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Re: K2F Magneto - adjusting points gap [Re: Deadstiffcatt] #384435
07/14/11 3:08 pm
07/14/11 3:08 pm
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Surrey UK
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Mattsta Offline OP
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Originally Posted By: Deadstiffcatt
In an earlier post, Alex commented on shimming the bearing. It would be worth checking out, as the paper gasket/shim is the many tabbed one seen in the picture, and it is very easy to tear a few of the tabs off-which could result in a snug fitting yet off centre bearing, further aggravating the differences in point gap. Although I have not tried it, his point of carefully shimming on one side will affect the point gap. It stands to reason that even a little too much play in the bearings themselves could cause this discrepancy.


Measured up the cam ring today. There's nowt wrong with it.

Magneto needs to come apart for inspection.

I'll be paying particular attention to the bearing cups and the concentricity of the two bearing relative to eachother. I'm pretty certain the armature is not bent


1952 Triumph T100 in a BSA A7 Frame
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Re: K2F Magneto - adjusting points gap [Re: Mattsta] #384481
07/14/11 10:03 pm
07/14/11 10:03 pm
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Mattsta Offline OP
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Found out what the problem is.

It's not the armature, the bearings, the camring or the insulation cup washers.

The camring housing is slightly egg shaped. When the moving CB rides over one side of the cam ring, the reaction force pushes the camring into a slight ovality on one side of the camring housing so the points open slightly less on this side. I retimed the engine on both cylinders with a strategically positioned 3 thou feeler gauge between the housing and the outside circumference of the camring, effectively filling this void. By doing this I achieved a constant 0.012" points gap on both sides of the cam. I timed the engine on the drive side cylinder. Refitted the magneto pinion and then retimed the engine on the timing side cylinder. I've repeated this procedure several times and my results are consistent.

The difference BTDC between the two cylinders is now around 20 thou (0.5mm) compared with 3/16" so I'm definitely on the right track.

As for a permenant fix, not quite sure what to do that is affordable and robust. I can't machine the housing back to roundness and concentricity without using an oversize camring. I'm wondering if it might be possible to contrive some kind of shim between the camring and the housing which will still allow the manual advance and retard to function properly.

Last edited by Mattsta; 07/14/11 10:44 pm.

1952 Triumph T100 in a BSA A7 Frame
Re: K2F Magneto - adjusting points gap [Re: Mattsta] #384486
07/14/11 10:35 pm
07/14/11 10:35 pm
Joined: Sep 2002
Posts: 7,813
Seattle
Alex Offline

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If you're running manual advance, this is going to be tough. If you have an auto advance, put a metal shim in there and, when you like it, glue it in with some loctite bearing retainer.

...at least that's what I would do.

CHeers.


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Re: K2F Magneto - adjusting points gap [Re: Alex] #384487
07/14/11 10:47 pm
07/14/11 10:47 pm
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Mattsta Offline OP
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Yes! It's a manual setup

Hmmm! What a bugger!

I have a solution for this dilemma but it's not gonna be cheap!

This does raise an interesting point which is that a lot of people must be buying new camrings (not cheap) and fitting them in old magneto housings which aren't round! If the housing is a fraction out, you are totally wasting your money.

Last edited by Mattsta; 07/14/11 10:51 pm.

1952 Triumph T100 in a BSA A7 Frame
Re: K2F Magneto - adjusting points gap [Re: Mattsta] #384507
07/15/11 1:41 am
07/15/11 1:41 am
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Deadstiffcatt Offline
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An interesting problem. In essence, you need to build up only a small section of the holder, at the same time keeping it round to allow for the movement of the manual advance.

Thoughts: Cheap: Jb weld, then smoothed or end bored to recreate that perfect round that is needed. How would the jb last under the movement of the manual advance?

Hard Chrome plating, again bored to the nice round shape you will need.

Check buddies for one that may have that end plate camring housing in better shape. At least you will know what to look for.

Re: K2F Magneto - adjusting points gap [Re: Deadstiffcatt] #384779
07/16/11 10:57 pm
07/16/11 10:57 pm
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Mattsta Offline OP
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An intersting problem!

All the more so because I'm a design engineer by trade and I'm racking my brains thinking about this. I design stuff for mass manufacture and I'm engaged in a constant battle to make stuff that is repeatable and accurate. There's usually just a couple of critical dimensions which will determine the functionality of a component.

One thing is absolutely clear here. Spending money on a nice new camring for your magneto isn't necessarily going to get you out of dodge. There's all kinds of variables here.

