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ted200008 09-02-2010 04:58 PM

harmonic balancer
i have a 71 350. i think its a 4 bolt main, but im not sure. anyways... the harmonic balancer is only 1 1/4" thick?? any other 350 that i have seen, the harmonic balancer is alot bigger? any explainations?

hvy barrel 09-02-2010 05:09 PM

I believe there were different size balancers used depending on the application. The main thing to be concerned with is the balance of the engine & the timing mark locations. A 350 is an internally balanced engine. You need to be sure to use a balancer for an internally or neutral balance rotating assembly. The 400 SBC was an externally balanced engine. The balancer is different for these engines. You also need to make sure the timing mark is compatible with the timing tab you are using.
I hope that this helps. I am sure there is more detailed information available on this subject.

ericnova72 09-02-2010 05:20 PM


Originally Posted by ted200008
i have a 71 350. i think its a 4 bolt main, but im not sure. anyways... the harmonic balancer is only 1 1/4" thick?? any other 350 that i have seen, the harmonic balancer is alot bigger? any explainations?

Most 350 engines from the 1970's have that balancer, only truck 3/4 ton 4-barrel engines and the higher performance Corvette or Z28 engines got thicker ones. Also, if you look at the thicker 1979-up balancers they are hollow on the back side of the ring, they look thicker but don't weigh any more than the thinner ones. The one you have was the most common seen in wrecking yards until the mid 80's when the 70's stuff started to become more scarce.

There are some really thin 7/8" thick or so balancers on some low performance 265/283/327 engines if you want to see a really small balancer.

techinspector1 09-02-2010 05:33 PM

hvy barrel has provided a good overview.
First thing you need to do is get the nomenclature correct. That piece on a 350 is properly called a harmonic damper. As such, it has absolutely nothing to do with balancing the motor. It is there to cancel out vibrations, or harmonics, that are generated by the twisting of the crankshaft due to power pulses from the piston. When the plug fires and jams the piston down in the bore, the rod throw on the crank twists out of shape a little. When it has sprung as far as it is going to one way, it springs back the other way as far as it can, then reverses and springs back the other way. Think of it as a plucked guitar string. If these harmonics are left uncancelled, they will eventually crack the crank and it will break (at least 50 miles from home when you have $2 in your wallet).

It is constructed with an inner hub and an outer inertia ring, separated by an elastomeric band of material which allows the inertia ring to rotate slightly clockwise and counter-clockwise in relation to the inner hub. On all internally-balanced motors like your 350, there is no eccentric weight on the damper. On some other motors, like the small block 400 and big block 454's, the motors are externally balanced with eccentric weights being placed on the harmonic dampers (now called harmonic damper/balancers and on the flexplate or flywheel.

There is a veritable plethora of different dampers for the small block Chevy. In addition to different thicknesses and diameters, there are also 3 different TDC ignition timing locations cut into the outer inertia ring. Unless you bought the car new and have owned it all its life, you have no idea if the damper and front cover (where the timing tab is located) are the same ones that came on the motor from the factory.

Fellows who are looking for immediate rpm gain, such as coming off the corner in oval dirt track racing, might choose the thinnest, smallest diameter damper they can find. Other fellows, who are building a streetable motor that they want to last for a while, might choose the largest diameter and heaviest damper they can find because it would be thought that such a damper would afford additional protection to the crankshaft.

I have used these guys for decades. They can provide you with a rebuilt damper in exchange for yours or you can buy one of their rebuilt units outright. They disassemble OEM dampers and damper/balancers and press them back together using new elastomeric material with hydraulic pressure. Once a stock OEM damper has been in service for a while, the elastomeric material begins to degrade and will sometimes allow the inertia ring to move in relation to the hub. This renders the TDC notch ineffective and any further attempts to time the motor with a timing light will be fruitless. I like to use these guys because I'm still dealing with an OEM hub, which I know is the correct diameter to provide a tight press fit onto the crank snout. Some aftermarket units are oversize, do not provide a press fit and will not be effective as a damper of harmonics. The unit must be tight on the crank in order to transfer the vibes to the inertia ring.
Click on harmonic balancers on the left of the page, then click on Chevrolet & GMC.

Like I said, there are 3 different positions for timing tabs and dampers on a small block Chevy, 12:00 noon, 2:00 O'Clock and 2:30 O'Clock, You must find TDC on #1 cylinder and install the proper damper with the proper timing tab in order to be able to time the motor with a light.

Also, let me help you guys out with this warning:

SSedan64 09-02-2010 10:45 PM

Good balancer Info, Pics to go with techinspector1's Info above>>

Dirty Biker 09-02-2010 11:52 PM

So a bigger damper is better for the crankshaft? So is there a benefit to using a larger eight inch one on a 327 or 283 vs a normal six something inch or is having the stock one rebuilt the best for these little motors? One 283 I have has the heavier 6 inch and one has the 7/8 incher. I mean, can anything bad happen from too heavy a damper? Thanks

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