Improving boring heads

[tonyfoale]

by the way I have been wondering since your original cyl-boring post how you hone them after boring.

I didn't always have the right tools, so in years past it was home made. Back in my teens when I couldn't afford a proper hone I made my own using old pistons. I would cut them in half vertically, fore and aft, mount them on a short shaft through the gudgeon pin holes, with a loose fit. Over that shaft was a compression spring to force the two halves outward. From the lateral centre of that shaft I would mount a drive shaft (~6mm) at rt. angles, this would be maybe 250/300mm long and I'd drive it with a drill.
To use, I'd cover the piston with valve grinding paste and run it in a dummy cylinder to charge the piston with abrasive. Abrasive particles would become embedded in the relatively soft aluminium. The "hone" would then be ready to use. This type of "hone" was only suitable for finishing the surface, it would not be able to remove a significant amount of metal. I had to bore the cylinder very close to size in the lathe first. With "proper" cylinder hones it is common to remove several thou for sizing after boring. Personally I see no point in leaving that much to hone out and I aim for a thou on diameter.

PS. This technique requires a full skirt piston, slipper pistons would not be suitable. The main contact is on the side of the piston halves and slipper pistons have no sides.