The BSA A7-A10 Forum

Technical => Lucas, Electrical, Ignition => Topic started by: snowbeard on 24.03. 2009 22:34

Title: timing with the head off?
Post by: snowbeard on 24.03. 2009 22:34
so whilst we are all thinking hard about timing issues, I thought I would ask this. 

if I put the pushrods back in to tell the compression stroke, shouldn't it be quite easy to time my mag with the head off?

just checking for drawbacks that I don't realize now...
Title: Re: timing with the head off?
Post by: Brian on 25.03. 2009 02:42
No drawbacks, its the best way to get the distance before TDC correct.
Title: Re: timing with the head off?
Post by: MikeN on 25.03. 2009 06:52
If you can use a dial indicator on the piston crowns and a mag base you will get it spot on.
Title: Re: timing with the head off?
Post by: groily on 25.03. 2009 09:20
 'Get it Spot On'
. . . if you get the points exactly in the right place, that is. See threads various about buzzers and meters, fag paper and feeler gauges, strobes even. And if you did time it on the wrong cylinder, not the end of the world - swap the HT leads and it'll be fine. Convention says rear pick-up to rh cylinder, but I haven't always bothered as I can't see it make a scrap of difference.
Title: Re: timing with the head off?
Post by: trevinoz on 25.03. 2009 20:22
Also check both sides and grind cam ring if there is a difference.
Title: Re: timing with the head off?
Post by: RichardL on 25.03. 2009 22:45

"Grind cam ring" is an interesting comment. I take myself as fairly handy in the shop, if sometimes making impatient mistakes, but I would be apprehensive of doing grind on my cam. Do you do your ownn and is it easier than some might think? New cams are available, if pricey, is this a "What have you got to lose?" situation?

Richard L.
Title: Re: timing with the head off?
Post by: snowbeard on 26.03. 2009 02:46
thank you all for the info, sounds like what I thought.  good point on not mattering so much if I mess up which is compression, but I'll give it a try not to!

so I don't have a micrometer, only a "guessing stick", or caliper.  I will likely try to use the degree wheel again, and double check by measuring.  I used a fag  paper the last time and it worked well enough, but holding the mag steady while tightening the nut was my problem.  I tried using the tiny bolt holding the points plate, but I fear over tightening. 

I've read a lot of the posts and it sounds like that is the problem.  too bad there's not a way to lock the mag without damage?

what about the grounding pickup? does that ride on a slipring like the HT pickups?  could one put something there to hold it?  obviously the slipring would be likely to score or crack, but I haven't had the armature out enough to know where else it might be locked without damage?

I'll be sure to check it on both sides of the cam tho, another great point!

from the impression I got from the shop, sounds like I'll have a decent time to get all this done.
Title: Re: timing with the head off?
Post by: groily on 26.03. 2009 07:44
The ground or earth pick up is a small brush and spring like the HT pickups and runs against the brass body of the armature at the drive end. Nothing there to hold things fast!
You're right to worry about holding things too hard with the cb centre bolt - 4BA and long and flimsy and not made for that. However, often a socket or box spanner on it, hand held, will be enough to get the mag pinion nut to start to grip on the taper the other end, and then you're nearly there.

Cam rings can certainly be ground - but not in my shed! Need more skill than I've got to get them the same, although imperfections can be removed carefully. I doff my hat to those who can do stuff like that accurately. Guess that's why the bits are expensive, relatively.
A guessing stick will do the job OK. A degree disc, the primary chain marking method in a previous thread and the plug-stop approach, where the piston position is calculated accurately and then a tool made from an old spark plug with a rod down it, set in the right place . . . etc . . . are all probably better - but they still depend on getting the points just so.

