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#467296 - 12/12/12 1:07 am Lucas SR1 Mag
Dave - North of 60 Offline
BritBike Forum member

Registered: 08/05/01
Posts: 26
Loc: Yellowknife, NWT, Canada
Any thoughts/comments on using an SR1 mag on a DBD project? Recommended or not? What to watch out for?

Thanks,
Dave

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BSA Gold Star eBay items

BSA Gold Star forum This board is dedicated to BSA Gold Star motorcycles.

#467339 - 12/12/12 10:34 am Re: Lucas SR1 Mag [Re: Dave - North of 60]
Kerry W Online   content
BritBike Forum member

Registered: 02/28/12
Posts: 756
Loc: Middle East,
Have used an SR1 on 350 and 500 engines. It is my Lucas mag of choice (the magnet rotates and not the delicate wire-wound armature..why build them like that?)- easy to set and most will have an option for a handlebar-mounted kill switch They seem to provide a good spark, with easy starting. Parts are readily available.

The only thing is to make a strap, the best version of which is a custom-made alloy bar, shaped like the angular mag top, with a hole in each end, allowing a threaded rod with nuts to pull the beam down on the mag, from the usual hold-down locations.

Spindle height is easily adjusted with the SR1 optional base plate. I have yet to try to do it, but expect that the coild could be replaced with a double ended coil, perhaps externally, to facilitate twin-plug operation..
_________________________
No generalisation is wholly true, not even this one.
Oliver Wendell Holmes

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#467347 - 12/12/12 12:18 pm Re: Lucas SR1 Mag [Re: Kerry W]
triton thrasher Online   content
BritBike Forum member

Registered: 08/07/01
Posts: 4455
Loc: scotland
Originally Posted By: Kerry W
magnet rotates and not the delicate wire-wound armature..why build them like that


Because magnets were too big to rotate inside a magneto, at one time.

Well, you asked.

The rotating magnet magneto got blamed for hard to start Enfield twins, rightly or wrongly.
_________________________
Amateur Loctite enthusiast.

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#467356 - 12/12/12 2:23 pm Re: Lucas SR1 Mag [Re: triton thrasher]
Magnetoman Offline
BritBike Forum member

Registered: 11/05/11
Posts: 1856
Loc: U.S.
Originally Posted By: triton thrasher
Originally Posted By: Kerry W
magnet rotates and not the delicate wire-wound armature..why build them like that
Because magnets were too big to rotate inside a magneto, at one time.
Ditto on what Triton Thrasher said.

Prior to the development of alnico in the late 1930s it wouldn't have been possible to make a rotating magnet magneto small enough for motorcycle use. However, that development, plus the increase in rpm of racing bikes after the war, made it essential. The problem wasn't with the wires, but with the forces on the armature that was held together with just two very long screws. The armatures were fine for lower rpms, but the mechanical forces on it go as the square of the rpm and they started coming apart as the speeds climbed.

The nature of the design makes rotating magnet magnetos tall boxes, whereas K2F-type magnetos are long tubes, so rotating armature magnetos couldn't just be substituted for rotating armatures in all applications. There had to be room on the engine in the right places for it to be possible. So, the older design remained in production through to the end. No doubt cost was a factor as well, but I'd have to dig out some price lists to know whether one was cheaper than the other to manufacture.

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#467370 - 12/12/12 3:55 pm Re: Lucas SR1 Mag [Re: Dave - North of 60]
Kerry W Online   content
BritBike Forum member

Registered: 02/28/12
Posts: 756
Loc: Middle East,
Well, you learn something every day...the nett result though, is that the SR1, while it seems to have been prevalent on stationary engines, and AJS/Matchy singles, fit and works very well on all BSA singles that I am aware off. The hold-down straps need thought, but it's not difficult.

Great for racing too, as the option of an on/off or lanyard-operated kill switch is a great thing if you want to avoid a surprise with a stuck carb slide and no valve-lifter fitted...
_________________________
No generalisation is wholly true, not even this one.
Oliver Wendell Holmes

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#467389 - 12/12/12 5:46 pm Re: Lucas SR1 Mag [Re: Dave - North of 60]
Magnetoman Offline
BritBike Forum member

Registered: 11/05/11
Posts: 1856
Loc: U.S.
Originally Posted By: Dave - North of 60
Any thoughts/comments on using an SR1 mag on a DBD project?
Be careful what you buy. There was the SR1 (or was it the RS1?) for use on stationary engines, and the RS1 (or SR1?) for motorcycles. They look pretty much alike.

