5 Speed gearbox. Part 4.

Drilling, threading and Milling
of Gear Box Case.

We now arrive at part four. The engineering work becomes a little more serious and more exacting as we progress through each stage. Part four is no exception, as we prepare the gearbox case to accept the special aluminium adapter plate (Still to be made) that will help to mate the Gemini/Isuzu 5 speed gearbox to the twin cam lotus bell housing.

There are certain mods that have to be done in order to be able to bolt the twin cam bell housing to the front of the Isuzu gearbox casing, in the photo above the first operation is to drill and tap the left hand post to 12mm-1.75, the right hand post was already threaded, because that is where the Isuzu clutch actuating lever pivot screws in. These two posts form part of the secure fastening structure to bolt the adapter plate to, and also the twin cam bell housing.

The drilling process is done in the vertical mill for accuracy, the 12mm tap is started manually in the drill chuck inserted in the mill, then the remainder of the thread is carefully cut by hand with the tap holder. The outer lip edge of the gearbox casing has also yet to be milled to proper dimension.

After the drilling and thread tapping operation, the gear case is mounted on a special holding plate affixed to the Vertex rotary table to enable the left and right hand posts to be milled perfectly round to 20mm Dia, the posts are designed to accurately engage into holes CNC milled into the yet to be made aluminium adapter plate.

You might begin to think that a five speed conversion is out of the question for you. It is true that the work and expense involved is considerable, you may not have the workshop facilities or skills to undertake this kind of project, however, we decided to record and share this information to the people out there who are confident in their ability to undertake the engineering involved in this gear box conversion project.

To continue reading please click and go to – part five article