Regular vs Heavy Duty Fan Clutch

(Paul) #1

Getting things lined up. not going electric for now, restoring all to original for now.

S1 XJ6
Living within typically insanely hot environment (TX).
Intend to get original AC setup going.

Debating between Regular or Heavy Duty fan clutch.
Hayden 2705 vs Hayden 2747
Regular slippage 20-30% cold, 60-70% when locked
Heavy Duty 20-30% cold, 80-90% locked
All other things equal.

Uncertain what to choose. Big difference?
Don’t need unnecessary power loss and poorer fuel mileage.
Having restored entire cooling system and block, cooling shouldn’t be too much an issue even within hot environment, but additional stop and go protection, especially using AC, would be helpful.
I have seen posts recommending the 2705, however curious anyone use of 2747.
XJS V12 guys recommend the 2747 apparently.

Anyway, thoughts?
Thank you

(Robert Wilkinson) #2

I fitted a black plastic XJS fan. I think I’m using the regular duty clutch. The fan is a lot lighter, so the clutch is probably more engaged in practical terms. I like it.

I have a S3 radiator and shroud BTW, but I think the plastic fan might work with the S1 radiator as well. I used the metal fan with the later radiator for several years.

(Paul) #3

Thanks for thoughts.
I have the metal fan, and well, hesitant to change. The early XJ-S used the same metal fans as the SI XJ6 too.

No need to worry about cracking, etc.
And per some fan testing, the link having been posted before here on UTube, those roadkill guys…I think the original S1 metal fan is ok.
Plastic fans can actually steal more horsepower ironically it seems.

I’ve always had older Jags and never had a problem with original metal fans in TX.
The earlier tech may be heavier and less efficient(fuel pumps, horns, etc), but they can be far more reliable than things today.

I have come across parts exchange notes here and post stating by the fellow the 2705 was too aggressive in his opinion, and listed it as heavy duty, which it is not.

Ah well, ordered the 2747 already…probably mistake, but will compare the two and see what I think and post results fwiw.
The heavier metal fan lent me the thought that a bit more oomph from a stronger clutch may be preferred.

Restoring everything and going crazy, I purchased aluminum radiator to replace, but upon cleaning the original, which was in good shape really, the brass tanks, soldering work, and “XJ6” and model info engraved on the bottom tugged at my heartstrings so am choosing to restore it instead…zzzzzz

So it goes.
Kind regards

(DavetheLimey) #4

I fitted the 2747 clutch with an improved black fan to my XJ-SC three years ago. Along with a new aluminum rad. Major overhaul of A/C also. Extremely happy with results.

(Frank Andersen) #5

The main drawback on engine driven fans is low flow at slow engine rpms, Paul - typical while idling in traffic. Accentuated with AC engaged…

At speed, the fan plays little part - ram air is sufficient for effective cooling. This is why electric fans stay ‘off’ while cruising peace and quiet…:slight_smile:

The fluid coupling slips with resistance, which increases with rpms - the intent is to protect the fan from excessive rpms. The fluid coupling, working properly, never ‘locks up’ - slippage just increase with increasing load. Indeed, if the fan is spun for testing; the fan should spin a bit before stopping - if not, or freely spinning the clutch is defective…

Basically, there is minimal slip at low engine rpms - temperature is sort of irrelevant…

xj6 85 Sov Europe (UK/NZ)

(Paul Breen pay palled it) #6

I found an aftermarket Dayco - the type Toyota use on Land Cruisers and similar. Steel fan (series 1) fits straight up. It has a temperature operated valve that locks up the fan when most needed. The original one I removed relies on viscous slip only? I live in Sydney - we’ve just had a bout of days at or about 40C (105F). Whilst I don’t go out in that I have driven at 35C. Modern AC works without electric fan assistance. Car does not overheat, also has recored radiator. Added some sound deadening under the bonnet - as there is a bit more fan noise in summer. All good - happy with outcom. Paul

(Frank Andersen) #7

The standard Jaguar coupling is viscous only, Paul…

…the lock-up version likely reacts to coolant temp(?) - and probably won’t lock up unless needed. With a steel fan the risk of overreving and bursting the fan under lock-up is greatly reduced

xj6 85 Sov Europe (UK/NZ)

(Carl Hutchins, Jr. ) #8

I have read of this option. It is a match to duty.

Commercial vs pleasure is one definition. Slow speeds, city streets vs highway, hearse, ambulance, cop car all commercial, need more fan operation.


(Doug Dwyer) #9

Right. This is “thermal type” versus “non-thermal type” fan clutch.

Jaguar has used both types over the years.

The thermal type, typically recognizable by the small temp-sensitive spring on the forward face of the clutch, is design to slip quite a bit until xxx-temperature is reached…at which point it engages firmly. The little spring actually operates a valve inside the clutch, allowing fluid to flow into a cavity and create the viscous coupling. Often the engagement results in a distinct increase in fan noise.

The non-thermal type, recognizable by the lack of the temp sensitive spring, is designed to be firmly engaged until xxx- rpm is reached…at which point is begins to slip.

