[Healeys] Laycock overdrive

Sun Aug 13 12:01:15 MDT 2023

 That was my thought, too.Gary
    On Sunday, August 13, 2023 at 12:32:36 PM CDT, Michael Salter <michaelsalter at gmail.com> wrote:  

 Re: I think the fact Laycock went from steel rings on the operating pistons to O-rings is telling. I would think that that was a cost cutting measure. 

On Sat., Aug. 12, 2023, 7:34 p.m. Bob Spidell via Healeys, <healeys at autox.team.net> wrote:

  I think the fact Laycock went from steel rings on the operating pistons to O-rings is telling. Both will wear, but the steel rings will score the cylinder.

 On 8/12/2023 9:40 AM, Hank Leach wrote:

 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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