[Healeys] Laycock overdrive

[Bob Spidell]

Two possible causes of this that I can think of:

1) O/D clutch is so worn it becomes jammed together (FWIW my BJ8's 
clutch was still serviceable at 205K miles)
2) something is preventing pressure on the operating pistons from 
bleeding down; not sure how this would happen, hopefully someone will 
chime in

bs


On 8/12/2023 2:19 PM, David Wirken wrote:
> Now that we are on the OD topic, I have a Healey 100 whose OD 
> won't disengage after I switch it off, it's not an electrical problem 
> and I'm really cautious about not putting the transmission in reverse. 
> The only way I can release it is to rock the car back and forth in 
> neutral until it pops out .
> It's probably time to rebuild, I presume.
>
> Any thoughts would be appreciated
>
> DW
>
>
>
> On Fri, Aug 11, 2023 at 11:11 PM Bob Spidell <bspidell at comcast.net> wrote:
>
>     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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