Gearing question

[Duncan R]

Hi All,

I have been running in my engine and have another 100 miles or so left before changing the oil. I have  noticed that the engine appears to be overreving (especially in first) I have to change up the box fairly quickly into top to get the revs at comfortable level. I think I have a 23T engine sprocket, not sure about the gearbox sprocket (I need to change the primary chain soon, so I will check when the cover is off) Do you know the recomended sprocket sizes all round? The bike is an A10 - no idea what spec, it has an alloy head and runs a single carb.

Thanks

Duncan

[bikerbob]

Hi there.
You do not say what year the bike is but according to Roy Bacon's book the A10 swinging arm had a 21T engine sprocket 1954-63
the A10R engine had a 21T engine sprocket 1954-63. According to the book the 23T sprocket was fitted to the Rocket Gold Star. But of course it also depends on what gearbox  clutch and rear wheel  sprockets are fitted to your bike.

[bikerbob]

Hi there again.
The correct sprockets for an A10 swinging arm  1954-63 are engine 21T clutch 43T gearbox 19T rear wheel 42T if the engine is A10R the engine is 21T the rest are as stated above.  I am assuming the  bike is either an A10 Road Rocket or a Super Rocket if it has an alloy head, you would have to check the engine number  to find out.

[Duncan R]

Hi Bikerbob,

Sorry I was a bit vague about the engine spec, the bike was imported from India and it was bought as a 1954 500 A7 SS.When the engine was stripped for rebuild it turned out to be a 650. The new pistons are 7:25 to 1, that is flash spec I believe. Thanks for info, I will check all the sprockets.

Thanks

Duncan

[bikerbob]

Hi there.
If you are sure it is a 650cc then the engine number would begin with CA10- if it is a Flash. If it is a Road Rocket which had an alloy head as opposed to the Flash's cast iron head then the number would begin CA10R. The A7SS Shooting Star 500cc engine would begin with CA7SS- Both 500 and 650 could have 7.25 to1 pistons. To get back to your original question about over revving in first gear assuming it is a 650 and you have a 23T sprocket on the engine and all other sprockets are correct then you should be experiencing the reverse the engine would be over geared because you should have a 21T sprocket on, also it is possible that you could have the wrong gearing in the gearbox check to see if it is stamped standard this would give you a guide but it could have been altered with different gears. The A7SS Shooting Star would have had a 18T engine sprocket.

[Duncan R]

Hi Bob,

It is definitely a 650 it started out as an A7SS (engine no is CA7 SS) looks like somewhere along the line it has been converted to 650. Cylinder head is still A7SS as the part no: matches, the chap who done my bottom end phoned me when he started it to say its not a 500 but a 650. It had a re bore and +20 7:25 650 pistons were fitted to the thick flange barrels. I bought the bike back from India years ago so what I know about its past history is a bit vague. Gearbox is stamped as STD but as you say internals could have been changed, the rest of the sprockets seem to match your info so I think I will give the 21T engine sprocket ago.

Trouble is I have never ridden another A10 so have nothing to compare it against.

Thanks for your help

[bikerbob]

Hi there.
I have an A10 and an A65 both have what I would consider to be a low first gear you just pull away then change into second.
As regards your bike it would seem that you have a 650 bottom end and barrels but a 500cc cylinder head this is not really an ideal situation as your bore size on a 650 is 70mm while a 500 is 66mm this means that your head combustion chamber diameter is 4mmm less than your 650 bore size. I would seriously consider trying to obtain 650 head they do come up quite often on e-bay. If you have not had the bike running before with this combination I would check to make sure the pistons do not hit the head.

[Duncan R]

Hi Bob,

The engine seems to run fine , there was definitely no evidence of the pistons hitting the head. Apparently the bike was in daily use around Delhi before I bought it and I added a few thousand miles whilst working in India. As you say not an ideal set up but it seems to work. I may try and pick up a cast iron head as they seem to be quite cheap compared to the alloy ones.

Cheers

Duncan