5 Speed gearbox. Part 3.

Isuzu-Gemini bell housing and
Gear Box Case ready for parting.

Parting of the bell and gearbox case.

We take a deep breath, because now its time to get a bit more serious about the project, IE, the process of parting the bell from the front section of gearbox casing, the point of no return, there’s no turning back from here.

So, first off, we need to separate or dismantle the Isuzu gearbox, this is a pretty simple job as its just a matter of undoing the set of hex bolts that secure the front, middle sandwich plate, and the rear part of the cases together. Note that the entire gear cluster is mounted to a thick aluminium central plate, in the photo below the rear half case is removed first… with the help of an impact puller tool (Not shown) to help crack the gasket seal, then the gearbox gear assembly is removed, the innards just slide out, its quite remarkable how it all comes apart, and lets hope it all goes back together just as easy.

The photo above displays the gear cluster assembly, its very neat the way its assembled onto the central carrier plate. Everything looks to be in excellent order, so we will possibly only replace the bearings, but even they appear to be in excellent serviceable condition.

The gear set design looks to be every bit as robust as the gear set in the Ford Lotus gear box, and should cope quite well when mated to a high horsepower twin cam engine.

Before we can wield the angle grinder fitted with a special aluminium oxide thin cut off wheel, designed for cutting cast aluminium, we fabricated a holding jig which is attached to the engine stand.

While it is easy to be overcome with impatience and attack the bell laying on the ground, the holding/mounting jig makes the operation very safe and so much easier to carry out, because some very careful measuring, marking out and cutting is required, and the rotating fixture on the engine stand makes the entire job very easy to do.

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The black line drawn on the bell housing is where the cut will be made. We are leaving a generous amount of metal to be milled off the case. Remember, its better to have a bit too much to machine, than not enough. The cutting tool was an old four inch (100mm) angle grinder fitted with a special thin aluminium oxide wheel that will cut through aluminium like butter, because it doesn’t clog like ordinary carborundum cut off wheels do. The bell was removed with less than five minutes cutting.

So now we have some cast aluminium scrap (Bell housing) to play with in the foundry, who knows what will be made from the left over bell housing. The picture above provides clearer detail of the face of the gearbox case. The next job is to clamp the case onto the milling machine table to mill the excess metal, the outer flange will be milled down to the same height as the machined bearing face.

After that, the case will be mounted to a special holding plate secured to the vertex rotary table in order to manually mill the outer diameter (20mm) of the two horns that protrude from the case, these horns will provide alignment and form part of the solid mounting required between the gearbox casing and the special adapter plate, and the new twin cam aluminium bell housing purchased from Burtons UK.

The holding fixture was made from scrap steel found around the workshop. It was only a temporary holding fixture, and after the cutting was completed, it was removed and modified to suit the twin cam bell housing, now that may sound like a lot of work, but if you want to do things correctly and safely, this is what you do.

The twin cam aluminium bell supplied by Burton’s is very well made and finished, but I didn’t like the fact that a rough uneven cast surface was left on the inside where washers and bolt heads will be torqued up, so we set the bell up in the milling machine and end mill face machined them the way they should be.

It is proper engineering practice to have a flat machined surface for bolt heads to be torqued up to, any deviation from this can lead to casting fractures and other issues. Having to do this extra work is fine if one has the workshop facility, but if not, it could be a problem if someone had to pay extra to have this work done at the local machine shop.

Side Note: We cant show every small detail of the work being carried out, because there is always a certain amount of behind the scenes machining, milling, or fabrication work of special holding fixtures being built, things that you simply don’t have time to record.

Click to continue to Part 4.