The Achilles heel in the system is not just the camring. It's the concentricity and fit of the camring in its housing and the concentricity with the bearing.

Assuming your armature is straight, your mag bearings are running true and the camring is accurately machined, you can still get undone by a worn camring housing.

I'm surprised some enterprising engineer hasn't picked up on this.

If your camring housing is just a couple thou out of round, you don't have a chance of getting the timing right on both cylinders and it seems, even a tiny (couple thou) eccentricity in the camring housing can make timing both cylinders accurately almost impossible, even if the camring is spot on accurate.

It makes you realise what a crap design it is in the context of modern engineering.


1952 Triumph T100 in a BSA A7 Frame
Re: K2F Magneto - adjusting points gap [Re: Mattsta] #384787
07/17/11 12:17 am
07/17/11 12:17 am
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Deadstiffcatt Offline
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So that explains your reference to it being an expensive fix! (Didn't sound like the words of a backyard, how do I keep it running mechanic.) However, it does seem as though an enterprising engineer may have finally stumbled upon this! YOU! As there were many manual advance units made, and many people like an ABSOLUTE restoration, perhaps a limited quantity run could come about. Certain firms that specialize in Lucas mag repair may be worth talking to.

As a thinker, one wonders how the egg shape could occur.

1. Poor tolerances at the factory originally: Is this a cast part, or was it machined? A worn or unclean mold for the casting could cause this from new. It would seem a machined part would be more consistant, as there would be a jig to hold the unit in place to machine the hole, yet by the same token, the jig getting some wear over several hundred parts in and out could cause enough slop to cause the egg shape as well.

2. Perhaps in the course of it's life, the mag was dropped just right to cause this misshaping to take effect? Even during a rebuild, a drop to the ground of just the housing and smacking a corner could also slightly effect the housing's roundness. Finally, the thought comes to mind of ovetightening retaining screws, causing a similiar warp as the carbs are so frequently exposed to.

3. Product use. Have you determined the point of wear? The thought comes to mind that one side of the ring housing would be subject to more force-the side that the cable attaches to. By it's design, the cable entry point would also be one that could permit small amounts of dirt in, and through the daily use of the mag and advance, would tend to wear that particular side more than the other.

So not that any of these thoughts solve the problem, but may lend to an explanation of how it occured....

A final thought: As I have previously noted on another thread, Lucas is a jealous god and by not performing the proper sacrificial rites when installing the unit, perhaps a previous owner pissed him off just enough to warp things a hair????

Re: K2F Magneto - adjusting points gap [Re: Mattsta] #384819
07/17/11 4:31 am
07/17/11 4:31 am
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scotland
triton thrasher Offline
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Originally Posted By: Mattsta


It makes you realise what a crap design it is in the context of modern engineering.


It was ok(ish) on single cylinder bikes. Their version of production engineering was to cheaply adapt single cylinder magnetos to twins. And anything that would last 20,000 miles was considered fully durable.

It's slightly better when the cam ring is fixed and auto-advance is used.


Amateur Loctite enthusiast.
Re: K2F Magneto - adjusting points gap [Re: Mattsta] #384828
07/17/11 7:36 am
07/17/11 7:36 am
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Maui Hawaii
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Magnetos are very cool. However, by the time you buy one for 250$ and send it for restoration(500$) you are near the price of a Joe Hunt and the end of these kinds of problems. Alternatively, you can buy and rebuild a distributor for 200$ total and also end the problems.

Bill


Bikes
1974 Commando
1985 Honda Nighthawk 650
1957 Thunderbird/T110 "Flying Tiger"
Antique Fans: Loads of Emersons (Two six wingers) plus gyros and orbiters.
Re: K2F Magneto - adjusting points gap [Re: Deadstiffcatt] #384830
07/17/11 8:02 am
07/17/11 8:02 am
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Mattsta Offline OP
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@Deadstiffcat

The cam ring housing is a machined casting.

The problem doesn't appear to be wear. The inside surface appears to be in good condition.

I reckon it's most likely that it wasn't machined very accurately. I suppose it's possible that it has been dropped at some point in its 60 year life too.

Difficult to assess where the eccentricity is without a CMM. You can't get a dial gauge in there to measure it. The gap is at the bottom and I've got a 3 thou feeler gauge between the ring and the housing to time the engine. I've managed to time the engine to within 10 thou between both cylinders.

The simplest solution is just to insert a shim and this would work fine except that when you operate the cable, the shim gets dragged around the gap between the two components. I could probably bond a shim in there but I don't think it would be a very robust fix.

I'm going to measure up the housing and model it in a 3D CAD system. I have an idea for a more permenant solution which could be marketed and sold.

