Gearbox mount with overdrive

[BobV12]

Hi all, very aware this has been done to death before, however I have a specific question- I have installed a compact Laycock overdrive on my standard V12 gearbox, and used a standard mounting spring and large top seat.
The result is the bottom washer and locking nyloc nut hangs down slightly lower that the large rubber grommet that contains the damping washer.
Is this ok or am I going experience the gearbox bouncing up and down? Is the only answer the fit the stiffer auto box spring and flatter top seat? I am hoping not.
Thanks
Bob

[MarekH]

The spring will always compress an amount equal to the weight acting upon it and that is fixed by the weight of the gearbox.

To change the height the gearbox sits at, you can just change the effective length of the bolt by shimming.

This is exactly the same principle as Gaz adjustable shock absorbers - the coil spring is always the same compressed length but the overall length of the "bolt" at the top or bottom of the bit that isn't over the spring is adjusted in length.

You can shorten the length of the through bolt using washers either top or bottom so there is no slack. Putting the shim at the bottom keeps the gearbox height where it is now. Putting the washer(s) at the top the spring makes the gearbox sit higher.

kind regards
Marek

[lowact]

I found that the gearbox with compact overdrive (& driveshaft) was too heavy for the manual gearbox spring. The spring was completely compressed and the gearbox sat down hard.

The weight of the manual box with overdrive is similar to the weight of the auto transmission so I used the auto spring; the box is perfectly balanced, i.e. the auto spring is half compressed; I can push the gearbox up and pull it down, it remains where you move it, the damper washer works perfectly.

With the box sitting level on the auto spring, with ability to move up, there were three over-drive clearance issues:

There is not enough clearance above the 4th gear OD enable switch. My solution was to bend down the lugs of the switch and to panel-beat a bump in my metal gearbox tunnel cover.

There is not enough clearance beside the speedo angle-drive (position is different on the OD). Panel-beating is not an option. My “solution” was to reduce the angle-drive’s side screw, gave me a couple of mm …

There is very little clearance above the overdrive, where the tunnel narrows behind the gear-lever. My “solution” was to modify the tunnel insulation (BD38357/1) and to hope for the best …

[BobV12]

Thanks guys, so noting the clearance problems you have had Colin when the box sits too high, placing shims into the gap shown in my photo and so picking up on Marek’s recommendation will avoid clearance issues (cos the rig will sit lower) and remove the slack. This will I hope discourage movement and the box bouncing up and down clouting the gearbox cover.

But otherwise I will go the auto spring option but of course testing it will have to wait til it’s all drivable.

Thanks
Bob

[MarekH]

The only new part here is the compact type-A overdrive versus either an BW12 autobox or a standard long tailed gearbox. That means you shouldn't have any new issues vis-a-vs the standard gearbox height. You may have clearance issues because the overdrive is physically bigger than the long tail it substitutes. If there is a height issue versus the tunnel, then you can drill an access hole above the two switches on the manual gearbox. Whlist you are at it, I'd fab up a small access panel at the very bottom front right of the tunnel cover. This will be directly over the clutch slave and will make bleeding the clutch so much easier.

You also need to check the integrity of the overdrive solenoid before putting it all to bed. It'll be a real bear of a job to unhook it and replace it once the overdrive is in the car. You also need a properly sized relay to run the solenoid. If it jams whilst operating, it'll pull ~18amps and fry your wiring.

I also run a compact type-A on my car.

kind regards
Marek

[BobV12]

Very helpful Marek thanks, will look at all of those recommendations

Bob