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I have installed a flex fan on my 261 and I don't know how much a flex fan flexes? In other words, how much clearence should there be between the fan blade and the harmonic balancer?

The fan will flex foward as it pulls air.If it clears when the engine isn't running, you'll be fine.

I have a 16" Flex-A-Lite on a '56 235, in my '49. The clearence is about an inch or less, I go 70-75 all the time, no damage.

One thing you could do to ensure the distance remains constant is to make sure your engine mounts are in good shape.

Is 1/8" enough room to go with?
Thanks!

I've got to ask again, is 1/8" enough room to go with on the distance between the fan and the harmonic balancer? Thanks!

What year truck do you have? What year 261 do you have? How are your engine mounts? Do you have the shorty water pump sold by Patrick's? Are you trying to run the stock fan? I wouldn't be comfortable with 1/8" clearance.

The engine mounts are a steel-rubber sandwich and are made to flex and dampen torque and vibration. IMHO, 1/8" is too little.

My truck is a 37 1/2 ton pick up with a 56 261 in it. The front mount is the standard older truck front mount, on the cross member under the timing cover. The rear mounts are on the bell housing. I have the short shaft water pump sold by Patrick's and others. I bought mine from [censored] Larrowe. I've got a 15" Derale Flex Fan mounted with a 1/2" spacer. I have another spacer that is 1/8" I can add if I have to. That spacer would put my fan about 1/4" away the harmonic balancer and about 1/2" or slightly less away from the radiator. Even when the 216 cu. in. engine was in my truck, I was working with no more than 5/8" from the stock fan to the radiator.
It's very cozy in there!

here is what i remember on my 327...
a flex fan (at least the one i used) is meant to pull less air at higher rpms - that means that it will flatten out -->i.e. the blades (or ramps) will not be as 'steep'. (air molecules act like the dukes of hazzard and like to jump off of ramps, propelled in the direction and angle that they are pushed)
that means that the outside edge of the BLADES will pull forward slightly at higher rpm. so if it clears the motor and associated parts behind the fan while not running - you are ok.

How about a 1 or a 2 inch spacer between the pulley and the fan?

6cylinders: If your Derale fan is anything like mine, it's not going to flex very much. Mine has some really stiff blades. They won't even flex when you push on them by hand. I don't have anymore room than you have described and have had no problems. Seems like it should work just fine.

Good Luck
George

Thanks, George! That was the last thing I had to work out and I guess I should have asked you first. Sorry!
Happy Thanksgiving!