Model 18 - What piston is this and which can I use to replace

They still had 5/8" gudgeon pins at that time - soon to change because pins could break under hard usage (according to George Cohen's "Flat Tank Norton" - worth having if you can find a copy).  He writes that after one broke on the Isle of Man he rebuilt with later components to fit a 7/8" pin.  He describes the changes if you must go that route.

Also they were moving from cast iron to alloy - yours looks like one of the alloy ones in his book.

What is the bore like, and the clearances?  Do you have a bore gauge to compare them accurately?  The piston is marked, but doesn't appear to have metal smeared across the ring grooves.  The marks might be a sign of dimension change, but (unlike grooves in the bore) they aren't likely to cause any difference in performance or oil use.

If the piston you have is about .007" to .010" less than a standard bore size (a bit above the bottom and at right angles to the pin), it might be easiest to re-line and re-use the original.  Perhaps someone can advise how practical this is, and if the barrel is thick enough at the bottom.

Ralf,

The George Cohen book is still for sale at the National Motorcycle Museum in Coventry. I saw it there last week on a visit. Check for information on their website or call them by phone

Be aware there are a number of inaccuracies in George’s book. So best if you can to corroborate any facts elsewhere that you are going to rely on.

Allegedly pre the main print run George was given a lengthy list of constructive observations re inaccuracies etc by a number of learned Norton types who had read the draft. George being the independent cove he was ignored these and went to print.

Notwithstanding the above, I find the book a delightfully self indulgent read and interesting.

H Ralph,

   This looks like an early 1930s racing piston, possibly for a CS1, and could be the type made by Specialoid at their factory at Brooklands race track.  As you can see these had 3 plain compression rings and no oil control ring. I used a piston like this in my Model 18 vintage racer and got turned away from the grid as it was smoking too much. The easy option was to get the bottom groove machined out to take a conventional oil ring and all was well. Your piston looks very oily and looks to have had a lot of blow-by on the skirt. I would think the compression ratio will be at least 9 to 1 which might be too high for unleaded petrol. If you can have the later 7/8" pin, most later ES2 pistons will fit, or if you go to 3/4" a BSA B44 low compression might work, but you will need to change the ignition advance to suit a lower ratio.

Piston should be a hepolite 2797 until end of 1930 model.

I had to get some made by Omega in 2019 in order to fulfil my requirements. I had them made with a more basic dome to slightly higher compression ratio but they can be CNC machined to standard crown.

The odd one appears on ebay from time to time.