Simon.Gardiner wrote:I'm not sure that the exact weight of a piston is as important as the size and clearance ... I'd suggest that values like con-rod length and balance factor are also 'nice to know's not 'need to know's - which may be why they're not often found, along with a lot of other information!)
The con rod length would be 'nice to know' because if I knew it I could calculate crankshaft rotation vs. piston position BTDC. That is important for getting accuracy and consistency checking the cam and in timing the magneto. It's true that piston weight and balance factor aren't easily found. But, that's not because these factors aren't important, which they are, it's because in the old days they were "irrelevant" because mechanics simply replaced scuffed pistons with new ones from the manufacturer so the balance factor didn't change. Unfortunately, with those parts no longer available we have to use aftermarket replacements whose weights differ significantly.
Quoting Phil Irving in 'Motorcycle Engineering' on why the balance factor is important: "Usually, a balance factor of 60-65 per cent, will be found to be correct, but it may vary widely; the factor for the MOV Velocette was as high as 85 per cent., while some engines have been down to 40 per cent... Because the eventual smoothness obtained depends so much upon the frame, a balance factor which suits one installation may not suit another... An instance of this was found in the original Vincent-H.R.D. singles. These engines ran very smoothly in the standard spring frame with a factor of 66 per cent., but the speed way versions, of which a very few were made, vibrated badly in a cobwebby dirt-track frame until the factor was reduced to 61 per cent., this not-very-large reduction making all the difference between a machine which was passably smooth and one which shook itself out of your hands."
Another quote showing that a few percent matters comes from A. Golland's 'Goldie': "Balance factor up to the year 1954 was 55%, but thereafter, 58 % was found more suitable..."
That was the year Gold Stars changed from a plunger to a swinging arm frame.
If I were only to use my Ariel for short club runs the extra vibration caused by having the incorrect balance factor might not matter. But, on a 4000-mile ride having unnecessary extra vibration not only would be uncomfortable it also would degrade the reliability of the bike.
john.whiting wrote:If you want that kind of detail on a 1928 motor,you will have to write the book yourself.
That thought has crossed my mind. I did write a book on Gold Stars (now in its 5th edition), but the amount of information on Black Ariels makes that about Gold Stars seem copious by comparison. Despite that, a shop manual might emerge from all of this, which would be of interest to roughly three other people...
To summarize, I'm not interested in answers to the questions I posed just to write in a notebook. They are important to the proper rebuild of a motorcycle that needs to be reliable for 4000 miles with minimal maintenance during that two-week period.