DB18 GEARBOX REMOVAL /REPLACEMENT
Remove the bonnet. Drain the radiator, remove the top hose and the air filter if it’s the oil-bath type. Remove the throttle control rod that connects the lever adjacent to the dashboard to the bell crank fitted just below the carburetor.
Under the car:
Disconnect the gear selector rod at the gearbox lever and tie it clear of the gearbox. Disconnect the propeller shaft at the rear axle.
Inside the car:
Remove both front doors, the front seats, the gearbox inspection cover and the front floorboard (not the toeboard).
Disconnect and remove the battery; disconnect the battery earth cable at the gearbox.
Disconnect the propeller shaft at the gearbox and remove the shaft from the car. (Disconnecting it at the gearbox end alone is insufficient; it must be completely removed from the car.)
Withdraw the speedo cable from the gearbox by loosening the clamp fastening the outer cable to the chassis and removing the thrust bolt from the speedo pinion housing.
Remove two bolts securing the speedo drive housing to the gearbox and remove the housing. (If there’s not enough room to withdraw it, it can be removed after the gearbox has just cleared the bell housing dowels by rotating the gearbox slightly.)
Disconnect the gear engagement rod at both ends by removing the two 3/8” BSF nuts.
Disconnect the gearbox tie rod from the gearbox cross-member by removing the split pin, castellated nut and withdrawing the bolt. (The tie rod can remain connected to the gearbox.)
Remove the gearbox output driveshaft assembly by undoing six 1/4” Whitworth bolts at the rear face of the gearbox, tap the driving flange rearwards with a hide or copper mallet until the shaft is clear of the gearbox. The entire assembly, including the driving flange, seal housing, ball bearing, speedo driving gear and the shaft itself will come away from the gearbox.
A warning here – the output driveshaft is bored out at its front end to accept two bushings. These bushings support the front end of the shaft to keep it concentric with the gearbox front (driving) shaft, and on removal of the shaft they will do one of 3 things: stay with the gearbox, stay with the output shaft assembly or one will stay with the gearbox and the other will stay with the shaft. Whatever they do THEY MUST BE RETRIEVED AND FITTED TO THE SHAFT PRIOR TO REINSTALLATION. (They’re a running fit in the gearbox so they’re easy to retrieve later by turning the gearbox front end up. They usually drop out.
Support the engine at the rear of the sump with a stout block of wood, and please bear in mind the combined weight of the engine and gearbox - make sure whatever you use is secure! Undo the 2 rear mount nuts projecting through the gearbox cross-member and undo the four nuts and bolts securing the cross-member to the chassis. To avoid damaging the rear mount’s threads remove it from the gearbox by undoing the two retaining bolts.
Remove four of the six bolts securing the gearbox to the bell housing leaving the upper two in place.
Support the gearbox as close as possible to its front face with a strong cable looped underneath it and round the chassis and remove the remaining two mounting bolts. The gearbox spigot is a dowel fit in the bell housing and it may be difficult to shift - ours certainly was - so I allowed the weight of the gearbox help break the seal while tapping a knife blade between the joint faces to separate them.
Once the gearbox is clear of the two bell housing dowels support the rear of the gearbox with a small trolley jack and ease the gearbox backwards until the driving shaft is clear of the flywheel. Lower the rear of the gearbox, then lower the front by gradually releasing the cable. The gearbox can now be manoeuvred away from underneath the car.
When the gearbox is out of the car it’s a good time to check for oil leakage from the fluid flywheel. If leakage is evident the oil seal fitted to the driving member must be replaced.
Replacement of the oil seal:
Undo the clutch and brake pedal pinch bolts and remove the pedals.
Remove the accelerator pedal rod. Remove the countersunk screws securing the toeboard and tie up the board sufficiently to allow the bell housing to pass.
Disconnect the starter motor cable and remove the starter motor.
Remove the bell housing bolts and tap off the bell housing.
Rotate the flywheel so one of the filler plugs is at the bottom, undo it and drain the oil into a container. (Theoretically this oil can be reused, but after many years’ hard work it probably deserves an honourable retirement.)
Make correlation marks on the driving member and the flywheel and undo twenty 5/16” BSF bolts securing the driving member to the flywheel.
Place a drip tray under the fluid flywheel to catch the residual oil and screw in a 1/4” Whitworth bolt at the three equally spaced jacking points and tighten them evenly until the driving member separates from the flywheel.
Remove the driving and driven members as a unit and place them on the work bench. Separate them, degrease them and degrease the bell housing.
Check the driven member shaft is free from scoring, particularly at the area where the oil seal runs on it.
