November 2013

Saturday, 23 November 2013

Engine and JT5 gearbox installation

Article Details

Finally the time has come to put the engine back in. The unknown quantity was the JT5 box, and how much this would complicate the installation.

Prior to starting I measured the JT5 and my Moss box. They are both 66cm long, although this is without the slip yoke fitted into the JT5 box, which adds a minimum of 6cm. The installation instructions warn against trying to install the gearbox with this fitted. It is recommended to attach the yoke to the driveshaft and install them in the tunnel before offering up the gearbox.

I have only ever installed engines from below, and this is how we planned to do it. I am fortunate to have a car hoist which makes the process a good deal easier.

3.8 JT5 01

With the engine on a trolley positioned under the car we lowered the body. It quickly became apparent that there was not much spare space; with the waterpump pulley a few mm clear of the picture frame the back of the gearbox was only about 10mm clear of the tunnel. Despite this we were able to get the engine in place on the engine mounts with a bit of judicious wiggling.

3.8 JT5 02

It quickly became apparent however that there was not going to be enough room to get the slip yoke into the gearbox. Detaching the driveshaft from the differential didn't provide enough leeway either. Whether installing the engine from above using an engine crane and angling it down would have made it possible to install the yoke is an interesting question.

3.8 JT5 03

After some discussion we decided to remove the IRS to give us enough room to manipulate the driveshaft into place. Even having done this it was still very difficult to get the splines to mesh and in the end we disconnected the slip yoke from the driveshaft and installed it by putting an arm up the tunnel much like a vet in calving season. Then the driveshaft was bolted back onto the yoke and the IRS reinstalled. Although it sounds complicated the removal and replacement of the IRS took about 20 minutes and was unavoidable.

3.8 JT5 04

When we fitted the engine steady bar it was not vertical. After some measurment it seems that the engine is actually fitted in the correct place and that the mounting holes drilled in the bell housing are about 10mm too far back. Given the heavy duty gearbox mount that comes with the JT5 box this shouldn't pose a problem but it is disapointing.

3.8 JT5 05

With the engine and gearbox installed it became apparent just how tight a fit the JT5 is. In many areas the clearance from the bodywork is only a couple of mm. "Some peening of the tunnel may be required" says the manual...

3.8 JT5 06

The supplied cables for the reversing light switch are jammed up hard against the tunnel. There is very little room to access the gearbox filler plug; around 25mm clearance will make removing the plug and filling the box tricky.

3.8 JT5 07

3.8 JT5 08

I am concerned at the lack of clearance particualrly around the bulkhead corners and the sides of the top plate. It will be quite hard to relieve these areas without changes that will make fitting the tunnel cover difficult. Depending on the amount of movement allowed by the gearbox mount it may be necessary to pack these tight gaps with some sort of rubber cushioning to stop rattles.

3.8 JT5 09

In summary then the JT5 box will go in but not without removing the IRS if you want to put it in from below. It might be possible to get it in from above by tilting the gearbox down and manipulating it back on the slip yoke but this would be hard with a gearbox filled with oil as it will spill out of the unsealed hole around the output shaft.

In comparison we installed a Driven Man box in a 3.8 coupe 2 weeks ago. This has the same fixed yoke as the standard box and while it is also a tight fit it was a much easier installation.