Friday, 12 October 2012

Back to the gearbox

Eager fans might remember the Gearbox Woes post from a while back, where I was flapping about the fact that the gearbox I had received in the Ariel kit appeared to be the wrong part number, probably from a twin or a VB, and that possibly the casing was wrong and the mainshaft too.

You might also remember that I concocted a plan to test the theory in a dummy build, to look at the chain alignments. Got all that?

Well, now that the engine is in manageable chunks, we can lift it into position in the frame and see if the sprockets line up.

To do that, we have to release the front engine plates since the curve in the crankcases means that it will not drop in from the top. Releasing these plates means that the lower frame rails are no longer tied to the front down tube and the frame is allowed to spring open - not a comfortable state of affairs, so a little support is needed from a box underneath.

With the bottom end in place, we can put all the engine studs back in position and begin to settle the engine & gearbox into position.

We swung the gearbox out of the bottom lug, since we believed that the modification Ariel had Messrs. Burman make for the Square Four was to machine the top & bottom lugs with a different offset. This proved not to be the case, and the gearbox swung back in with not problem.

We know from the parts lists that the Square Four carries different gearbox casing & mainshaft part numbers; My theory is that since the SQ4 uses a 4.00" (WM3 rim) rear tyre, and the twins and singles a 3.25" or 3.50" (WM2 rim), then the gearbox sprocket must be 1/4" further outboard of the centreline, and the mainshaft must be longer to allow the clutch to maintain it's position in relation to the gearbox sprocket. To accommodate the different widths of the tyres, see? If the SQ4 engine is wider than the twins (it is clearly wider than the singles) then the mainshaft must be even longer.

But is it? having put it all back together, we find that the gearbox sprocket appears to align with the rear wheel sprocket (as we had seen months ago) and the clutch journal on the mainshaft appears to sit in the right place in the primary drive casing.

Promising? we need to sort out the clutch to find out.

More later. 

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