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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=E2=80=AFPM Bob Spidell <bspidell@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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<div dir=3D"ltr">Now that we are on the OD topic, I have a Healey 100 whose=
OD won't=C2=A0disengage after I switch it off, it's not an electri=
cal 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 .<div>It's probably time to rebuild, I pr=
esume.</div><div><br></div><div>Any thoughts would=C2=A0be appreciated</div=
><div><br></div><div>DW</div><div><br><div><br></div></div></div><br><div c=
lass=3D"gmail_quote"><div dir=3D"ltr" class=3D"gmail_attr">On Fri, Aug 11, =
2023 at 11:11=E2=80=AFPM Bob Spidell <<a href=3D"mailto:bspidell@comcast=
.net">bspidell@comcast.net</a>> wrote:<br></div><blockquote class=3D"gma=
il_quote" style=3D"margin:0px 0px 0px 0.8ex;border-left-width:1px;border-le=
ft-style:solid;border-left-color:rgb(204,204,204);padding-left:1ex">re: &qu=
ot;... The health of the 8 springs that keep the unit in direct drive <br>
is important to keep the clutch from slipping...."<br>
<br>
I believe the shop manual admonishes owners to replace these when <br>
overhauling the unit, even if the old ones look and feel fine (some say <br=
>
not so). I think my old ones are in a box on a shelf somewhere if <br>
anybody wants them.<br>
<br>
Note the O/D pump is a positive displacement pump that will continue to <br=
>
produce pressure--driven by the full power of the engine--unless there <br>
is a pressure relief mechanism.<br>
<br>
<br>
On 8/11/2023 10:34 AM, Hank Leach via Healeys wrote:<br>
> Guys-this is a very lively topic and one of sometimes great mystery <b=
r>
> and confusion.=C2=A0 The unit is complex but the end result is working=
well <br>
> or not functioning at all.<br>
><br>
> On the subject of pressure required to operate the shift into <br>
> overdrive there are many schools of thought. Some people replace or <b=
r>
> stretch springs or add washers to increase the pressure needed. The <b=
r>
> health of the 8 springs that keep the unit in direct drive is <br>
> important to keep the clutch from slipping.=C2=A0 The pressure supplie=
d to <br>
> overcome the strength of these 8 spring , and effect overdrive, relies=
<br>
> on adequate hydraulic pressure. The change from direct to overdrive <b=
r>
> needs to be smooth, not harsh, and if pressures are high the unit will=
<br>
> snap into and out of OD rather than the smooth interchange we are used=
<br>
> to while driving.<br>
><br>
> When applied to the Austin Healey 100, the Laycock overdrive was <br>
> initially a 32% reduction unit. It was soon found out in these early <=
br>
> Warwick cars that 32% was too much reduction.=C2=A0 DHMC swapped units=
<br>
> three times in the original DMH tour car until they finally found a <b=
r>
> combination that produced the smooth shift and complimentary gearing <=
br>
> desired.<br>
><br>
> It is difficult, today, to find any information on this original 32% <=
br>
> gearbox, however if you examine the booklet (attached) supplied by <br=
>
> Laycock for the early cars, you will notice that the casting allowed <=
br>
> for a large accumulator piston (seen on p6 in brochure). After some <b=
r>
> research and modified engineering, the company came up with this alloy=
<br>
> "sleeve" called the piston housing (photo) which partially f=
illed the <br>
> chamber of the original large accumulator casting and allowed for a <b=
r>
> smaller accumulator piston that we see in all the later 28% reduction =
<br>
> units. That size piston, in proper condition, produced from 350-450 <b=
r>
> lbs of needed hydraulic pressure to overcome the springs influence.<br=
>
><br>
> I'm confident that the remark made by Geoff Healey in his writings=
<br>
> about the early transmissions "harsh shifting" revolved arou=
nd this <br>
> huge accumulator piston producing too much pressure. Additionally, <br=
>
> there were three various accumulator springs supplied which adjusted <=
br>
> the pressure. The early boxes produced about 350lbs and the later <br>
> about 450.<br>
><br>
> Today, Overdrive Spares supplies that sleeve housing with only one <br=
>
> rubber O-ring as it is basically a filler part and not under extreme <=
br>
> pressure. The accumulator piston within is under that pressure. They <=
br>
> supply a modified piston using O-ring technology. So, it is not <br>
> unusual to get good pressures over the normal 450 lbs required. If it =
<br>
> works well don't fix it.=C2=A0 The only concern is if the pressure=
does not <br>
> release when overdrive is not called for. A clean unit, with proper <b=
r>
> 30W oil, will allow that to happen. All this some old theory and <br>
> history to ponder. Hank<br>
><br>
<br>
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