Mysteries of the Laycock de Normanville Overdrive

INTRODUCTION: All Austin Healeys which are equipped with overdrive have Laycock de Normanville overdrive units. There were three different models used (WN 1260, WN1292, WN1308) but they were all type A with ratios of approximately 32.4%, 28.5% (on BN1), 28.5% (BN4 and BN6) and 21.89% (3000 series). A 21.89% ratio was also a factory option on the BN1. The O/D is functionally divided into three sections: Clutch section, the oil pump section and the gear section.

The engage/disengage operation of the unit is driven by hydraulics with pressure derived from a reciprocating pump which is activated by following a cam on the third motion shaft (main shaft) from the gear box. The pump builds up pressure in the system. When the O/D is not engaged, oil accumulates in a reservoir (the end of which is a movable piston). As pressure builds up in the reservoir and the piston moves (against resistance of a large spring), a pressure relief hole in the side of the reservoir is exposed. The pressure relief hole bleeds oil back into the system. Thus, the final pressure of the system is stabilized. When the operating valve is moved to the O/D engagement position, the direction of the oil flow is altered allowing the oil in the reservoir to be pushed out with the force of the large spring behind the accumulator piston and directed to a second set of two small operating pistons which overcome the resistance of an array of springs. The pressure of the oil pump is also redirected to the operating pistons. The array of springs hold a conical clutch in contact with the annulus until overcome by the two operating pistons. When the two operating pistons overcome the spring resistance, the clutch is released from the annulus and engages the stationary brake ring. This action brings the sun wheel to rest allowing the annulus to over run (overdrive) a one-way clutch. The overdrive ratio is determined by the planetary gears which engage with the sun wheel and annulus.



The overdrive unit requires sustained pressure over 420 psi to properly stay engaged. The factory spec is 470 to 490 psi. You have a leak in your system. Think of the hydraulic system as a water hose. If there is a leak anywhere in your garden hose, the water pressure at the end of the hose is weak.


Trial and error with patience is the best method which I have found. There are a number of suspicious places where the system usually loses pressure:

A. Based on my discussions with a number of experts, the non-return valve spring is the most likely culprit. With use and age, this spring can weaken and allow the oil to leak back around the ball/seat on the return stroke of the oil pump. The OEM spring has about a 5.5 lb. installed pressure. I have experimented with more than a dozen spring combinations and have found that 5.5 lb. is a minimum. Based on my experience, a spring with an installed pressure between 6.0 and 9.0 pounds will function correctly. Although there is some disagreement among the experts, I found that too strong of a spring will cause the non-return valve to not open on the compression stroke of the piston. Thus, there is no pressure in the system.

Del Border has written an excellent article in which he suggests that the fabrication of a longer plunger to go with an older non-return valve spring usually increases the spring rate enough to solve the problem. His solution has worked in many cases. He increases by thickness of the plunger by .94 inches which raises the installed pressure of the OEM spring by about .5 pounds. (from 5.5 to 6.0 pounds). On an older spring, it probably raises the pressure back to the original 5.5 pounds. In my unit, I use a spring with an installed pressure of 7.75 pounds. TROUBLSHOOTING: A leak here will cause a pressure drop with the O/D engaged (if it will even engage) or not engaged.

B. The next most likely culprit is the O-ring(s) (BJ8 has two O-rings, older cars have one) on the outside of the sleeve insert which is in the accumulator piston reservoir (a blind bore). The A type O/D unit which is used in the Austin Healey is designed for an accumulator piston which is 1.75 inches in diameter (as used in a TR4). Austin Healey decided that the stroke from a piston of this diameter would be to strong for comfort so they sleeved the blind accumulator bore with a removable sleeve and used a smaller piston (1.125 inches in diameter). The sleeve has O-rings for sealing against the blind bore walls. These are suspect. TROUBLESHOOTING: A leak here will cause a pressure drop with the O/D engaged or not engaged.

C. The next suspect is the operating pistons. These are the pistons which overcome the pressure of the spring array to disengage the cone clutch and engage the brake ring simultaneously. There are two of these pistons. They are 1.375 inches in diameter. In earlier cars, these pistons have steel piston rings, five on each piston. On the BJ8, the steel piston rings were replaced with O-rings. TROUBLESHOOTING: A symptom of a leak here is that the pressure is good when the O/D is not engaged but drops dramatically and does not rise when O/D is engaged.

D. The next suspect is the accumulator piston. It has four steel piston rings. A ring could be broken or the bore in the sleeve could be deeply scored. If one is using the “competition” piston (which is really just the TR4 piston), there is no sleeve to suspect so the finish on the blind bore and the rings on the large size accumulator piston must be checked carefully. As with all piston rings, the gaps must be spaced around the piston � not lined up. TROUBLESHOOTING: A leak here will cause a pressure drop with the O/D engaged or not engaged.