Neither type fully locks up, unless outright failure/seizing occurs. There is always some slippage.


(Paul) #10

Thank you all for taking time.
Yes, I appreciate and understand the differences between viscous and non-viscous fan clutches (thermal/non-thermal), just pondering choices of HD or RD.

And unusually, intuition tells me the earlier viscous version is wiser than the thermal which seems a quirky contraption. However all dismiss its quality, robustness and effectiveness. I guess it makes sense when you think about it.

Again, I own a '72 S1 XJ6 with the old saloon type metal fan.(same as on 420/MkX, S1 XJ-S and others no doubt such as 340 etc).

@Breen60 : Paul do you remember the part no. for that Dayco? Regular or Heavy duty? Do you recall?

At any rate, I don’t wish to return to a topic discussed ad nauseum, in part by me!
These threads are worth perusing for those interested.

In the end it is personal preference/experience to large extent.
Just need to experiment.

I am re-hashing the same thought process/dilemma, and again returning to the same conclusion… Restore all to original spec…

Although the elec fan upgrade does appeal to me, I’ve never had a problem in the past with my old Jaguars running as original. And I have restored entire block and cooling system already, and anticipate no problems even in TX heat.

YET … I am drawn to the idea of efficiency of course and as a perfectionist, the best option.

But I still read reviews of electric fans dying, and in TX traffic I can’t have that.
I may install a small elec. fan in the front of the AC condenser when I get there.

And if I replace with good elec fan unit, I need new alternator, new wiring, etc…$100 here and $100 there adds up, and I have much else to do yet.

An interesting note, I have read from apparent well respected Jaguar restorers addressing those who are considering adding electric fans to the older models such as the Mk2 and XK that the airflow has a dead area at those grilles /front radiator areas at 70mph.
Vis a vis, continuous fan operation is necessary even at speed to pass air properly and cool on those models.

Anyway, reading past threads there are some discrepancies I’d like to address.

The 2705 recommended for the S1 XJ6 is not heavy duty, it’s regular duty.
~3" long.

I received the 2747, and it’ll be too long at 3.5" for my Series 1 XJ6 and I’ve returned it.
(Amazon, free returns and job security for someone I guess)
It is also 7.5 " diameter…too large for this older metal fan style.

The clutch on my car was perfect at 2.5" in length. And I wouldn’t go larger than 7"dia.
And I think going longer than 3" will be too long.

I ordered the Hayden 2765 (Regular Duty) per past posts I’ve read here. ~2.5" long.

HOWEVER, one must be aware that on these older cars the pilot hole at clutch base (where attaches to water pump) is .63" (5/8"), this 2765 model has .75" hole.
Therefore it needs that bushing that many come with which fits within to attach to the earlier models (.63 water pump shaft) such as mine.

Strangely past posters stated how difficult it was to find this Hayden model 2765.
I don’t know how that’s possible. Internet has MANY vendors for them.

However, just came across a post at Summit Racing directly replied to by Hayden describing a shorter version of the 2747 (heavy duty)…the 2947. (see below)

A perfect option for those wanting a heavy duty. Again, many V12 XJS guys prefer HD…
I’m uncertain and am going to play with the Regular Duty and see what I think.

This 2947 is not listed on the Hayden Fan Clutch part list dated 2007 found on their site. Must be new.
(VERY useful list I might add. Lists ALL dimensions…)

They recommend the 2705 (regular duty) for 71-73 XJ6 models…but I have to be a wiseguy and try something else…

Of course I understand they do not “lock up” entirely.
I am simply trying to find some perfect balance, not wishing to sap HP, be too loud, steal mpg…load the motor unnecessarily,etc.

Going electric, well, one needs a good quality fan$$ or you are gambling/wasting time, then I must update alternator, wiring, install relays and so on…and I already have a lot to do yet and it adds up and that can be never ending.
And I already gotta deal on an original rebuilt original Lucas alternator from a Lotus …so heck with it.

So we’ll see how it goes…

In the end, for S1-S3 XJ6 Hayden options are:
2705 Regular Duty .63 pilot hole, 3"length (S1 specifically) 7"dia
2765 Regular Duty .75 pilot hole (needs bushing) ~2.5" length 7"dia
2747 Heavy Duty .75 pilot hole (comes with Bushing for .63) ~3.5" length
I measured 7.5" dia. on mine received.
2947 Heavy Duty Same as 2747 shorter version. How short? Pilot hole dia?
Dunno need to call Hayden.

Again, Regular Duty locks up ~60-70% hot, Heavy Duty 80-90% when hot.
Horses for courses.

Ok, that’s all I know.

EDIT: Hayden 2947 Specs found. Possibly great HD clutch option for S1 XJ6 and earlier metal fan models.