Last edited by Mattsta; 07/17/11 8:11 am.

1952 Triumph T100 in a BSA A7 Frame
Re: K2F Magneto - adjusting points gap [Re: triton thrasher] #384831
07/17/11 8:03 am
07/17/11 8:03 am
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Mattsta Offline OP
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Originally Posted By: triton thrasher
Originally Posted By: Mattsta


It makes you realise what a crap design it is in the context of modern engineering.


It was ok(ish) on single cylinder bikes. Their version of production engineering was to cheaply adapt single cylinder magnetos to twins. And anything that would last 20,000 miles was considered fully durable.

It's slightly better when the cam ring is fixed and auto-advance is used.


Yeah. It's not a problem at all if you only have 1 cylinder to worry about.


1952 Triumph T100 in a BSA A7 Frame
Re: K2F Magneto - adjusting points gap [Re: Mattsta] #384853
07/17/11 11:50 am
07/17/11 11:50 am
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Vic. Australia
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Pete R - R.I.P. Offline
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You can still correct this by filing the face of the cam housing that bolts to the magneto body.Because the cam ring is further out than the bearing,you can tip it to where you want it.

A shim in the right place would do the same thing.

Re: K2F Magneto - adjusting points gap [Re: Mattsta] #385099
07/18/11 10:24 pm
07/18/11 10:24 pm
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Mattsta Offline OP
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The saga continues!!!!!

I decided to dismantle the magneto and investigate further

Measured camring. 47.6mm dia all round or 1.874" measured in several locations.

thickness of camring on the raised internal profile: 3.75mm (0.1475") both sides. Thickness on lower profile 2.70mm (0.1065") one side, 2.65mm (0.1045") on tother. The raised part of the cam is what dictates the points gap if everything else is Concentric.

Cam ring housing..........

Not as bad suggested in previous post. 47.64 - 47.67, (1.876" - 1.877")

So I wasn't satisfied that the camring housing was the problem after all.

So stripped the mag and checked bearings and armature. Spun the armature on both bearings and no visible sign of runout. You can detect a tiny amount of runout just with your eyes.

Checked straightness of points packplate screw. Seems fine.

Assembled points backplate on the armature with screw and spun armature on timing side bearing.

Eureka!

The points backplate is visibly running out of true by a couple thou.

Not quite sure how to fix this yet because there is a raised key on the taper on the back of the backplate and this feature is vitally important. So I can't just turn the backplate taper true on a lathe. Off to see an machinist friend tomoz to see if he has any smart ideas!


Last edited by Mattsta; 07/18/11 10:30 pm.

1952 Triumph T100 in a BSA A7 Frame
Re: K2F Magneto - adjusting points gap [Re: Mattsta] #385134
07/19/11 2:01 am
07/19/11 2:01 am
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If at first you don't succeed, try a bigger hammer.

Which may have happened at one point. Sometimes it has been recommended to gently tap the plate 'home' to make sure it is fully seated. As the plate is a softer metal,(brass, if memory serves) this could account for a slight wobble, as someone's version may have been a graceful 'WHACK!'causing a very minor bend to the tapered end of the back plate.

Of course, the opposite may also apply. Perhaps the backplate was NOT properly seated, which could also give rise to the wobble. (By testing it on timing side bearing, do you mean out of the body using only the 1 bearing? If so, the natural amount of bearing play with NO support to the opposite end of armature could easily give the same wobble. Obviously, to properly test it would need to have both bearings supporting the armature, and I figure you already know that, but still worth mentioning.)

Given the overall large outside diameter of the backing plate versus the much smaller tapered diameter of the taper itself, just a tiny fraction of the taper being not quite home (similiarly minor bend) when installed in the armature would magnify dimensionally at the backing plate, easily being .002.

I would try fully reassembled mag less backing plate, then installing plate slowly and keeping an eye on runout as it goes in, once home, if still off center, a gentle tap at the right place with a block of wood and a very small flat tip punch to possibly center things up. NOT Wham!!!!, but *tink*. THEN tighten the retaining bolt, again checking for runout/discrepencies in point opening between cam lobes.

Re: K2F Magneto - adjusting points gap [Re: Mattsta] #385155
07/19/11 6:06 am
07/19/11 6:06 am
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Having a bent points plate won't give any difference in timing,between the cylinders.

Even if the points heel is bent out to a different radius,that new radius still remains a constant distance from the armature axis until the point when it strikes the cam.Differences can only be caused by the cam lobe positions.

Last edited by Pete R; 07/19/11 6:11 am.
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