5 Speed gearbox.Part-5.

Gear Box case-to bell housing
aluminium adapter plate.

The next stage:
Machining the 40mm thick aluminium plate.

The adapter plate is the main sandwich plate component of the five speed gearbox conversion project, for without it, nothing further will happen. we wont be able to mate the Bell and the gearbox together.

The initial plan was to have the entire adapter plate CNC machined, but as things happened, the machine shop we normally engage for our CNC machining work was incredibly busy, so we decided that for the bulk of the lathe turning work we would have to handle that, and leave the final CNC detail work to the HAAS four axis CNC machining centre when time becomes available.

The header photograph above shows the front and back of the plate in CAD model form. It is not that difficult to understand, but many of the dimensions and specs are critical for best fit.

In case you are wondering about the odd shape of the aluminium block?

The reason, is because the central bore of the plate is required to be offset towards one end of the plate, and when the plate is mounted and rotated in the four jaw chuck, the off set end corners of the aluminium plate were hitting on the bed of the lathe and the apron, so back in the mill it went to machine the corners off, to allow sufficient clearance, and also to machine four shallow pockets for the chuck jaws to grab hold of, and then the centre pop mark which is the centreline of the bore which had to be aligned with the centre of the lathe, the adjustable four jaw chuck makes that quite easy to do, and the face of the plate was also checked for run out with a dial indicator gauge.

In reality, it would have been an advantage to have a larger capacity lathe than what we are working with, the saddle cross feed just covers the total swing of the work piece, another reason why it would have been much easier and simpler to do the entire job in the big four axis CNC mill, but it is what it is, and this set up will have to do for now, and it will work quite OK.

Photo above shows the completed bell housing locating boss, and the protrusion boss where the thrust bearing slide spigot will be located via press fit arrangement. A lot of lathe turning time went into the plate.

This photo now shows the gearbox side of the plate. In effect, the machining represents and mimics the shape of the original Isuzu bearing and oil seal locating holding plate.

This is the gearbox side of the adapter plate, we’ve now arrived at the stage where it is possible to carry out a dry build (Trial Build) of the gearbox casings, including the gear train assembly, the adapter plate, and the bell housing. Note the modified gear lever set up.

So now we get a better look at the overall set up. This gearbox is dimensionally correct to fit snugly into the Lotus elan chassis, the conversion work to mate the gear box to the aluminium twin cam bell housing creates a very neat and compact installation, utilising the original clutch slave cylinder, and starter motor.

You may note that at the rear of the Isuzu gear box there are two flange plates that can be used for the rear support mount. After some careful measurements of the existing mount in the elan, we have decided to use the forward mount rather than the rear most mounting flange.

It has also been said that the original Lotus elan rear gearbox support mounting bracket can be turned 180Deg and it will work. Well, we’ve found that not to be the case… it does come close, but it is not correct, so the next step is to design and fabricate a dedicated rear gearbox support mount bracket for the Isuzu gear box, and the rear rubber isolating mounts are courtesy of BMW Transmission Mount Set BMW 1 Series E87 (2004-2011) 2.0L 125kW | 120i(PT# 22316799330 X 2). From only Euro Parts Melbourne Australia.

Learn more-Continue to Part 6 installment.