[Healeys] Laycock overdrive

[Hank Leach]

Guys-this is a very lively topic and one of sometimes great mystery
and confusion. The unit is complex but the end result is working well
or not functioning at all.
On the subject of pressure required to operate the shift into
overdrive there are many schools of thought. Some people replace or
stretch springs or add washers to increase the pressure needed. The
health of the 8 springs that keep the unit in direct drive is
important to keep the clutch from slipping. The pressure supplied to
overcome the strength of these 8 spring , and effect overdrive, relies
on adequate hydraulic pressure. The change from direct to overdrive
needs to be smooth, not harsh, and if pressures are high the unit will
snap into and out of OD rather than the smooth interchange we are used
to while driving.
When applied to the Austin Healey 100, the Laycock overdrive was
initially a 32% reduction unit. It was soon found out in these early
Warwick cars that 32% was too much reduction. DHMC swapped units three
times in the original DMH tour car until they finally found a
combination that produced the smooth shift and complimentary gearing
desired.
It is difficult, today, to find any information on this original 32%
gearbox, however if you examine the booklet (attached) supplied by
Laycock for the early cars, you will notice that the casting allowed
for a large accumulator piston (seen on p6 in brochure). After some
research and modified engineering, the company came up with this alloy
"sleeve" called the piston housing (photo) which partially filled the
chamber of the original large accumulator casting and allowed for a
smaller accumulator piston that we see in all the later 28% reduction
units. That size piston, in proper condition, produced from 350-450
lbs of needed hydraulic pressure to overcome the springs influence.
I'm confident that the remark made by Geoff Healey in his writings
about the early transmissions "harsh shifting" revolved around this
huge accumulator piston producing too much pressure. Additionally,
there were three various accumulator springs supplied which adjusted
the pressure. The early boxes produced about 350lbs and the later
about 450.
Today, Overdrive Spares supplies that sleeve housing with only one
rubber O-ring as it is basically a filler part and not under extreme
pressure. The accumulator piston within is under that pressure. They
supply a modified piston using O-ring technology. So, it is not
unusual to get good pressures over the normal 450 lbs required. If it
works well don't fix it. The only concern is if the pressure does not
release when overdrive is not called for. A clean unit, with proper
30W oil, will allow that to happen. All this some old theory and
history to ponder. Hank
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