Learning by degrees

[orabanda]

On the last occasion I had the head off my rigid A10, I decided to compare the degrees of crankshaft rotation, with the pistons vertical movement.
A timing disc was used on the crankshaft (SRM cush drive nut), and a dial indicator on the LH piston.

I only recorded the fractions commonly quoted by BSA in relation to the A10.
Further, I assume their "11/32" is angular; measured using a tool screwed into the spark plug hole. The next time I have the timing disc on this machine, I will also take the angular measurement, and add a column to the spreadsheet below.

1951 BSA A10  ZA10 8083         
Piston position in degrees BTDC, and inches         
Measured using degree wheel on crankshaft, and dial indicator on piston (head removed)         
         
BTDC (in inches)         Degrees
Fraction   decimal      
  9/32   0.281                31
 19/64   0.297                32
  5/16   0.3125        32.6
 21/64   0.328                33.3
 11/32   0.344                34
 23/64   0.359                35
  3/8            0.375                36
 25/64   0.39                37
 13/32   0.406                37.2
 27/64   0.422                38
  7/16   0.4375        39
 29/64   0.453                40
         
The best timing setting for the following two machines, as proven on a dyno, was:

1954 S/A A10:           30.5 degrees BTDC
1951 plunger A10:      31 degrees BTDC

Best = highest power output with the greatest torque.

Hope someone finds this useful.











   

[mike667]

interesting - thanks!

[MG]

Very useful information indeed.
Interesting that max. power would be achieved at around 31 deg.

[orabanda]

Extra info:

55 Flash:  67-356 cam, 276 carb, iron head, 7.25:1 pistons
51 Rigid:  67-334 cam, 276 carb, iron head, 7.25:1 pistons, runs leaner on LH pot (hope to cure with anti-induction bias gasket)

[orabanda]

Woops, shouldn't have posted the Rigid details!
the 51 plunger has the same engine configuration as the 54 GF: 67-356 cam, 276 carb, iron head, 7.25:1 pistons

Time to go to bed (& dream of BSA's; hopefully not nightmares!!).
Dopey Dick

[RichardL]

Richard,

At high risk of personal embarrassment, I think I must disagree with the thought that BSA's measurements BTDC were with respect to the angle of the spark plug hole. I am rather certain that would have resulted in different timings for each type of piston. Yes, I think one (or, you all) might argue that they had a timing in mind for each different compression ratio, and that may well be the case. I just think it would have been weird to design for such a timing approach. Also, I would be most curious to know if plug hole BTDC measuing devices were even marketed in the early '50s.

Richard L.

[bsa-bill]

Also, I would be most curious to know if plug hole BTDC measuing devices were even marketed in the early '50s.

Richard -there was, it was called a pencil and a pen knife   

On a more serious note, do the figures used by BSA hold up with modern fuel anyway.

Mucho thanks orabanda for the figures I intend to cut and paste and store in the shed

[RichardL]

Yes, and with said pencil (assuming it was of small diameter) you could reach down to the piston verically, instead of on an angle. I checked the service sheet showing factory tools ( http://www.scribd.com/doc/22470305/BSA-Factory-Service-Sheet-No-711 ) and no TDC locator is shown, I thought it might be too generic, but then I noticed the valve grinding tool, which is really generic.

Richard L.