I need a fan clutch for my 68 427,
looking at web sites and ebay, I am lost as
to which clutch will work. Any thoughts? Thanks, Bruce

There are several different fan clutches, depending on year and application, but essentially they break down into small and large bolt patterns. I think '68-'71 uses the small bolt pattern (I don't know what the pattern size is). Other than that, it's the fan blade that mounts on it that changes the most with differing applications.

BTW - if you buy an aftermarket fan clutch at Pep Boys, AutoZone or Advance Auto (probably a LOT less expensive)...they usually come with slotted mounts instead of a specific bolt circle so they can fit many applications.

not only are there different aplications but there is also a diference in shaft lenghts ... i found out the hard way... the shaft was too short and it hit the radiator hose

There are so many good articles in here, this site is a great help. After reading this post, I checked my fan clutch (I don't have an over heating problem)and I noticed that my fan turns when the engine in cold or first started. So I'm not sure if mine needs replaced or not. With the enginge off and cold, I can turn the fan by hand; however there is enough resistance that it will turn at engine speed when I start the motor. Should it not turn at all when I first start it? It appears to be the original on my car.

It's when the engine is hot you can tell more about the fan clutch. At normal running temperature and low speeds the fan should pretty much lock up with the engine and run at equal rpm's. Shut the engine off and the fan should stop after a few revolutions. If it keeps spinning then the clutch is worn out.

There are thermostatic and centrifugal fan clutches available. They are supposed to do the same thing, but by different methods.

Your clutch sounds like it's operating normally.

It is operating normally. Fan clutches are either speed or thermal sensitive.

Both use a viscous silicone fluid to create resistance to turning, yet allow slippage.

Speed sensitive will increase air flow as the engine speeds up. When enough resistance is met from the fan and no more additional air flow will help, the clutch slips to prevent consuming more engine power and creating noise.

Thermo fans, which most of us have, can be identified by a small coil spring on the front of the fan clutch.
These are heat sensitive. The coil is really a bi-metallic thermostatic coil. When the radiator get hot, the hot air coming off of the radiator heats the coil, and it closes a valve. This creates much more resistance and increases fan operation and air flow. These are the better of the two, they also work as speed sensitive so you get the best of both concepts.
Thermo fans cut engine drag when not needed, but still supply all the necessary air when hot, without excessive fan drag


Failure of either type will do one of two things. One is reduce fan operation, and cause overheating. The second is locking and not slipping. The car won't overheat, you have plenty of air flow. You will hear a roaring noise from the fan. This can be quite loud. You will also lose engine power and fuel mileage. It makes a noticable difference.

This is the same concept as a flex fan. Instead of a clutch, the blades have more angle to move more air at low rpm. As fan speed raises, the blades will move more air than necessary and make noise. To prevent this the blades flatten out from the air resistance so they continue to deliver a maximum air flow with an even resistance.

The clutch fans work better in general, with the thermo having the clear advantage. Both cut engine drag. Why the non-thermal? Less Cost.

Flex fans are even cheaper, and their other advantage is they are light weight. Less to rotate, less power draw, if they move enough air for your application. It can be a good choice. The disavantage they are a constant resistance, so they never reduce drag as do the clutch types.

Thanks!