I have had shimmy while crossing train tracks at slow speed and I have had it at 40 mph rounding a curve and hitting a bump. Speed was not the trigger. All of the possible causes discussed here had been eliminated. It happened on both a 1922 Packard and a 1929, I suspect that the very large heavy wheels are a factor even though they had been balanced but I never found a definite cause and it rarely happened.

What type of wheels? Wood spoke, disc or wire? Front or back?

How about a photo of a wheel.

These cars steer beautifully if the wheel alignment is correctly set up so I do not see any point in locking the trunnion springs. Braking is also very good.

On the subject of wheel alignment, I found that the front axles on all of the 3 1920's cars I restored to be out of alignment. Not visible to the naked eye but all needed significant correction to the kingpin inclination angle. This work is easily carried out by truck repair specialists.

I drove only one of my cars before the front axle was aligned and it did not steer well but was excellent after only minor correction to the kingpin inclination angle.

The shutter are bolted to the front of radiator, so the radiator surround must first be removed. The shutter thermostat opens and closes the shutters, the spring is a relatively light spring to assist in closing the shutters.

Peter,
From memory all 4 trunnion springs were the same size when I assembled that area on my 740, however I suspect that the previous owner (Don) had them made and was probably not aware of any differences. Sorry, not much help.

I suspect that the 443 may be longer in the hood. For example, the 5 passenger phaeton bodies for a 640 and 443 are 1 11/16" difference in length from firewall to rear axle centerline. See attached drawings (which may not be clear enough to read). The wheelbases are 140 1/2" and 143", a difference of 2 1/2". The bodies and radiators on these two models could be positioned to accommodate these differences while using the same hood or the hood lengths are different.
I would tracking down owners of each model and asking that they measure the length of the hoods.

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Peter,
To the best of my knowledge, all 6th Series 626/633 cars had the same engine bay length as the 5th Series 526/533 cars regardless of whether they had a side mounted or front mounted water pump. The front mounted water pump on the 626/633 cars was drastically redesigned to take up far less space than the pumps on the earlier cars so that the eight cylinder engine would fit in the space previously used on the 5th series 6 cylinder cars. The engine in the 6th series standard eights is a very tight fit, length wise.
David

Packard called this a 2 passenger Coupe.

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I doubt you will find any wood patterns for your car, attached is the Packard body illustration for your model. If you have it professionally enlarged to the maximum size then draw scale grid lines on it using dimensions on the drawing , you can scale off missing dimensions.
Unless you can find an identical car with exposed wood this is about as good as you will get in my view.
The copy does not look very clear, send me your email by PM and I will try to send a better copy.

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I agree with Peter, I have had the same experience with my 633's and the 126 but not as often.
Those starter motors are about the easiest to remove of any car in my experience. Just one easily accessed bolt.