One of the most common questions on fan clutches
is, What is the difference
between a Standard Duty, a Heavy Duty, and a Severe Duty fan clutch? The following
will attempt to shed some light on this issue:
First, we must understand the operation of a fan clutch. Normal operation of a fan
clutch is to be engaged on start up. After a few minutes it should disengage and quiet
down. When the air coming across the radiator reaches a set temperature, a thermal
spring inside the assembly will turn, causing the clutch to re-engage.
When installing a fan clutch make certain that the clutch you are using is designed
for your application. Fan blades with a pitch of under 2 ½, require a Standard Duty fan
clutch. Blades with a pitch of over 2 ½ can use either Heavy Duty or Severe Duty
Standard Duty fan clutches are usually installed on cars and light trucks designed
with good air flow. Due to minimal airflow requirements, the fan blade equipped on
these vehicles, have a relatively low blade Pitch. The Pitch on a fan blade can be
measured by laying the fan blade on a flat work surface. Measure from the work surface,
to the highest portion of the fan blade. The pitch on Standard Duty fans is typically under
2 ½. Since these blades are under 2 ½, the fan clutch is designed to disengage with
minimal resistance. These fan clutches are designed to turn the fan at 60 70% the speed
of the shaft it is attached to.
Heavy Duty Fan Clutches are applied on vehicles with a higher demand for
airflow. As you would expect, increasing the pitch on the fan blade will increase airflow.
If the pitch of the fan blade is increased, the resistance on the fan clutch will also need to
be increased. A fan blade with a pitch of 2 ½ or over, will require the use of either a
Heavy Duty or a Severe Duty fan clutch. If the wrong fan clutch is installed, the fan may
spin freely and fail to produce adequate airflow to cool the engine. Heavy Duty fan
clutches turn the fan at 70 90% shaft speed.
Severe Duty Fan Clutches have the same operation as Heavy Duty fan clutches.
The difference between them is in the Working Area. As a fan clutch spins, it creates
heat, the larger the working area the easier the heat is dissipated. Severe Duty fan
clutches typically have 65 sq. in. of working area, Heavy Duty clutches have 47 sq. in. of
working area. Due to the larger working area, the Severe Duty fan clutch runs cooler
providing a longer life expectancy.
Over the years I made a personal list of fan clutches
that can be used on the 455 powered GS. Info is from various sources plus
my own personal experience. You can see that many are identical specs, I
have no doubt there are only a couple of manufacturers and everyone just
puts their name on the box.
Clutches listed by part number, standard (S), heavy duty (HD), severe duty (SD), fan height, pilot hole depth, pilot hole diameter, then notes/comments.
Pilot hole depth less than .75 inch usually requires cutting shaft on waterpump shorter with die grinder or drilling out clutch. When cutting water pump shaft dont overheat it. Take your time.
Pilot hole diameter was originally 0.63. Some clutches come with 0.75 holes and supply a bushing to fit the 0.63 as well. This works ok.
Most fan heights are 1.51 or 1.63, on a factory 455 GS of 71/72 vintage with the long water pump (ie, A/C or HD cooling) this puts the fan almost completely inside the shroud which is not ideal but works ok. Make sure shroud is mounted/centered properly or you could have contact with larger fans (20). Standard (short) pump and 1.51 fan height puts the fan half in/half out of shroud which is ideal. Only one clutch has short fan height, the Hayden 2765 at 1.09 which would put fan half in/half out on long water pump 455 cars. Have not tried it yet.
Most of these are not originally for our GS cars. Applications (when known) are posted to help source parts.
If your thermostatic clutch engages too early (cool), you can modify them by unclipping the coil, straighten it out with pliers, then rebend about 3/16 shorter, trim off the excess (die grinder)., and re-clip the end. The shorter coil will delay the opening to a higher temp (some clutches end up pretty close to rad so seems they are constantly on) so it wont be constantly roaring (great for cooling, sounds crappy). Please note this can ruin the clutch if you take off too much or bend something so be prepared for that if you plan on playing
On another note, Hayden and others make some very good 6 bladed fans for these clutches. Dont get all wrapped up with the 7 blade 18 stuff if you cant find one, the new fans have wider chords and good design, they flow a tremendous amount of air. No worries about cracks and bent blades either. Dont forget the 71/72 HD cooling/AC cars originally used a 5 blade fan. Larger fans turning slower are more efficient than smaller fans turning faster. Some of these may not be available anymore. Ive had great luck with Hayden and Four Seasons.
AC Delco 15-4320 SD (no info) AC Delco 15-4216 (GM p/n is 12306569) (no info)
AC Delco 15-4949 (no info)
GM12529772 (HD, not sure on sizing, rpm, temp, 85 Olds diesel)
Airtex 2797TA (no info)(same as Hayden 2797?)
Everco A8198 S, 1.51" fan, 1.03 pilot, .63 dia, (works great)
Everco A8189 HD, 1.63 fan, .63 pilot, .63/.75 dia (bushing) Everco A5570 SD, 1.63" fan, .63 pilot, .63 dia
Four Seasons 36952 S 1.53 fan, 1.03 deep, .63 dia (Rock Auto) (GS specd) (identical to Hayden 2705)
Four Seasons 36956 HD 1.62fan , .63 pilot, .63-.75 dia (bushing) (Rock Auto)
Four Seasons 36704 SD 1.63 fan, .63 pilot, .63 dia (Rock Auto)
Hayden 2765 S 1.09 fan, .56 pilot .63/.75 dia, rock auto
Hayden 2705 S, 1.53 fan, 1.03 pilot, .63 dia, (Rock Auto) (GS specd) (identical to FS 36952)
Hayden 2747 HD 1.62fan, .63 pilot, .63/.75 dia (Rock Auto) Hayden 2797 SD 1.63fan, .63 pilot, .63 dia noisy (after coil mod excellent, 190 degrees)(Rock Auto)
Imperial 215045 SD, ?? fan, .63 pilot, .63 dia (same as Napa 271301?) (85 Chev 3/4ton, 350)
Imperial 215046 HD, ??fan, .63 pilot, .63 dia (pics show busing so .75 as well?) (85 Chev 3/4ton, 350)
Murray 271301 (same as NAPA 271301?) (no info)
NAPA 271305 S 1.51 fan, 1.03 pilot, .63 dia NAPA 271303 HD 1.63 fan, .63 pilot, .63/.75 dia (bushing)
NAPA 271301 SD 1.63 fan, .63 pilot, .63 dia, (85Chev 3/4ton 454, low temp) (same as Hayden 2797?) (excellent)
TRW FC 135 no info