Check the driving member’s bushing for security and wear – although it should be a press fit they can work loose over the years.
Remove three 3/16” Whitworth bolts securing the seal retaining plate to the driving member, collect any brass shims that may be present, the oil seal distance piece and finally tap out the oil seal. The original type of oil seal for these is impossible to source, but a conventional oil seal is equally good; in fact Daimler went to this type of seal in the 1950s.
The bore for the seal is 2.0” and the diameter of the driven member shaft is 1.30”, and although an oil seal of this specification is not easy to source, a standard oil seal of 2.0” x 1.250” will suffice and it costs $8. (I can live with the 0.025” radial undersize.)
Press the new seal evenly into place and fit the distance piece. Shimming is not as critical as with the original type of seal – merely adding enough shims to ensure the distance piece nips up the oil seal without undue pressure when the seal retainer is refitted is all that’s required.
Manufacture a thin paper gasket for the flywheel to driving member joint faces.
Fit the driven member to the driving member, smearing grease on the shaft to avoid tearing the lip of the oil seal.
Apply a light coating of jointing compound to the flywheel and driving member joint faces and place the gasket on the driving member. When refitting the assembly to the flywheel it’s a great help to manufacture two guide pins to screw into the flywheel 180 degrees apart.
Align the previously made correlation marks and install the driving member’s 20 bolts with their shake proof washers and tighten evenly. Refill the fluid flywheel with SAE 30 oil.
Installation of the bell housing is the reversal of removal.
(What a cheap little cop-out that phrase is!)
Before the gearbox is refitted remove the inner speedo cable and grease it. (If your speedo needle wavers you may find this will cure it.)
Clean and inspect the gear engagement rod end brass bearings for wear and grease them.
Inspect the gearbox tie rod rubbers and replace them if they’re worn, and I can speak from personal experience here - don’t use heater hose! Reinforced rubber of this type will cause problems to the gearbox housing because its unyielding nature applies a load on the gearbox stud sufficient to damage the threads in the gearbox. Clark’s Rubber sell plain tubular rubber of the correct diameter and you need less than 2” of it. (They gave me that much gratis.)
Reinstallation of the gearbox is the reversal of removal, remembering to fit the two bushings to the output drive shaft. Apply jointing compound to the output drive shaft seal housing and gearbox joint face. When installing the shaft in the gearbox there are 2 sets of splines to engage, so when the shaft has fully engaged the rear (reverse drum) splines it may need twisting a little to engage the front (first gear drum) splines; a light tap on the driving flange with a hide or copper mallet is now all that’s needed to seat it, BUT PLEASE DO NOT, ON ANY ACCOUNT, USE UNDUE FORCE!
Once the ball bearing is seated in the gearbox housing the drive shaft’s splines are fully engaged, but there will remain a gap between the oil seal housing and the gearbox housing. This is normal - it’s the result of the oil seal’s volute spring holding the seal housing rearward, but it will pull in on the gearbox when its six retaining bolts are tightened.
A warning: tighten these bolts evenly and make sure the spring’s front seating ring has been centralised; if it’s not central further tightening of the bolts will trap it and crack the housing.
That’s about it, really.
I now want to share my experience with our car after we fitted the gearbox I’d built.
When I selected and engaged reverse nothing happened. Repeat, with the same result.
Hmmm…not good, not good at all. In fact, very bad.
I mowed the lawn as a diversion and I cogitated.
Reverse worked OK without the propeller shaft, but with nothing doing when the car was under load… maybe adjustment of the reverse gear brake band?
Next thought: maybe a composite gearbox fitted with good but used brake bands is the problem.
A used band taken from one of three gearboxes may not be quite concentric with a running gear assembly made up from parts taken from all three gearboxes…slight wear variations between the three.
Well, either I remove the gearbox and strip it all over again or I wear the reverse brake band in.
A no brainer, that; run it in and see what happens.
I set the engine speed to about 1,500 RPM with reverse engaged. Nothing.
Then a little smoke from the gearbox.
Then a bit more smoke was followed by slight movement of the propeller shaft. I stopped the engine and let the reverse band and annulus cool off.
I tried again after about 15 minutes at a slower engine speed.
Much less smoke this time and a little more movement in reverse. After several further attempts, allowing a healthy cooling off period in between, reverse slowly came good, and now - touch wood - it’s not slipping at all.
Overall, this was a fairly brutal fix that still makes me wince, but it seems to have been effective enough. I’ve now test driven it round the block a number of times and the gearbox is behaving well. Time, however, will tell….
PS: it’s now been well over a couple of years and reverse is fine.