The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => Lucas, Ignition, Charging, Electrical => Topic started by: owain on 29.06. 2018 18:33
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I'm wondering if anyone has any tips about how to tighten the magneto sprocket fixing nut (that double threaded nut!) to the magneto spindle without the spindle moving out of place. It seems like a bit silly to set the piston 11/32" before TDC and set the magneto so that the contact breaker is just about to seperate, only for the magneto spindle to rotate out of position when tightening the magneto sprocket fixing nut. I have the auto-advance sprocket btw.
The only thing I can think of doing is trying my best to hold the contact breaker as best as I can in my hand whilst tightening the nut, but it still moves slightly out of position and I'm not super thrilled about all the torque being applied to the contact break when grasping it and tightening the nut. Has anyone got any tips for setting the timing in a more efficient and precise manner?
Cheers
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Try loosely tightening the ATD. You may be able to tap the points plate to the correct, (fag paper release) position using a blunt screwdriver as a drift and then go round and fully tighten the ATD. Recheck the timing after that, of course.
By the way, clean and adjust the points BEFORE timing.
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G'day owain.
I get the points to the right position and use a small timber wedge between the post and cam ring to hold it. Then bring the motor to firing position and gently place the AA unit on the shaft and start the thread. I the use a large socket to give the AA unit a short sharp tap (not belt or whack just a tap) then tighten the nut. Even doing this it may move a tad so I fudge the timing this tad to be correct when tight.
I love my Boyer *smile*
Cheers
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Hi Owain,
I think this is a real common problem.
I found out that every time I did this, I was off 7-8 degrees when done. I started to cheat the system 7-8 degrees. After that I hit it second try.
I think its just important to be gentle and patient no matter which way you try to do this.
Lee
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Interesting.
I have always used a piston stop once the correct position has been determined.
You can still be out by the backlash in the gear train.
But I use a timing disc so I can determine if there is any great difference.
Engines with timing set by piston drop are not all that fussy unless you are running 11:1's on 85 octane.
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I have had a couple of fibre (timing) gear failures through the years, and in hindsight have probably contributed to them, by tightening the magneto nut against the gear train. I now realise that side loading on the fibre teeth from a spanner, is not going to do their mechanical integrity any good.
So, these days I use a similar procedure to Mighty Musky, to ensure there is no loading against the fibre teeth when tightening the nut.
If you are re-fitting the magneto drive gear, then it's not much more effort required to remove the K2F and slot the three mounting holes. This will give you approx 18 degrees of rotation (9 degrees advance, 9 degrees retard if you fit the gear onto the taper in the middle of the arc of rotation.
Carefully offer up the gear and follow Musky's procedure of a sharp tap to seat the taper, before tightening the nut against the ATD body (not the fibre gears). Time with strobe light, and rotate K2F body until you have 30 degrees BTDC for iron heads, and 32 for alloy.
Then brumble down the lane!
Richard
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Hi Richard,
How long do you file the slots.? 18 degrees is quite a wide range, so must be fairly long slots.
Colin
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I took Richards mod on board, and it may be a bit rat-tail bastard rough , but works for me.... *work* *beer*
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G'day Col.
It's only 9 degrees at the mag to give 18 at the crank.
Cheers
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Hi Muskrat, Oh yes of course *good3*. So 9 degrees isn't much slot width. About 4-5mm eh???
Colin
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Hi Colin,
I will post some useful info and pics tomorrow.
Richard
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The hole(s) are arced to a length of 0.7" I initially used a file, but now use a drill press and dividing head.
The magneto casing fits over the mandrell (see pics & drawing) - remove the shaft end bearing & insulating washer first.
Then dividing head is mounted to drill (or mill) table, and mandrell / K2F casing clasped vertically. After centering any one of the three mounting holes under the drill arbor, use an end mill to slot the hole say 8 degrees each side of the mounting hole centre.
After slotting all three, remove from the mandrell, clean up the slots with a file, and trial fit on the RH crankcase.
File off any high spots, until the casing can swing fully in each direction.
Reassemble Magneto
Happy trails!
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No one has mentioned the trick of replacing the earth brush housing temporarily with a bolt to hand tighten against the armature to prevent it moving during tightening of the magneto/ATD nut. It's always worked for me but I accept that there may be a downside I'm not aware of.
