Will a J type OD from a Dolomite Sprint fit a Stag

[sujitroy]

Being in the US, sourcing an OD gearbox will fetch as much as a complete car. So I have sourced a manual Stag non-overdrive gearbox in the US and have acquired a Dolomite Sprint OD with box in the UK
Will the Sprint's OD fit without any mods to the internals? I plan to use the Sprint's output shaft . These part no. look the same.

Sujit

[Motorsport Micky]

Check it out with the experts, Overdrive Repair Services in Sheffield, they have many Laycock ex personnel working for them and will know and how to do it.

https://www.overdrive-repairs.co.uk/

Micky

[Guest]

You don't need to ring ORS.

The mainshaft (J type) is identical to from the sprint to the stag.
The overdrive is also the same as is the adaptor plate.

The sprint OD unit 25/115842 is similar enough to be used on the Stag without change.
I don't remember the number of the STAG unit, but the sprint unit already has high oil pressure.

The worm in the back of the OD is different and so is the speedo drive pinion.
These are not major worries.
The sprint has 6 starts on the worm.
I suspect the one on the stag is like the saloon (5).

I often used bits of Sprint transmission to change TR6 to overdrive. (last one in December).
Thank goodness there was this source of bits or it would have been impossible over decades.

[flying farmer]

I have a Dolomite sprint overdrive on the back of the Stag box in my TR and it has done the last 20 years and about 100,000 miles without problems. IIRC the Sprint overdrive has a lower operating pressure than the Stag unit and tends to engage a bit slowly under full throttle in third, but that engine produces 50lbft more torque than a standard stag engine, let alone a Sprint engine. It could be that it is getting tired, it probably has the best part of 200,000 miles on it now without a rebuild.
I did have to change the speedo drive worm to suit the TR, the Stag one is probably different. The last J type I fitted to a Stag was a saloon unit and I swapped the drive gear worm for what was supposed to be a Stag one, but it was miles out and I ended up having to fit a 2.5S speedo to correct the reading. I think I ordered the wrong one from ORS on the strength of what was listed on their website, so check carefully with several sources before you copy my mistake!
Neil

[trunt]

To find the "starts" its exactly that, look down the hole and rotate the output shaft 1 complete turn and count how many times you see a "start" of a coiled tooth

The Stag speedo drive ratio is 3:1 so normally 5 start to 15 tooth gear, so if its a 6 start you would just need to get a 18 tooth gear which is available.

Where did I get that 3:1?? since I did all the investigation work on that, I now see that Rimmer and the parts list are showing a 18 tooth gear.. with 5 start drive Strange!


Its wheel turns per mile X diff ratio divided by speedo drive ratio =tpm at the speedo

15 tooth/5 start
835 x 3.7 / 3 =1028

18 tooth/5 start
835 x 3.7 /3.6 =858

that's a heck of an under reading even if you use 175 tyres with a 1000 tpm speedo..



Terry

[trunt]

I don't remember the number of the STAG unit, but the sprint unit already has high oil pressure.

Hmm interesting, didn't think of that! I have a TR6 OD ready to fit and that seems to be 430/460 psi, Stag is 510/560. Would it be a matter of changing the relief valve?

The only real difference on the parts list is NKC35 for Stag. NKC36 for TR6 , I thought "stronger spring" but the springs are the same number.

Terry

[Guest]

To find the "starts" its exactly that, look down the hole and rotate the output shaft 1 complete turn and count how many times you see a "start" of a coiled tooth

The number of starts is stamped on a lug on the end of the overdrive.
Most people don't know that.
Sprint has 6, like the Spitfire 1500 and Dolomite 1850.
TR6 has 8 (dunno why!).

The solid dashpot is the part that does most of the improvement in behaviour of the operation of the J type.
As in most overdrives this stuff is set with shims anyhow.
SAH used to sell this as an improvement.
Who knows why, because most of them fitted it from the factory!

On the A type OD, by the time the Stag came along, they only had the small accumulator anyway, (as did the Jag Mk2s) so most of the stuff about the Stag being "high performance" is nonsense.
Only the TR4 and TR5 had the BIG accumulator which gives that whopping fierce activation you need for high torque.

The main improvement in the Stag drive line was the clutch which used a 9.5" DS - (same as Jaguar XJS-6), instead of the smaller 81/2" used on the Dolomite and TR6..

In my view they should have had the larger Stag clutch on the TR5/early TR6 too, because they are always dying,-
Otherwise why use the large clutch spline size common to the Stag/Jaguar Mk2/E type?

The TR4 has a clutch well over engineered for 100bhp, while the TR5 which uses the same was always a marginal clutch cover, despite all the hype surrounding the Laycock units, (which are the same as used on the 2L Vitesse!)

