[Healeys] Laycock overdrive

Sun Aug 13 09:24:36 MDT 2023

Thanks for the advice, I have verified the small hole in the valve is not
obstructed and I have increased the hole up one size larger as advised by
another mechanic. I will remove the valve as suggested to verify it's not
pressure locking it engaged.

DW

On Sun, Aug 13, 2023 at 6:01 AM Michael Salter <michaelsalter at gmail.com>
wrote:

> 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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