When all is said and done, I set timing with the head off if I have a head off; otherwise, the old way with rod down hole and fag paper, or with a plug stop on engines for which I've made them, and never had any real trouble. Friend of mine with a V twin sets his with a strobe, but he's clever and had already designed and built an electronic rev counter with a sender mounted on the cb assembly, which gives him a point of reference, because he made it that way. But That Is Extreme and a classic example of 'I Did It Because I Could' from a bloke who Can. He also has 3 degrees of variability on his cam ring, ATD mag, having made an adjustable eccentric in place of the standard securing pin that holds the ring. Also Just Because - but useful certainly for minute adjustment without deranging the sensitive internal relationships of the mag unduly. Not worth the effort for most applications - but he did it because magnetos on V twins are very sub-optimal and every little helps. But even he has no way of locking the armature in place to set the timing, although I wouldn't put it past him!
Title: Re: timing with the head off?
Post by: MikeN on 26.03. 2009 10:15
My manual compettion mag also has a stop peg that is (theoretically) an adjustable eccentric although in practice its not because you cant get at it to adjust it when its on the m/c.
 I always thought it was a standard fitment?
 I purchased a new CNC ground cam and couldnt understand why I couldnt get my firing events equal. I checked it and it was prefect.Further investigation revealed wear in the mag end cover/bearing housing so it was slack and thus eccentric .So I bored it and fitted a phos/bz liner to bring it back which improved matters.
  It is important that the cam ring housing is absolutely concentric to the armature bearing but difficult to check with out a lathe and dial guage.
Title: Re: timing with the head off?
Post by: groily on 26.03. 2009 11:52
Yup, the stop peg IS eccentric, but it's tiny and only gave the Lucas people a tiny amount to play with when finally fitting up the instrument (I suppose). And the way it's assembled, with the cover over it, the screw is not meant to be disturbed, I'd suggest. By removing the thing, drilling and tapping a larger hole in the case, a larger eccentric (and a locknut to hold it, can be put on by machining the bottom part of the screw off centre. Haven't done it myself though, as no real need and I'm not THAT curious.

Many mag restorers do say 'It's the Bearing or Housing, Stupid' - not the cam ring. It's been said to me, and correctly.

However, keen to find something to do this grey morning, I had a play with a spare ATD mag to see if or how the armature could be held in position for getting the pinion back on. See the attached pix, which are pretty self explanatory and it's dead easy: I replaced the upper left hand end housing screw with a bit of turned up bar, male threaded at the bottom to replace the original and keep the housing on, female threaded 2BA on top. Height from housing (ie ecl lower threaded portion) - 19mm. On top attaches ashort bit of flat steel bar, plain drilled to be attached to the screw already made, and with a second hole tapped 0BA (could be anything though), with its centre 18mm from the centre of the firts 'ole. An 0BA screw, with a tapered point on the sharp end, screws through the bar and up against the brass mount for the tail end of the spring for the moving point. Some wiggle room on the bar is needed - by making the hole where the 2BA screw attached a little larger than the thread - to allow for variations if fitting different points and for cam ring wear etc. Works a treat. Just had to file flat and centre pop the top face of the brass mounting to allow the pointy 0BA screw to grip accurately. Doesn't take any real pressure to hold the armature exactly where you want it.

For a manual mag, the same procedure could work, but using the upper right hand screw on the end housing as the cable is obviously in the way, or one could use the lower housing screw - harder to get at on the bike though. Dimensions may also vary - I haven't measured up a manual mag.

If used with a buzzer or whatever would give 100% guaranteed cb position, and be a cinch. I'm going to make another one for manual mags.

Hope the attachments attach - not my thing!
Title: Re: timing with the head off?
Post by: groily on 26.03. 2009 11:58
Well, I couldn't make a pic of the thing on the mag attach apparently - but you see what it is anyway.
Title: Re: timing with the head off?
Post by: groily on 26.03. 2009 12:00
2 down, one to go
Title: Re: timing with the head off?
Post by: snowbeard on 26.03. 2009 14:41
wow, "necessity" mothers invention once again! very nice!   *respect*

so for commercial production we can slot the end with the wiggle room for greater adjustability, eh?  *contract*

I need to get some right tap and die sets I see... but I think I will dig thru my very limited spares and see if I can come up with something similar!! maybe I can figure it out to create a bend in the flat that will capture a bit of the plate itself, thereby limiting the possibility of the punch and point end slipping!

very nice bit of fettling there, much better than drilling and tapping the housing for a stop bolt!

there's a beer or two waiting for you when you hit the states (or my state at least)!!  thanks!
Title: Re: timing with the head off?
Post by: groily on 26.03. 2009 16:15
Bit rough Snowbeard, but made a prettier steel bar with more wriggle room this afternoon. Don't need much wriggle in fact to get plenty of degrees rotation of the cb unit to cope with any wear on heel of moving point or on cam ring. I thought about modifying the brass bit to put a little peg in to locate the bar, even maybe with a thread on and a small nut - but didn't like the look of the job and don't have too many cb units to ruin out of damn fool curiosity. However, a small hole, few mm deep, for a screw with the end turned right down to a parallelpin, rather than tapered, would make for a more positive register than a mere dot! But tis only a prototype and there be other better ways, I'm sure. Shall we reserve them for the commercial production run? *smile*

Actually, apart from needing a 2BA die for the bottom of the long pillar screw, any threading tackle will do,,obviously The actual hex I used was a chopped up bit of old 8mm rubbish allen key from some cheapo source, as I don't have any BA-size hex.