I have never understood any of the logic of any of that.

[trunt]

The solid dashpot is the part that does most of the improvement in behaviour of the operation of the J type.
As in most overdrives this stuff is set with shims anyhow.
SAH used to sell this as an improvement.

OK you cant tease me like that.. shims??

The number of starts is stamped on a lug on the end of the overdrive.

TR6 has 8 (dunno why!).

Yup 8 but the real late ones went to 7, that's what I counted and apparently I was wasting my time as there is indeed a no 7 stamped on the lug! Thanks for that tip!

[Guest]

This is the early type dashpot assy/valve. (not very positive action).
This is tweaked by changing the main assembly by using a higher rated one.

I found a photo of this online.

This is the bit that sets the final pressure:-

The little port opens underneath the end nose of the plunger.
The one shown here is one of the "weaker" ones.

The whole thing can be stripped out and replaced from under the car if need be, and replaced with the solid type.

This is the only photo of the SOLID ONE I can find.

I have a small pile of overdrives to rebuild this month, mostly J type.

Most people don't realise I have been building transmissions for these for so long.

I started doing overdrives well before ORS or OVD Spares even started,- servicing the rather weak GT6 D type units.
At that time there was nobody doing them at all.

The GT6 D type has a design fault, which is why they were a prime candidate to replace with my ground breaking J type conversion.
(jumps out of OD on LH bends)

Nowadays it's considered to be a "normal banal upgrade"
































As you can see, it's a somewhat simpler assembly and the dashpot is sealed with an O ring, rather than a series of grooves.
The larger spring sets the idle/residual pressure so that the pistons remain constantly filled with low pressure oil.

This was a major improvement compared with all the older type OD units.
The main high pressure mode is set by the strong small spring, which is itself tweaked by a few small shims which you can't see on this photo.

In the competition unit built by Mr Britton at Layock I had for my Vitesse, (which was stolen by a guy in Sutton Coldfield), the operating pressure was raised to 680psi, and the oil ways drilled out.

Even Ernie at ODS didn't know what Mr Britton did, and there is no longer anyone around able to build an overdrive unit to this spec.
(They are all RIP).

[trunt]

Good stuff, thanks

I had to study those pics a while to spot the difference, but still not sure, is that an extra hole in the plunger or a reflection?
My OD is pretty late (1976) so I assume its the solid type.

And could it be the length of the pin that sets the operating pressure as the springs seem the same? I would imagine a longer pin would need more pressure to open the valve.

Apologies for all the questions, I'm semi retired now and have too much time to ponder these things!!

[Guest]

tweaking J type overdrives part 2/

The early dashpot works on a different basis.
The early relief valve is preloaded and has tiny screw up the middle, which I have seen come undone and the whole lot explode internally.

The small weak spring up the middle and the dashpot preload spring act together to make a residual pressure system which acts against the 4 really heavy springs on the sliding member and fills the pistons to, if I remember rightly,- about 1-2 bar.
The later dashpot assembly is much more direct in action.

For those that are interested in really technical details, I happen to know the oil pump is machined with something like a 8 degree chamfer which influences its efficiency quite a bit.
The reason for this is, simply the darn thing is prone to pump rattle when cold.

For my competition unit of course (which of course I am very offended about being stolen - Mr Mike We.. (x) ver), we dispensed with all the stuff like pump rattle and the pump shank was machined straight.
The action of the overdrive is very much influenced by the diff ratio.

If the propshaft is going round really fast when you fit a low ratio axle, the pump and the amount of oil it can debit per second is vastly increased.
Being as I used a 4.55:1 axle, this meant at peak power the prop was going round at 10 000rpm (scary I know).

In effect it meant the action of the overdrive unit in 4th gear was absolutely instantaneous.
When you are talking about changing the speed of a heavy crankshaft going round at 7000-8000rpm, changing by 1500-2000 rpm you are talking of considerable amounts of energy dissipated....

On the Stag which is fitted with 14" wheels and a lowish axle ratio (3.7:1) the overdrive will work optimally fast at 70-90mph as a result.

FYI there were several weaknesses identified on the J type overdrive.

