No no brain fade...should have said Halibrand type. Brian made the casting as he could not afford a real one. Still the same one in the car but he must have had a good look at a real one because the centres are 2 1/4 inch as is Halibrand and the Halibrand gears fit exactly. I still use Brian's original ones which are a mixture of helical and straight cut with a spline welded in the centre. I also use the lovely new ones which fit perfectly.The diff is Model B4 and makes a lot of noise. The photo shows the casting you would have seen and also what it looks as I use itName: 1955 BCM Special Diff assy 001.jpg Views: 1151 Size: 65.7 KB geared up but apart in the photo. You can change the ratios in 5 minutes. Chris Read.