to a USN requirement from the late 1960's Both of these helmets are fitted with a dual lens visor system, refered to as the ramshorn, and both came with the leather and nylon strap webbing suspension system, from the factory .
The SPH-3 shell was fiberglass/resin and the 3B was Kevlar. Both these type underwent a variety of modifications while in US military service. Most notable here is the webbing being removed in place of a V-Tech liner that allowed for a liquid rubber to be poured in to the liner and fitted to the wearers head allowing for best fit and overall impact protection. These V-Tech liners are easily identified by the overall black leather liner seen when viewing the interior. These SPH's were now re-designated HGU-64/P's

SPH-3B with factory ramshorn dual lens visor system
 
HGU-64's are often seen wearing either the factory dual lens visor system aka the ramshorn or the single lens visor system that supports a NVG ANVIS/PVS device.
When a SPH-3 has undergone the liner upgrade and gotten the single lens visor housing with the NVG ANVIS/PVS mount it is now know as the HGU-59/P.

HGU means Head Gear Unit

HGU-64/P with factory ramshorn dual lens visor system

These single lens visor ANVIS/PVS systems have been fitted to a variety of helmets over time and for specific end use. SPH-4's have been seen wearing them as have 4B's and HGU-64's. I've even seen this visor system fitted to a HGU-55/P the standard issue helmet for the UASF and US Navy fixed wing community. This system can be easily identifed by the mount in the center of the visor housing along with the center extension for the turn knob used to deploy and retract the lens.

SPH-4 with factory ramshorn dual lens visor system installed

SPH-4 with single lens ANVIS/PVS mount and battery box

SPH-5's the 5 is made from a Graphlon shell, a composite of graphite and nylon that allows for maximum impact protection and extreme lightweight comfort for the wearer. These are often fitted with either a single or dual lens visor system and ship with the TPL liner from the factory. This is the current issue helmet for the US Coast Guard (SPH-5CG) and the most widely used helicotper helmet in the commercial market today. This helmet can most often be identified by a overall black nomex retention harness that sets it apart from its brother the 4B, wearing the green nomex retnetion harness! Canadian Forces also utilize the SPH-5 cuurently in the form of the (SPH-5CF)

SPH-5 with ANVIS/PVS mount, lip light and battery box

These bits of information will hopefully help you become informed on the numerous SPH helmets often seen here on eBay for auction.

Recently the US military has adopted the most current rotary-wing flight helmets for front line use.
The HGU-56/P has become the standard issue helmet for the US Army, US Air Force and for helicopter pilots within the US Coast Guard. The 56 is made from lightweight composites that bring the lightest weight to the wearer and offer the optimal protestion from impact damage. The 56 comes with a variety of accessories to further enhance the helmet system. The maxillofacial shield is one such piece that allows for overall lower face protection from blast and debris while also blocking ambient noise for normal communications in high noise level enviroments.

HGU-56/P with ANVIS/PVS mount, lip light and battery box

HGU-56/P with maxillofacial shield, finished in overall matte black as worn by 160th SOAR crews

The HGU-84/P has become the standard issue helmet for the US Navy and the US Marine Corp. The 84 is made from lightweight ballistic nylon, kevlar and epoxies. Offer the wearer overall lightweight and impact protection. NAVAIR requires helmets to be overall taped with reflective tape allowing for easy spotting in open water enviroments should the aircrew go down at sea.

HGU-84/P with bungee dual lens

Further advice for you potential buyers is to ask the seller some questions to futher aid you with your choice to bid or not!
Most helmets are fitted with a white information tag located on the inside of the shell. This tag usually has size informtion on it along with manufacturer and lot numbers. Not all helmets will have these tags, having been removed for a number and variety of reasons over the years. But it doesn't hurt to ask if it is still there...and ask for a picture to be sent to you if it is! This way you know what you might be getting.

If there is a single picture in the auction, ask the seller to provide more and at different angles as well.

Ask the seller if there is any visable damage to the helmet and or visor system that might not be readily seen in the images. A cracked shell does no one any good at all making the helmet worthless to collector and potential user alike.

Want to wear your helmet, even if not for flight use? Get your size then!
The three common sizes are Regular, Large and Extra-Large. 7 1/4 and below should look at Regulars. 7 1/4 thru 7 1/2 should look at Large. And those with bigger than 7 1/2 set your sights on Extra-Large.

However make sure to consult the helmet fitting chart for the style helmet you are think about, as they do vary considerably from style to style!
The 56 alone is offered in at least 6 different sizes XS-XXL

For those that plan in using their helmet, never do so with knowing that it is flight-worthy and/or has been inspected by a ALSE certified tech,  finding the helmet worthy of flight ops.

Hoping that this information might help you get some of your questions answered and yourself just a bit more knowledgable about SPH helmets in general.

Regards,
SavoyZeroSix