[Healeys] Laycock overdrive

[Bob Spidell]

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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://autox.team.net/pipermail/healeys/attachments/20230812/d7a5b51c/attachment.htm>