[Healeys] Laycock overdrive

Sat Aug 12 10:40:10 MDT 2023

Bob-good point. I just got a set of springs from Moss (from OD Spares
in UK, I imagine). The cost was $99 or about $12/spring which in the
scope of things is really not relevant. Holding that clutch in tight
contact to the brake ring is paramount and just imagine the number of
miles in direct drive that the car has experienced...many at high RPM.
The other side of the brake for overdrive position is the "lazy side"
as the operating pistons under hydraulic pressure have plenty of clout
driving mostly steady RPMs. Yes, springs wear out.
I place them on the bench and line up the lengths. They should measure
4-1/2" for the long springs and 4-1/4" for the short ones but
invariably there is differences in the resting length. After 50+ years
of pushing you would tend to get tired. When in overdrive, the springs
are greatly depressed. That is asking a lot for spiral metal to
accomplish. So, yes they may look good, but have poor stamina-oops, I
just describe all of us. Hank

	-----------------------------------------From: "Bob Spidell" 
To: healeys at autox.team.net
Cc: 
Sent: Friday August 11 2023 9:19:34PM
Subject: Re: [Healeys] Laycock overdrive

 re: "... The health of the 8 springs that keep the unit in direct
drive
 is important to keep the clutch from slipping...."

 I believe the shop manual admonishes owners to replace these when
 overhauling the unit, even if the old ones look and feel fine (some
say
 not so). I think my old ones are in a box on a shelf somewhere if
 anybody wants them.

 Note the O/D pump is a positive displacement pump that will continue
to
 produce pressure--driven by the full power of the engine--unless
there
 is a pressure relief mechanism.

 On 8/11/2023 10:34 AM, Hank Leach via Healeys wrote:
 > 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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