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(I) have a feeling someone mentioned that a while ago- maybe was you *smiley4* ? What thread is it ?
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G'day Ted.
Never thought of that one.
Cheers
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Thanks Richard,
That is enough for me to go on. I see it does not leave a lot of meat at the slot ends does it.?!!
Colin
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There is enough however.
You can choose to reduce the length of the slot.
I use heavy duty (or high tensile) 5/16"flat washer under the nut.
Post some pics when you finish
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Where I am I can't read other posts. I use a slice of pink pencil eraser jammed between the two posts and the cam ring. This provides stability (because of the fri ction) while moving toward the points-open position and prevents running down the slope just as points open. Also, allows the pinion bolt to be loose enough make adjustments while avoiding stress on the brass locator in the points block. Finally, once in position, tap home the pinion with a socket over the bolt, then slowly sneak up on tightening the bolt. Fingers first, then light wrenching. Now recheck timing. Good? Final wrench tight and check again.
Well, that's about how I do it, at least.
Richard L.
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...then slowly sneak up on tightening the bolt.
I like that! Should I be tunelessly whistling?
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Placing a screw or bolt in the earth track is a good idea, not thought of that.... BUT, I bet this is why so many earth tracks have deep grooves in them, like in photo, I've often wondered how the hell had that happened! I'd think that a brass screw or bolt would be safer than steel one. Or certainly some kind of flag or visual reminder that it is there before starting.
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If pressing the auto advance onto the taper with your thumbs doesn’t make it stick sufficiently that you can tighten the nut without the armature turning, then the tapered faces are not in good condition. They can be ground into each other with valve grinding paste.
Avoid straining the fibre gear teeth, by sticking a screwdriver through the auto advance yoke and pushing against the turning force of the spanner on the nut.
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...then slowly sneak up on tightening the bolt.
I like that! Should I be tunelessly whistling?
It wouldn't hurt.
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Ted, I like your solution of putting a screw into the earth brush hole. The thread is 5/16 BSF. I agree with LJ that a soft screw needs to be used. I would recommend a nylon screw if it is available, or fit a plastic tip to a steel screw.
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G'day Rick. *welcome*
This is an old thread but thanks for your input.
Now get over to introductions and tell us a story.
Cheers
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Tis an old thread - but just worth maybe reverting to LJ's pic for a moment while the thread has folks' attention.
The track on the armature has not been created by the use of a bolt to hold the thing still when setting the timing.
It's been machined down to eliminate the dimples that are caused by the return current from armature to mag body via the earth brush at the firing point(s) of the magneto. The dimples can become deep over time and they affect the smooth running between brush and brass. One dimple is usually worse than the other on a twin (or a four for that matter) owing to the opposed polarity of the sparks.
The armature in the picture is a sad case owing to the depth of brass removed, which is pretty much to the depth of the steel dowel pins used to assemble the armature. Filling in dimples with hard solder or braze and machining back to round isn't something I have experimented with - nor do I want to as a true fit on the coil is important and distortion is a risk. Probably 'do-able' though in extreme cases. In 95% of cases a light turn in the lathe is enough to recover the brass end cheek and the majority of rebuilt mags for our bikes will show signs of this as it's part of the routine.
It is said that the use of an auxiliary earth brush on the rear of the contact breaker can reduce the risk of deep dimpling caused by the main brush, as the earth path is shared, but I have to confess to mostly using cb assemblies not so equipped on my own bikes. None of the later 'low inertia' steel cb units has the facility for a brush in any case.
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I have never used a bolt in place of the earthing brush but were I to do so I would not use a steel or brass bolt as they colud mark the brass on the armature so creating the possibility of premature wear on the earthing brush. I would remove the brush and spring then turn a piece of nylon so it was a push fit in the earthing bolt and the correct length so when tightened down it would grip the armature, it would not have to be very tight to hold it while fitting the ATD onto it's taper.
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Hi All after reading this and needing to time another make (triumph), I found a spare earthing bolt, a small piece of dowel and glued that in. It appears to hold really well *smile* though not actually used it yet as other problems manifested themselves *fight*