Timing Tool

[BSA_54A10]

Draw it on a sheet of squared paper .
If you consider the engine to be a right triangle with the con rod as the hypotenuse it will all make sense.
If you plot piston drop vs degrees you end up with one of those nice sine curves they tried to teach you about in high school.
See even way back then you wee being preparred for a future fixing motorcycles.

[Triton Thrasher]

Struggling a bit to figure how rod length factors into this.

With a shorter rod, the piston changes direction more suddenly, to put it simply.

[jachenbach]

Went for a nice long ride and thought about it (no traffic) and was able to picture it more clearly.

[stev60]

Everybody has their own way of setting the correct piston height and Greybeard is correct in what he says about the angle of the plug hole in relation to the piston crown, I did once measure the angle and draw it all out on paper and there is a difference in relation to the angle of the plug and the vertical drop of the piston. How I overcame this was to use an old plug break off the ceramics and modify the center of the plug to accept a 8mm dia rod, tighten the plug in the head then bring the piston up to TDC then using a junior hacksaw blade mark the rod take rod out put a decent groove in the rod using the junior hacksaw then measure your timing distance accurately from the bottom of the groove  put another groove in the rod so that your correct distance is to the bottom of both grooves when you set the piston at TDC take it down to the top of second groove this then compenstes for the angle of the plug hole. As I said earlier everybody has their own way and this works for me.  Also as an addition I was told many years ago that when setting correct height of the piston you should always take the piston down past the correct  level and then bring it back up to the correct mark this I was told eliminates any wear in the timing gears. In regard to the original post you only have to look at who makes it to make me doubtful look at the width of the timing grooves if you used the bottom of one and top of the other you could be off with your timing.
I bought the same timing tool but the graduations made it a little difficult. the fit was a bit sloppy as well. I did exactly what is described here using the sparkplug with ceramic broken out, they machine easily and makes a more accurate reading.

[muskrat]

My timing plug. Now I have Boyer on both I just use it to find TDC on the degree disc. Turn crank one way till stops, mark disc, turn other way till stops, mark disc. center between two marks is TDC. Use timing light  to get 35 degrees on the A7 and 32 on the Cafe.
With the magneto I had the plug set at the right distance before TDC and brought the piston up to it to set the points.
Cheers

[BSA Biker]

I too have that tool and yes it is a bit floppy to say the least, plus the graduation lines are pretty much useless. So having fitted 3 small rubber grommet rings over the lower inner part section and then brought the piston up to TDC it was then simple to measure and fit another ring on the top section at the right height (3/8") and drop back the crank and slowly bring up the piston until just touching the tool. Yes it might not be 100% accurate but it seems okay to me. You can of course check the position using the old pencil method afterwards, something I have done, not much difference.

[stev60]

Ha, I used the pencil the first time and worked spot on, using that graduated tool similar to Muskys gave the same result , Im not an expert but the engine will tell you 

[hdawson]

I bought this tool a while back and found it quite useful however beware of the o ring which luckily broke in my hand and not while in the plughole. 

[bsa-bill]

Next time your in Costa or other coffee shop grab a handful of the slim wooden stir sticks, they're thin enough to go vertically down the plug hole, and long enough that you can use the join between the rocker box and the head as a reference point to mark TDC and you preferred timing point

[Peter Gee]

I really prefer TDC with the head off and using a dial gauge to find TDC..in conjunction with a timing disc on the primary. Then work backwards to find BTDC for the setting.  Luckily the manual adv/retard on A7SS's means even if using the TDC tool mentioned that is not at 180 degrees  to the flat-top  piston, you'll have a 'sweet spot' you can manually set when underway to give the best timing. Those ADV/RETARD ;levers had a purpose!

[bsa-bill]

I really prefer TDC with the head off and using a dial gauge to find TDC..in conjunction with a timing disc on the primary

No argument there, dial gauge gives much more accuracy as to TDC, but for those occasions when timing is required and the head is on and staying on, I'll forgo a bit of accuracy, always a bit of remedial adjustment via the points gap

[duTch]

 

.... Then work backwards to find BTDC for the setting. .....

 Ok,  but as far as I'm aware,  general thinking is to go back past the timing mark and come up to the mark, which takes any slop out of the gear train

[Topdad]

hi Peter, whilst I  think your attention to detail is admirable I think you are over complicating our engine. It was after all mostly a commuter bike and as such needed to be "fixable " at the side of the road and whilst i take time to time it properly i certainly wouldn't take the head of to do it . As i keep saying I (SORRY ) set mine with a pencil and a fag paper ,always a new one !  My bike as always been a 1 to 2 kick starter even after the winter lay up.

[Greybeard]

My bike as always been a 1 to 2 kick starter even after the winter lay up.

A quick start will be handy when the bikes all pull away for your local DGR this weekend!

[stev60]

I agree with the comment on the advance retard lever, makes it easier to start by retarding a bit and when hot you can find that right spot, using the old fashioned ear. Retarding a bit on a hill seems to work as well.