How to balance cooling fans

[Mark Nunn]

My ’26 Runabout has a severe engine vibration at any speed over 30 mph. It really rattles my teeth. While saying a little prayer that the vibration is not coming from the transmission, I noticed that my fan hub bushings are shot. I could wiggle the fan blades about 3/16” front-to-back at blade ends. I removed the fan and hub assembly today and confirmed the need for new bushings.

Before I commit to putting the fan back on, is there a way to test it for balance? Two of the four fan screws are 1/8” longer than the others and need to be balanced. Heads are the same but lengths differ. But what about the fan itself? There is no central hole or divot to use a cone balancer. What have you guys done to balance them?

[Norman Kling]

The problem might be the looseness of the bushings. There is a ball bearing fan hub available which would eliminate the need for oiling the fan. They run very smoothly. Also can replace the bushing and shaft and have one like the original which will need to be oiled periodically. I don't know of a way to balance the fan, but the blades should all be bent in exactly the same position. Also if the two shorter bolts are the same length and the two longer ones are same length, you can place them diameterly opposite each other and it would balance. Don't tighten the belt too tight. When you have it correctly, the weight of your hand should be just enough to move it a bit.
Also try running it without the belt just long enough to be the vibration was actually the fan and not something else.
Norm

[Scott_Conger]

I am betting you will kill the vibration so much you won't worry about what remains.

With as much wear you have, it is undoubtedly orbiting about the spin axis at higher speeds while never reaching critical speed and smoothing out. This will result in significant banging around. All of that will disappear with properly fitting bronze bushings (if you can, make them from bronze...brass is almost a waste of time). I use 660 bronze for work like this. Do not become convinced that a dead-close fit is OK...it will seize. Conversely, don't fit too loose. If you can line ream to .001" over nominal shaft and then line-lap to .0020"-0025" clearance, you should be in great shape as you should maintain a good hydro-dynamic bearing if using oil and the cross-hatch finish will retain microscopic oil film for startup and shutdown. Purely reamed or slick surface finish on the bronze or even the shaft could bring trouble. Myself, I would not be tempted into using grease as that is an entirely different story. Keep the belt slack enough to allow the fan to spin some when the engine is off and you give it a whirl.

Finally, load the reservoir with a good spindle oil at first, like South Bend Lathe "C" (Mobile DTE 26). Within 50 or so miles, parts will be permanently married to a happy clearance and you can (carefully) switch to a heavier oil. I'd avoid way oil or rear end oil for at LEAST a few thousand miles as these will NOT get to where they belong with the clearances I've advised.

[TXGOAT2]

Blade pitch needs to be as close to the same all around as possible, and all blades need to run in the same plane. With ball bearings or a good set of bronze bushings with a few hours run time, the blades can be roughly balanced by removing the fan belt and giving the fan blades a spin with your hand. Note whether any particular blade tends to stop at the bottom. If one does, it's heavy. File a little metal off the outer edge of that blade until it stops in a random position when you spin the blades, or another blade begins to stop on bottom. If another one now stops on bottom, lighten it a little.

[TXGOAT2]

Engine vibration can be aggravated by loose or missing engine mounting bolts. U joints can also set up strong vibration. Loose body bolts and steering column bolts can cause more vibration to reach the driver. Have you tried parking the car in "neutral" and raising the engine speed to match the speeds at which the vibration gets bad?

[Mark Nunn]

Engine vibration can be aggravated by loose or missing engine mounting bolts. U joints can also set up strong vibration. Loose body bolts and steering column bolts can cause more vibration to reach the driver. Have you tried parking the car in "neutral" and raising the engine speed to match the speeds at which the vibration gets bad?

Thu U-joint is new (100-120 miles) Body bolts are tight. I completely rebuilt the steering column, only keeping the column housing and steering bracket with new bushings.. reving the engine in neutral produces the same vibration. I'm hopeful for fan imbalance.

[TXGOAT2]

Definitely eliminates the U joint. I'd take the fan off and try it again. Also check the crankshaft pulley. Oil the front motor mount and check the rest for tightness.