A/ high power loss, much higher than the A type overdrive that preceded it, because of high tooth contact area. (Solved or improved with the 27% unit)
B/ misalignment of the pistons, because they are much shorter, leading to high O ring wear.
C/ pressurisation of the unit internally leading to vast oil leaks. (a serious problem on the GT6 which never officially fitted it).
D/ leaks down the top 2 rear overdrive studs, because of a design fault (they are inside the gasket area, which is why they were fitted with copper washers).
E/ failure of the sliding member resulting in it exploding into pieces (solved by shot peening them, but was v common in Volvos).
F/ failure of the rear pressed in steel insert into the annulus with the sleeve coming adrift - FATAL
G/ failure of the rear pressed in bush in the annulus leading to the bore going oversize....needs oversize bush etc.
H/ swarf being centrifuged into the planet gear bearings via the flinger ring - high mileage
I/ noisy gears in the annulus/planet gears,- very annoying whines esp on 25% units....
J/ failure of the solenoid due to weakening of the return spring, resulting in it being stuck in overdrive when asked to go back out.
K/ failure of the later unidirectional clutch thanks to them making a forged version of it, which doesn't work once the metal cage wears a little bit...
L/ Oil quality influences overdrive action. Fully synthetic oil is quite different from normal because it's less viscous at lower temps...

This list is not exhaustive, but covers most of the stuff.
now I don't have to remember it - cos it's now written down for posterity.

[trunt]

It took a while but I think I finally got a handle on the theory, I had a hard time with the plunger until I realised that the pressure was pushing on the plunger itself, against the spring. I do enjoy working these things out!

I asked ORS about the low/high pressure type OD and they felt the lower one would be fine. My thoughts too, unless there is a weakness or some extra power from the engine. I'll tell you in about a year, when I get mine running, I'm happy I went with a new cone clutch though!

So just to complete the info here are a couple of pictures, one is the rear showing the Starts stamped (7 in my case) and unless corrected, a picture from the parts manual showing the "old" type relief valve and another (Volvo parts) showing the solid type

[dasadrew]

tweaking J type overdrives part 2/

The early dashpot works on a different basis.
The early relief valve is preloaded and has tiny screw up the middle, which I have seen come undone and the whole lot explode internally.

The small weak spring up the middle and the dashpot preload spring act together to make a residual pressure system which acts against the 4 really heavy springs on the sliding member and fills the pistons to, if I remember rightly,- about 1-2 bar.
The later dashpot assembly is much more direct in action.

For those that are interested in really technical details, I happen to know the oil pump is machined with something like a 8 degree chamfer which influences its efficiency quite a bit.
The reason for this is, simply the darn thing is prone to pump rattle when cold.

For my competition unit of course (which of course I am very offended about being stolen - Mr Mike We.. (x) ver), we dispensed with all the stuff like pump rattle and the pump shank was machined straight.
The action of the overdrive is very much influenced by the diff ratio.

If the propshaft is going round really fast when you fit a low ratio axle, the pump and the amount of oil it can debit per second is vastly increased.
Being as I used a 4.55:1 axle, this meant at peak power the prop was going round at 10 000rpm (scary I know).

In effect it meant the action of the overdrive unit in 4th gear was absolutely instantaneous.
When you are talking about changing the speed of a heavy crankshaft going round at 7000-8000rpm, changing by 1500-2000 rpm you are talking of considerable amounts of energy dissipated....

On the Stag which is fitted with 14" wheels and a lowish axle ratio (3.7:1) the overdrive will work optimally fast at 70-90mph as a result.

FYI there were several weaknesses identified on the J type overdrive.

A/ high power loss, much higher than the A type overdrive that preceded it, because of high tooth contact area. (Solved or improved with the 27% unit)
B/ misalignment of the pistons, because they are much shorter, leading to high O ring wear.
C/ pressurisation of the unit internally leading to vast oil leaks. (a serious problem on the GT6 which never officially fitted it).
D/ leaks down the top 2 rear overdrive studs, because of a design fault (they are inside the gasket area, which is why they were fitted with copper washers).
E/ failure of the sliding member resulting in it exploding into pieces (solved by shot peening them, but was v common in Volvos).
F/ failure of the rear pressed in steel insert into the annulus with the sleeve coming adrift - FATAL
G/ failure of the rear pressed in bush in the annulus leading to the bore going oversize....needs oversize bush etc.
H/ swarf being centrifuged into the planet gear bearings via the flinger ring - high mileage
I/ noisy gears in the annulus/planet gears,- very annoying whines esp on 25% units....
J/ failure of the solenoid due to weakening of the return spring, resulting in it being stuck in overdrive when asked to go back out.
K/ failure of the later unidirectional clutch thanks to them making a forged version of it, which doesn't work once the metal cage wears a little bit...
L/ Oil quality influences overdrive action. Fully synthetic oil is quite different from normal because it's less viscous at lower temps...

This list is not exhaustive, but covers most of the stuff.
now I don't have to remember it - cos it's now written down for posterity.

Top post!

[trunt]

Tech area drew? Maybe you can edit to a single post

[dasadrew]

Yep. Willdo.
Drew