  • Mounting Hole/Stud Qty: 4
  • Bolt Thread: 5/16" - 18
  • Clutch Type: Thermal
  • Color: Raw Aluminum in Color
  • Description: Fan Clutches
  • Drawing Type: C
  • Duty Rating: Heavy Duty
  • Fan Bolt Circle : 3.250"
  • Fan Mount Dia. : 2.622"
  • Fan Mount Height : 1.09"
  • Material: Aluminum
  • Meets or Exceeds OEM Specs: Yes
  • Mount Dia. : 2.59"
  • Mount Type: Flange
  • Mounting Hardware Included: Not Included
  • New or Remanufactured: New
  • Original Equipment Replacement: Yes
  • Overall Dia. : 7.2"
  • Overall Height : 2.707"
  • Performance Rated: Yes
  • Rotation: STD
  • Style: Heavy Duty
  • Type: Thermal

(Frank Andersen) #11

‘Sap hp’ is somewhat strong, Paul - it takes little, and steals very little mpg. The advantage of electrics is that they run only when needed - but the are audible. But they still needs engine power…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

(Doug Dwyer) #12

There are no guarantees in life :slight_smile:


(Doug Dwyer) #13

Well, there ya go! Problem solved.

As hobbyists we are generally drawn towards making things better. There’s nothing wrong with that…especially when choices exist that really will be an improvement. And, it’s part of the fun. But, often, “as designed” is, simply, good enough.

Over the many years I’ve conjured up more upgrades and improvements than you can shake a stick at. Looking back, though, a great many of them left me feeling quite good while the car itself couldn’t care less, if you get what I mean. :slight_smile:


(Paul) #14

Well that’s something. Whew. Can you imagine the damage?!

Yes, indeed…true dat!

Recall reading through ole’ Bernard Embden’s S1 XJS site and his fixing the fan solid…
That was one thing I disagreed with and thought not such a swell idea.

Fwiw, I come across too many reviews and posts of elec. fans failing, plastic blades coming detached, motors dying in a year, etc. …even Mishimoto, Spal, Derale…BUT all new cars use elec. so what do i know?

It’s been 20yrs since i did this stuff.
Jeez, call me old fashioned? I’m simply going to add a small one before AC condenser.

There decided!

Have a nice day…and thanks all for the thoughts. Sometimes one becomes too wrapped up in this stuff…



PS: Didn’t see your other post. Had to add.
Thank you for that…spot on and well said. Could not have been more prescient.

(Paul Breen pay palled it) #15

I am in Japan until Thursday, Paul. In the meantime I will try to access my PayPal records for details and provide. Paul

(Paul Breen pay palled it) #16

Here is a link - ebay australia.

(Paul) #17

Thank you very much for that Paul.

Just logged in to say don’t worry and have a safe travel.

Japan would be a lot of fun and interesting.

Well traveled, but never made it to Asia all those yrs abroad.

Kind regards


PS. Mamma mia, those are expensive, wonder why?
Dayco 115765 (should the link disappear) is 2.75" long. Ideal i think…

Hayden is a well respected brand.
They recommend the 2705, 3" long.

I got the 2765, stated ~2.70" as i recall.
Arrives in couple days. Will update fwiw.
$45 USD shipped…2705 is similarly priced.

Nevertheless, useful info. Thanks again.

(tony) #18

Hi Paul…does that fit the S1 XJ ?

(also 420G / MKX / 420)

that is fitted with the smooth faced viscous clutch, NOT the later spring/thermal unit that is a bit thicker, and an absolute mongrel to fit on the earlier cars?

That would be a good breakthrough, as the older units are NLA, and especially helpful to us owners of the above Saloon models !

any mods needed?..or fitment issues?

are you able to be more specific about what Toyota models, or is it across many models?

In Australia, you can also run a Commodore 5.7L V8 manual Radiator, and Ford BA electric twin fan, for a cheap combo

I have a few older units as spares, one is failed, and i was going to open iot up and see what the issue is, they can be repaired, but it is not economically viable if a replacement can be found.

edit…that ebat unit you posted 22hrs ago is the LATER unit, and will not fit the early S1 XJ or Saloons without a bit of trouble…the bolt holes line up…the problem is the measurement in the pic “overall height 66mm” . The problem is worse on the Saloons…I think I have ~70mm clearance on my 420G

(tony) #19

I will respond further to your long post later, if I dont go paint now, my mrs will hospitalise me,
here is a excerpt from S2 Parts Manual showing early fan, and a adaptor arrangement for fitting later fan to early cars, wont work on Saloons though as clearance is not enough

The 2947 sounds like it will work

I have more questions (and factoids) as well

(tony) #20

picture of VC, one from 420G, one from S1 XJ6, they are dimensionally identical

here is a diagram I drew some years ago

edit diagram temporarily deleted, as some measurements are misleading, however “hat height”, which is critical to me, not so much an XJ owner is correct at ~ 54mm.

a new diagram will be inserted later

(for my ref, the clearance on a 420G between water pump hub face to radiator is ~72mm).

I believe this distance is somewhat more on a S1 XJ6

The above pictured VC is used in S1 XJ6, ( maybe some S2, according to parts manual?),
late MKX, (early ones used a fixed fan) & 420G and 420 Saloons, and is NLA, so a replacement that at least fits perfectly would be good