Overdrive

[Mr Tiger]

The thread on buying a Stag has reminded me of some questions I have about overdrives.

Do all A types operate on 2nd, 3rd & 4th and all J types on 3rd & 4th only?
Are A types available in 22% and 28% and J types 25% only?
Is there a visible difference between them? I can't seem to find any plates on mine to identify what type it is.
Is one type stronger or more reliable than the other?
And finally, the one that has really been puzzling me - does a 25% overdrive mean the revs increase by 25% when you go from o/d to direct and decrease by 20% when engaging o/d?

I'd be grateful for any info.

Thanks,

Chris

[garagewidow]

i'm not certain on some of this but,
A type on 2,3 and 4th not sure but generally J type is 3 & 4.but it depends on what g/box you are looking at as they are electro/mechanically operated you can engineer it to engage them on any gear(except reverse of course or strange things happen).the large tr/saloon/sprint/stag boxes may or may not have the provision to add an extra inhibitor switch in the top gearlinkage housing(there are bosses in the casting,some may have a blank plug fitted).the sprint one certainly has,i added an extra switch to enable o/d in 2nd.
J types also come in 28% flavour (stags and pi saloons?)
the A type is physically larger and fitted to early tr's and perhaps other models.A type has vertical solenoid?,J type has horizontal on the l/h(passenger side-rhd vehicle).
logically 25% would mean an increase of that figure input > output.1 turn in is 1.25 turns out.engaging o/d reduces engine revs for better,quiter cruising speeds.
avoid the fragile D types.the J type is quite compact and fairly robust if not as much as the A type.

hope this helps.

[Mr Tiger]

Thanks for your reply. You've given me plenty to consider. I'll need to check whether mine has a vertical or horizontal solenoid.

I've just been though the handbook for the car. It states the ratio for 4th is 1.00 (:1) and o/d 0.82

I've not heard of an 18% overdrive but if the revs decrease by 18% on engaging then they will increase by 22% on disengaging.

This does seem counter intuitive but perhaps this is the way these are graded i.e.

22% drops revs by 18%
25% drops revs by 20%
28% drops revs by 21.875%

I could be completely misunderstanding this of course.

I'd be grateful if anyone could clarify.

Thanks,

Chris

[//j17]

Regarding overdrive identification just spotted this on the Jigsaw site while looking for something else:

[quote]
WHAT DIFFERENTIAL DO I HAVE?

There are a number of ways of identifying a differential – wither by counting the crown wheel teeth and dividing by the number of pinion teeth (eg c/w 35 pinion 9 = 35/9 = 8.39), or by counting the number of turns the front flange requires to turn both output flanges through 360 degrees (or one output flange through 720 degrees) – just more than 3 ½ means 3.63, just more than 4 = 4.11

By just looking at a differential you can often narrow down the ratio – for convenience we can split differential units into four types:-

- TYPE A – square front pinion flange, 5/16 bolt holes all round (1/2AF spanner size), Small quarter shaft diameter inside, Casing No’s G, Y, FC to 120,000

- TYPE B - square front pinion flange, 5/16 bolt holes all round (1/2AF spanner size), Big quarter shaft diameter inside, Casing No’s GE, FC from 120,001

- TYPE C - square front pinion flange, 5/16 bolt holes all round (1/2AF spanner size), 3/8 bolt holes on output flanges (9/16AF spanner size), Casing No HB

- TYPE D – Round front pinion flange, 3/8 bolt holes all round (9/16AF spanner size), Casing No’s FH, FK, FR, FD, HC, KC, KD

Type A & B are indistinguishable from the outside but Type A should not be fitted to Herald 13/60 as the quarter shafts are liable to break. Type C (which was only used for the Vitesse 6 originally) is very prone to quarter shaft failure

Having decided which differential you have you may now want to alter the ratio. The following table gives rounded figures for the miles/kilometers per 1000 rpm

RATIO mph/1000 (4th) mph/1000 (o/d 4th)
4.875 13.50 17.00
4.55 14.50 18.00
4.11 16.00 20.00
3.89 17.00 21.00
3.63 18.00 22.50
3.27 20.00 25.00

[LoudV8]

I added 2nd OD to my Stag, Sprint and 2500S, which all have J-type ODs.

There is a blanking plug in the top of the gearbox selector housing above the 1st/2nd selector rod. This is where the activation switch screws in. The rod has a notch in the right position to activate the switch when 2nd is selected.

It is the same type of switch that is used for the 3rd/4th OD activator and reversing light. It just needs wiring in parallel with the 3rd/4th switch.

[caziques]

//j17 said:

g overdrive identification just spotted this on the Jigsaw site while looking for something else:

[quote]
WHAT DIFFERENTIAL DO I HAVE?

There are a number of ways of identifying a differential – wither by counting the crown wheel teeth and dividing by the number of pinion teeth (eg c/w 35 pinion 9 = 35/9 = 8.39), or by counting the number of turns the front flange requires to turn both output flanges through 360 degrees (or one output flange through 720 degrees) – just more than 3 ½ means 3.63, just more than 4 = 4.11

By just looking at a differential you can often narrow down the ratio – for convenience we can split differential units into four types:-

- TYPE A – square front pinion flange, 5/16 bolt holes all round (1/2AF spanner size), Small quarter shaft diameter inside, Casing No’s G, Y, FC to 120,000

- TYPE B - square front pinion flange, 5/16 bolt holes all round (1/2AF spanner size), Big quarter shaft diameter inside, Casing No’s GE, FC from 120,001

- TYPE C - square front pinion flange, 5/16 bolt holes all round (1/2AF spanner size), 3/8 bolt holes on output flanges (9/16AF spanner size), Casing No HB

- TYPE D – Round front pinion flange, 3/8 bolt holes all round (9/16AF spanner size), Casing No’s FH, FK, FR, FD, HC, KC, KD

Type A & B are indistinguishable from the outside but Type A should not be fitted to Herald 13/60 as the quarter shafts are liable to break. Type C (which was only used for the Vitesse 6 originally) is very prone to quarter shaft failure

Having decided which differential you have you may now want to alter the ratio. The following table gives rounded figures for the miles/kilometers per 1000 rpm

RATIO mph/1000 (4th) mph/1000 (o/d 4th)
4.875 13.50 17.00
4.55 14.50 18.00
4.11 16.00 20.00
3.89 17.00 21.00
3.63 18.00 22.50
3.27 20.00 25.00

Glad to see this information is still available, I wrote it about 1990 - not sure I ever gave Jigsaw permission to use it (but I don't usually argue with someone who classes a 20 piece KFC bucket as a personal snack).

[//j17]

caziques said:

Glad to see this information is still available, I wrote it about 1990 - not sure I ever gave Jigsaw permission to use it (but I don't usually argue with someone who classes a 20 piece KFC bucket as a personal snack).

I'd probably back you to out-run Mark Field