Timing a Comet using a Degree Disc - what does everyone do?

[miked]

Hi,

I'm in the process of recommisioning my 1950 Comet hopefully in time for 1000 Bikes, and I'm at the stage where I need to set the ignition timing.
I'd prefer to do ithis using a degree disc as that's what I'm used to, but there doesn't seem a way of mounting it to the crankshaft except to make some kind of adaptor that screws into the primary drive shock absorber (large femail thread in centre).
I just wondered if there was an accepted method, and if an adaptor is required could anyone tell me what the size of the thread I need to make it screw into - I have limited measuring and manufacturing equipment and my brain works in metric...

Many thanks in advance,

Mike

 

[A-BCD]

Remove the big end quill on the timing side and a degree disc can be simply tapped into place in the mainshaft. You will need a tapered spindle on your degree disc, starting at about 5/16" dia and about 6" long.

 

[timetraveller]

And I made the spindle for the centre of this disc from copper so that it cannot cause any damage.

 

[miked]

Thanks for the replies, I'll have a look into that.
As i said my manufacturing facilities are limited, so it may be a case of improvising something this time.

Cheers,

Mike

 

[b'knighted]

My timing disc has its central "bush" in rubber so that it can be turned on its spindle. I use a pushrod as the spindle and find that its taper push locks adequately. I use a bit of coathanger wire from the footrest hanger as a pointer.

Using a screw adjustable piston stop, rather than a sliding TDC finder, I find bottom dead centre by noting the disc reading with the piston up then rotate to the next stop. Halving this movement lets me find BDC and having set the engine to BDC, I then zero the disc to suit. Once I have checked that it now turns the same amount in each direction I can set the engine to the appropriate firing point. Once this is achieved I reset the piston stop to match the piston height. It can then be carried and used to accurately set the firing point without resorting to the disc.
I made the piston stop by breaking all of the ceramic material from an old plug and tapping 3/8". A length of studding is inserted and a lock nut fitted. I filed a flat on the thread and a step on its flange to give a gas escape. It could probably done with a bit of M6 studding nutted above and below the plug body although resetting to firing point would be much more fiddly.
My piston stop remains set up specifically for my Comet. I suppose I should have made another two and set one for each cylinder of my twin as I can adjust their sparks individually.

Cheers,

 

[Black Flash]

+1 for Ian,
he is exactly spot on, can't say it more clearly. we do not know each other, live about 600 miles apart, made exactly the same piston stop and use exactly the same method. Surprising that there are two people with the same opinion on a Vincent topic!

Bernd

 

[b'knighted]

I joined the club in 1977 - I had to agree with someone eventually!

 

[davidd]

Mike,

If you are limited by equipment, you can use a piece of threaded rod and just shape the taper with a bench grinder. I made one of these in the seventies and I used it last week.

David

 

[timetraveller]

Not only do I agree as well but would point out that if you make the piston stop long enough it stops the piston about half way down the stroke which is where the piston is moving most rapidly up and down for any angular rotation of the crankshaft.

 

[Tom Gaynor]

The one thing you don't actually need to FIND, is TDC. It's bloody difficult too, with about 3 degrees of crank movement either side before the piston moves. More accurate is to calculate where TDC is on your timing disc (and I use the same piston stop / timing disc set up as below), so that exactly where 34 degrees BTDC (or whatever) can be found. So turn the piston stop well in to find 30 or 40 BTDC, note the reading on the disc, turn the engine backwards (or forwards if appropriate) until you hit the stop again, and note that reading too. Exactly between the two is TDC. Subtract 34 (say) from that, turn the engine to that point, and set the timing. You know where TDC is, but didn't actually find it...
A refinement is then to screw the piston stop in until it it hits the piston (still at timing point), then count the turns out until the piston just scrapes past. Note this number. Now you can time the motor WITHOUT a timing disc. Just screw the piston stop in until the piston just touches, which is TDC +/- 3 degrees, turn the motor back, screw the stop in the number of turns burned into your memory, and when the piston hits it you are at exactly 34 degrees.

The explanation is that at TDC there is very little change of piston height per degree of crank rotation, whereas at 34 degrees BTDC, there's lots. I checked this on a Rudge with a sliding stop (centre plug made it easy). I found TDC more or less by judging when the rod was at max height, marked it, withdrew it, and made another mark 12 mm - Rudges are timed on piston drop - above it. Then I set the 12 mm mark against my "sight" and checked the degrees. Bang on!

Practical tips:
1) NGK plugs use a ceramic that probably resists nuclear weapons. Use a Champion or Lodge plug to make your stop.

2) Cut an airgap in the piston stop pin with a hacksaw to release compression.

3) If tempted to make a sliding rod, DO NOT make it from stainless. There is no way to fish a dropped stainless rod with a magnet. The head has to come off. Don't ask.

4) Don't forget the number of turns + 1/4 turns you memorised...

5) Francois Grosset makes and sells a timing disc mount that can be used static, or when the motor is running. It screws in in place of OP 9/2, and allows oil circulation while timing. You also need Superman's eyesight, or a strobe.