[Healeys] Laycock overdrive

[Michael Salter]

Assuming that you have confirmed that the bleed off hole in the operating
valve is not blocked, something to try would be to remove the operating
valve plug (cautiously) while the unit is stuck in overdrive.
This would determine if the problem was being caused by residual oil
pressure or a mechanical fault.
If the overdrive does not slip whilst disengaged it's very unlikely that
there is anything wrong with the 8 clutch springs.
Incidentally I have rebuilt dozens of "A" type overdrive units and have
never encountered the need to replace those 8 springs.

M

On Sun., Aug. 13, 2023, 1:33 a.m. Bob Spidell, <bspidell at comcast.net> wrote:

> 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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