As a non engineer to a non engineer..
The first motion shaft has a 'constant mesh' with the layshaft via the 14:28 cluster at the front of the gearbox.
This constant mesh can be changed to make a closer ratio gear box - e.g. 17:25 or whatever. To change it you need to replace the existing first motion shaft with one that has been machined by taking down the 14 existing teeth and shrinking/welding on top another cut down gear with a different tooth count. To make this work, as you say, you need to replace the 28 tooth gear at the front of the layshaft. That's why replacements come as pairs on ebay or from Andy Bird, say.
You will notice that you can use an old 16 tooth and an old 26 tooth gear as donors to make the rings for a new constant mesh 16:26 gearbox which is a really nice compromise. I think it's the one they call a 'Rolt' cluster and it's really nice and driveable on the road. A sort of modest close ratio set up.
You can change the second gear cluster too, but that's for the cogniscenti.
The first gear & reverse cluster on the layshaft are made from one lump of metal. The gear on the third motion shaft slides either forward or back when you move the gear lever either back or forward (respectively). When the the gear on the third motion shaft is forward it takes drive from the layshaft via the first gear component and off you go. When it is backward it takes drive from the idler gear which is itself constantly meshed to the layshaft lump. Because the idler sits to one side and slightly above the layshaft it can accommodate the third motion shaft gear without losing 'distance between centres'. You can see on your diagram that your 26 tooth gear can never reach the 12 tooth gear on it's own!
You can get a new first gear cluster with a modified (lower) tooth count if you want a really low first gear and again this has to be fitted by removing the layshaft and replacing the existing one.
Generally the second gear cluster takes most abuse in an A7 gearbox and many of these gears are worn. Generally the constant mesh pair are in good condition whenever they are taken out.
Top gear is simply a matter of joining together the first and third motion shafts without allowing any mesh between lay shaft other than through the well-named constant mesh gears, which are, of course, always engaged. Clearly you can only use top gear if you prevent all other links between lay shaft and third shaft.
Your question 2 - Yes.
I'm waiting for a proper engineer to come and correct some of this sometime soon but there's a start..
Next step is deciding the rear axle ratio for any given combination.
I once bought a gearbox box of bits from Beaulieu for about £20 - someone had taken it apart and soda blasted the casing - had lots of fun putting it back together again - but there are quite a few components that get abused, broken or worn in a three speed box so I was lucky with mine.
The first motion shaft has a 'constant mesh' with the layshaft via the 14:28 cluster at the front of the gearbox.
This constant mesh can be changed to make a closer ratio gear box - e.g. 17:25 or whatever. To change it you need to replace the existing first motion shaft with one that has been machined by taking down the 14 existing teeth and shrinking/welding on top another cut down gear with a different tooth count. To make this work, as you say, you need to replace the 28 tooth gear at the front of the layshaft. That's why replacements come as pairs on ebay or from Andy Bird, say.
You will notice that you can use an old 16 tooth and an old 26 tooth gear as donors to make the rings for a new constant mesh 16:26 gearbox which is a really nice compromise. I think it's the one they call a 'Rolt' cluster and it's really nice and driveable on the road. A sort of modest close ratio set up.
You can change the second gear cluster too, but that's for the cogniscenti.
The first gear & reverse cluster on the layshaft are made from one lump of metal. The gear on the third motion shaft slides either forward or back when you move the gear lever either back or forward (respectively). When the the gear on the third motion shaft is forward it takes drive from the layshaft via the first gear component and off you go. When it is backward it takes drive from the idler gear which is itself constantly meshed to the layshaft lump. Because the idler sits to one side and slightly above the layshaft it can accommodate the third motion shaft gear without losing 'distance between centres'. You can see on your diagram that your 26 tooth gear can never reach the 12 tooth gear on it's own!
You can get a new first gear cluster with a modified (lower) tooth count if you want a really low first gear and again this has to be fitted by removing the layshaft and replacing the existing one.
Generally the second gear cluster takes most abuse in an A7 gearbox and many of these gears are worn. Generally the constant mesh pair are in good condition whenever they are taken out.
Top gear is simply a matter of joining together the first and third motion shafts without allowing any mesh between lay shaft other than through the well-named constant mesh gears, which are, of course, always engaged. Clearly you can only use top gear if you prevent all other links between lay shaft and third shaft.
Your question 2 - Yes.
I'm waiting for a proper engineer to come and correct some of this sometime soon but there's a start..
Next step is deciding the rear axle ratio for any given combination.
I once bought a gearbox box of bits from Beaulieu for about £20 - someone had taken it apart and soda blasted the casing - had lots of fun putting it back together again - but there are quite a few components that get abused, broken or worn in a three speed box so I was lucky with mine.