E. The last suspect is the operating valve. The valve must be positioned properly when it is seated. It actually sits on a lever arm mounted on a shaft which runs through both sides of the O/D housing with another lever arm outside of the housing on the operating valve side and a third adjustable arm on the solenoid side of the housing. If the operating valve is not positioned in its up and down plane correctly, oil can bleed by causing a pressure drop. TROUBLESHOOTING: The symptom for this would be low oil pressure when the O/D is not engaged but good pressure when the O/D is engaged.

F. If all else fails, inspect the oil pump. IMPORTANT: Always remove the non-return valve prior to removing the oil pump. Failure to do so can result in damage to the oil pump housing as well as the O/D case. The steel ball in the non-return valve is harder than the oil pump housing and if not removed can cause a score in the oil pump housing. The housing, in turn, can then gouge the case as it is removed. A gouge here will cause loss of pressure when you reassemble the unit.

When inspecting the oil pump assembly, check the roller for slop. Also, look at the cam on the third motion shaft. Wear in either of these areas will cause a shortened stroke on the oil pump plunger resulting in non peak pressure. One final note on the oil pump. Laycock made a series of plungers for the oil pump which were defective � they were short by .030 or so resulting in a shortened stroke and low pressure. This may be why BMC put washers in the bottom of some of the accumulator pistons at the factory. If you have forever had problems which could not be sorted out, there is some chance that you might have a short plunger on your oil pump.


If you have eliminated all of the leaks, you should have at least 400 psi pressure. The Triumph spec for the same unit is 390 to 400 psi. But remember, they are using a larger piston. Thus, the Austin Healey needs the higher pressure to work properly. The pressure in the hydraulic system is regulated by the strength of the accumulator spring. The accumulator piston is pressed against the large spring behind it when the oil accumulates in the reservoir. The accumulator piston only moves about one-half inch before its movement uncovers a pressure relief bleed hole(s) in the housing (one large hole for BJ8, several smaller holes in earlier models). Thus, the stronger the spring, the more pressure needed to move the piston far enough to uncover the relief hole(s). In order to increase the operating pressure of the system, you must increase the pressure on the accumulator spring.

EXAMPLE: If you have 400 psi in your system and are using the OEM accumulator piston with sleeve, your pressure on the accumulator piston spring is about the same as your psi (the piston has .9934 sq. in. of surface area with hydraulic pressure of 400 pounds per square inch. Springs are rated in pounds per linear inch). The spring pressure and hydraulic pressure have reached an equilibrium. Thus, your spring pressure will have to be increased by 70 to 90 pounds to raise the operating pressure to 470-490. Since spring pressure is linear, we can calculate the amount of additional compression needed to bring the spring up to the desired pressure. First measure the length of the accumulator spring. Next, measure the depth of the accumulator blind bore (with the piston at the bottom) from the inside bottom of the piston to the top edge of the housing. Compare this measurement to the spring length. The difference is the installed compression of the spring. Mine was about .25 inches of compression. Thus, when the accumulator piston reaches the relief hole(s), it is compressed .75 inches (.25 inches of installed compression plus the .5 inches of compression needed to reach the relief hole(s)). This means that the spring rate is 533 pounds per inch (400/.75) for a non leaking system which pegs at 400 psi. To raise the installed/compressed spring rate to 470-490 (to equal the target hydraulic pressure), the compression of the accumulator spring must be increased by .1313 to.168 inches (70 or 90, respectively divided by 533). This can be done with washers in the bottom of the accumulator piston. Before using shims, always check your accumulator spring for total available compression travel so you do not cause coil bind. You can measure the distance between two of the coils and multiply by the number of gaps to figure the total available compression travel.

The figures are different with the larger “competition” piston and related springs since the surface area of the piston is 2.405 sq. in. instead of .9934 sq, in.. but the results are about the same.


You must make, borrow or buy an 0-1000 psi pressure gauge (glycerin filled) which is screwed in place of the cap covering the operating valve. This will monitor the pressure of the system.

NOTE: The O/D oil pump only operates while the third motion shaft is turning. A car at rest in neutral will not have any O/D pressure being built up from the O/D pump.


The competition accumulator piston is the same as the TR4 standard piston. It was used in the works rallye cars so it was dubbed the competition piston. The piston is larger in diameter (1.75 inches) so has about two and one-half times the surface area of the OEM piston. The result is that the push of oil from the accumulator piston reservoir is much greater in volume which causes the operating pistons to engage the O/D instantly instead of slowly engaging the O/D system. The result is a very positive engagement of the O/D like shifting into another gear and quickly releasing the clutch.


Most of the aftermarket suppliers have replacement parts.


Oil pump failure in O/D units is rare. They operate in an oil bath. Check the other symptoms first.