The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => Lucas, Ignition, Charging, Electrical => Topic started by: Blackbeeza on 24.02. 2023 20:20
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Hi All , For my 1960 A10 G/F :
I am trying to remove the mag from the timing case and have come up against a problem . The left hand thread on both the securing bolt and the auto advance unit are badly damaged / partially stripped and therefore the automatic extract feature of the bolt will not work .
I have ordered a new bolt in the hope that the new threads will re-form the threads in the auto advance unit . Can anyone inform me what the specification ( diameter , TPI and thread form ) of the left hand thread is so that , if necessary , I can get a tap or thread chaser to help repair the thread in the auto advance unit ?
Any other ideas for getting this auto advance unit off if the threads are permanently damaged ?
Thanks and regards - Ron Little
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3/8 BSF
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3/8 BSF is the thread on the armature. Don't know what the LH thread is.
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Hi Ron,
You can remove the mag by removing the 3 holding nuts on the mag flange
screw the mag nut back onto the armature finger tight and undo 1 - 1 1/2 turns
pull the mag as far as possible off its studs until the AA is against the timing case
a tap with a smallish hammer on the AA nut will break the taper loose, then unscrew the mag nut from the mag threads,
John
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The left hand thread is 9/16" x 20tpi.
In the cases I've dealt with, the female thread has still been OK. It's more rugged than the raised thread on the centre bolt/nut.
I've cut them at 55° assuming they're BSF but can't swear that's right - don't have a new 'bought' one to compare.
A standard thread chaser won't help a lot on the female thread though - and even Tracy Tools, may their name be forever praised, don't seem to have a suitable tap.
When buying new, be sure to get 'Beesa length' not Trihard size by mistake. The Beesa one is longer owing to the fatter tufnol gear..
As to getting out the one that won't self-extract . . . I don't know an easy way because the thing can't pass out the inner end even if you decapitate it, owing to the female taper on the main unit. So I've destroyed the centre bolts by drilling away until they're wafer thin and then collapsing them.
Maybe Cheeserbeezer has a more cunning and less time-consuming method?
All I know, and have whinged about it many times, is that anyone who thought the first turn of a raised left hand thread could make a durable 'shoulder' for extraction needed his head examining.
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Thanks to everyone that came back to me - again I'm astounded at the level of knowledge in the members of the forum.
Looks like I've got to cross my fingers and hope I can make a silk purse out of a sows ear !!! But , at least you have given me hope - everything has been done before by someone.
Regards - Ron Little
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Maybe Cheeserbeezer has a more cunning and less time-consuming method?
All I know, and have whinged about it many times, is that anyone who thought the first turn of a raised left hand thread could make a durable 'shoulder' for extraction needed his head examining.
I've had to deal with this a few times. Don't cringe but I have managed to remove ATDs with levers when they won't self-extract. Rocking from side to side does it. Fortunately the back of the crankcase half is solid against the mag so there is plenty of material to lever against, but this is not a recommendation!
Regarding removing the centre bolt, again, no quick fix, just keep drilling until there isn't much left then break the outer skin of the nut and collapse it. I find the left hand tap I have will repair the female threads so, with a new nut, all is well again.
A problem not previously mentioned is that the threads on the end of the armature are often a bit short so, if the left hand thread on the nut is a bit mangled, the inner thread of the nut skips over the last thread on the armature before it extracts, hence the problem. I find this is as much the cause of the problem as any other. If Lucas had made the armature two threads longer the problem would be much rarer. I usually remove short or damaged threads on the armature and replace with the correct thread by cutting the old threads off, drilling the end of the armature on the lathe and tapping 3/8 BSF about 3/4 inch into the shaft. I fit a high tensile bolt with threadlock and cut the head off at about two threads longer than original. This makes a superb repair and you get the benefit of new threads and longer threads to prevent the ATD extractor failure.
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A problem not previously mentioned is that the threads on the end of the armature are often a bit short so, if the left hand thread on the nut is a bit mangled, the inner thread of the nut skips over the last thread on the armature before it extracts, hence the problem. I find this is as much the cause of the problem as any other. If Lucas had made the armature two threads longer the problem would be much rarer. I usually remove short or damaged threads on the armature and replace with the correct thread by cutting the old threads off, drilling the end of the armature on the lathe and tapping 3/8 BSF about 3/4 inch into the shaft. I fit a high tensile bolt with threadlock and cut the head off at about two threads longer than original. This makes a superb repair and you get the benefit of new threads and longer threads to prevent the ATD extractor failure.
+1 for that!
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I've found trying to extract a recalcitrant mag nut is facilitated by holding the ATD in a lathe chuck and the nut (bolt?) in the chuck of the tailstock.
I then apply extractive force via the tailstock whilst rotating lathe chuck in appropriate direction by hand. Works for me.
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Hi All , For my 1960 A10 G/F :
I am trying to remove the mag from the timing case and have come up against a problem . The left hand thread on both the securing bolt and the auto advance unit are badly damaged / partially stripped and therefore the automatic extract feature of the bolt will not work .
I have ordered a new bolt in the hope that the new threads will re-form the threads in the auto advance unit . Can anyone inform me what the specification ( diameter , TPI and thread form ) of the left hand thread is so that , if necessary , I can get a tap or thread chaser to help repair the thread in the auto advance unit ?
Any other ideas for getting this auto advance unit off if the threads are permanently damaged ?
Thanks and regards - Ron Little
I've had this quite a few times now. Getting the ATD off when it won't self extract is relatively simply, but takes patience. Simply get a small hammer and tap round the unit without using brute force. This will 'encourage' the unit to loosen on the taper and, sooner or later, will pop off. Done this around four times now and haven't had an issue yet. NB, don't tap on the bits of the ATD that look delicate!
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I got over this problem by, undo the extractor nut until it is loose then drill and tap the bolt to accept a 1/4 dia bolt. By then doing up the 1/4" bolt in (whilst holding the old extractor bolt) and up to the armature shaft it acts as an extractor. Worked for me.
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I also had to remove my ATD a few weeks ago to reset the timing. I discovered belatedly that the extractor thread was stuffed. After loosening the outer bolt from the mag thread, I gripped the end of the outer bolt with a vicegrip and put a long 5/8" extension socket bar through the gap in the vicegrip. After putting a wooden block beneath the lower end of the bar, I gave the top end of the bar a good tap with a light hammer (to minimise damage to the mag bearings), and the ATD came off easily. I tried to ensure the force was perpendicular to the shaft. This is the ATD I had spent hours on getting a large advance angle for the electric starter. Next time I will try Mugwumps modification.
Col