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That Scuttle Shake

By Mark C. Lambert

Originally published in the Austin-Healey Magazine, February 2000

In Donald Healey's day, "scuttle shake" referred to a pronounced vibration caused by excessive flex in a vehicle's body structure. It frequently occurred in wood‑frame-bodied vehicles at high speeds. Since many sports cars of that era suffered from scuttle shake, Donald and Geoff Healey were determined to eliminate it in the car that was to become the Austin‑Healey. They succeeded by using the relatively new building method of welding the steel body substructure to the frame. Unlike the wooden structure bolted on top of the frame (as used in the T series MGs, etc.), this welded superstructure added torsional strength to the rigid crucifix type frame.

The term "scuttle shake" has been abused by current Austin‑Healey enthusiasts who assign it to any vibration that they have failed to rectify and are resigned to live with.

If your Austin‑Healey suffers from any pronounced running vibration, then it has been weakened by rust or accident, or has a wheel, tire, drive shaft or brake drum imbalance. You do have a problem, but it's not scuttle shake. If your car's problem is rust, it's a parts car until the values of the sound cars double their current levels. If not, your car can be made vibration‑free throughout its full speed range.

To correct vibrations, begin with the heaviest components and work your way downward. First, check the wheels and tires. After these are confirmed correct, go directly to the drive shaft. Every Austin-Healey I've owned or serviced has needed some part of the following procedure and the results have been rewarding.

There are two causes of drive shaft vibration: a damaged tube or an out‑of‑center joint cross. If your car suffers from either, you'll feel a vibration through the entire car that increases with speed.

As far as I can determine, the tubes were true at manufacture. Any problem with the tube is likely to damage (accidents, road debris, dropping, etc.). This is not the case regarding the yokes, however. Manufacturing error was apparently too wide, causing some shaft assemblies to spin out‑of‑true.

A proper check of the shaft is done with a dial indicator. Position the car overhead on a wheel lift and raise the rear tires just enough to allow the drive shaft to be turned by hand. Take the readings about one inch from the yokes. A shaft no more than .006" out-of‑true is OK.

By the way, in DMH's racing days, mechanics often balanced drive shafts using radiator clamps. They would jack up the rear wheels and, with the engine running in gear, slide under the car with apiece of chalk. Holding the chalk as steadily as possible, they would touch it against the turning drive shaft in several spots. If the shaft was "out", the chalk marks would be heavy on one side and light to none on the other. Then they would position the hose clamp nuts to serve as weights opposite the heavy chalk marks, thereby balancing the shaft. I know people who have done this and lived to tell about it. They agree the dial indicator method is far less dangerous.

I have checked about forty cars. All had good crosses and slip joints. Most were in the .020" range. Only two cars had damaged tubes. Two others were less than .006" out. The rest had the error in their yoke machining.

This article was inspired by a shaft that was .052" out at the front yoke, .035" out at the rear, with a good tube (.005"). This car felt good up to 55 mph where a slight vibration began. The vibration became pronounced at 70 and unnerving above 75 mph. I noticed the indicated run‑out corresponded to the position of the shaft yokes, not the flange yokes, so I suspected this problem too, was in yoke machining. However, it's always best to begin with the tube. I removed the assembly and took it to our local drive shaft shop.

Many drive shaft shops claim to "balance" drive shafts, but few actually do. Most only check the tube for trueness, remove any dents, and replace joints. Fortunately for us, balancing is unnecessary because our car speeds do not require that degree of precision. Unless the tube is heavily damaged, they can probably get it to within .006".

After the tube is OK'd, the drive shaft must be installed onto the flanges of the transmission and pinion to be checked. This is, of course, best done on the car.

With the U‑joint circlips removed, mark the position of the runout on the shaft. Using a hammer and soft drill, center the yokes on the crosses until a good reading is obtained on the dial indicator. As the U‑joint caps are moved in their yokes to achieve this, make certain they are pushed flush against the crosses. When centered, the clips will no longer fit, thus illustrating that the yokes were machined in error. How do we secure the caps without clips? We tack weld.

Die‑grind a small kerf into each yoke down to the circlip valley. Welding only in the kerfed area simplifies future replacement. Any vibration remaining is in the brake drums. Take these to a clutch specialist and have them dynamically balanced on a pressure plate balancer. Afterward, your Austin‑Healey will be as smooth at 110 mph as an Acura.

If only the Acura sounded like a Healey.

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