Recommended piston to bore clearance for new 750 re-bore

If they have not got the pistons and the manufacturers recommendation, they should not be boring anything. 

Respectfully – when my MK2A 850 needed a rebore, I used pistons from Mick Hemmings (I don't recall the make, I have it somewhere) that came with the manufacturer's leaflet recommending the clearance. I take it you don't have anything like that? Anyway I took the work to a local machinist who refurbishes car engines and had no specialist knowledge of Nortons or the pistons I was using. He did a fine job – Norman White passed it anyhow, while doing more general work on my engine. So I wouldn't limit your options unnecessarily, so long as you go armed with the specs you need.

... as many reborers aren't familiar with our relatively crude air-cooled engines and would be looking at clearances suitable for modern water-cooled engines which would be significantly tighter.

I always go for clearances on the high side ("racing" clearances we use to call them) as I don't do the miles I once did and don't want to spend the remainder of my life running in. A thou or two extra on the bores means they stand much less chance of picking up.

One tip I was given (but have never tried) is to pop the pistons in the oven at around 200C and measure them while hot (quite tricky without burning your hands). This should give you the size they'll be when running.

Ian:

Would you not also have to heat up the cylinder to account for any bore increase (or shrinkage) before measureing?

Mike

The clearance needed is directly attributed to the expansion of the piston in relation to the expansion of the barrel. Different alloys expand differently. There is an added issue that cylinders don't expand the same amount top and bottom.The top can get much hotter. I would be concerned at less than 4 thou with the normal pistons.  8 thou on a Rudge!

... you probably would, but as I say not something I've ever felt the need to do. In reality the barrel will expand much less than the piston as the coefficient of expansion is likely to be higher plus it will get hotter.

hello well 0.004 will be too much  clearance   the right clearance is 0.003 thou  as my 650 twin is this clearance  from new,   and so was the Atlas  with BHB pistons fitted as standard   Commando were Hepolite  AE pistons  as fitted as standard  and the first over size was plus  10 thou  and then 20 thou but do not over bore pasted plus 40 thou,  for safety reasons   and all bores are tapered  from the bottom up  so your bore will be narrower at the top  and one half thou bigger at the bottom  and you need to re-machine the taper for were the rings fit in the bore at the bottom edge, Norton pistons are tapered to  bottom skirts are bigger than the top edge  and the skirts are made to expand  with engine temperatures   and ring end gaps are important too   8 to to 10 thou is the norm  ,for Norton Commando rebores  Steve Maney is your Man                  Yours  Anna J