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#467418 - 12/12/12 8:38 pm Re: Lucas SR1 Mag [Re: Magnetoman]
gREgg-K Offline
BritBike Forum member

Registered: 09/01/07
Posts: 797
Loc: Ottawa, Canada
Originally Posted By: Magnetoman
Originally Posted By: Dave - North of 60
Any thoughts/comments on using an SR1 mag on a DBD project?
Be careful what you buy. There was the SR1 (or was it the RS1?) for use on stationary engines, and the RS1 (or SR1?) for motorcycles. They look pretty much alike.


The SR1 was certainly used on the Royal Enfield 500cc Bullet, and the SR2 appeared for a while on their twins.

Though bulky, they are good mags. Contact breakers (points) are getting difficult to acquire, and the rear cover (distributor cap) for the SR2 is both scarce and expensive.

.. Gregg
_________________________
Spyder Integrated Technologies
Lucas & Miller Magneto & Dynamo Restoration
SMITHS Chronometric Restoration
magneto@spyder-it.com

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#467745 - 12/15/12 2:29 am Re: Lucas SR1 Mag [Re: Kerry W]
Dave - North of 60 Offline
BritBike Forum member

Registered: 08/05/01
Posts: 26
Loc: Yellowknife, NWT, Canada
I was wondering about the height - come to think of it I saw one of the base plates you mentioned on eBay last week. Will have to see if it is still there.

I was also wondering about mounting because of the shape of the SR1 - if anyone has a pic of one mounted I would appreciate that.

Thanks for the reply.

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#467750 - 12/15/12 3:36 am Re: Lucas SR1 Mag [Re: Dave - North of 60]
Kerry W Online   content
BritBike Forum member

Registered: 02/28/12
Posts: 756
Loc: Middle East,
SR1fits fine behind even a big-fin engine. Will see if I can find a photo.
_________________________
No generalisation is wholly true, not even this one.
Oliver Wendell Holmes

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#468004 - 12/16/12 5:32 pm Re: Lucas SR1 Mag [Re: Dave - North of 60]
Ryan C. Offline
BritBike Forum member

Registered: 08/05/01
Posts: 60
Loc: Vancouver, BC
You can also use a modified Fairbanks Morse mag. About 10 years ago I made up both flange mount and base mount FM for my Gold Stars. I did not incorporate any advance/retard ability. Starting in full advance, you just have to kick with authority or bump start like the racers do. I have always kick started with no problems. I believe ABSAF are now making a mag with FM parts and adding an auto advance. Check their website.

Ryan

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#468019 - 12/16/12 6:38 pm Re: Lucas SR1 Mag [Re: Ryan C.]
Peter R Offline
BritBike Forum member

Registered: 11/08/09
Posts: 1070
Loc: Netherlands
Read here about the new mag developped by ABSAF.

http://www.absaf.nl/products_magnetonew.html
_________________________
Peter.
1974 Commando 850
1972 Trident T150T
1961 Goldie DBD34
1969 Benelli 250 sport special

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#468069 - 12/17/12 1:01 am Re: Lucas SR1 Mag [Re: Ryan C.]
Magnetoman Offline
BritBike Forum member

Registered: 11/05/11
Posts: 1856
Loc: U.S.
Originally Posted By: Ryan C.
You can also use a modified Fairbanks Morse mag.
When I was waiting for my friend's Bosch magneto to arrive to rebuild for the Cannonball Run I picked up most of a Fairbanks-Morse and a Joe Hunt magneto to modify/adapt one of them in case the Bosch turned out to be hopeless. Based on these magnetos, my concern with using a FM on a Gold Star is the magneto's cam, which is very abrupt, as well as the mass of its points assembly. Although these magnetos work fine at the lower speed of a V-twin, I definitely would run tests with a strobe before I would be confident the points could withstand operation at 3750 rpm (7500 rpm engine) without either floating or having the rubbing block break. Unfortunately, the cams on both are machined into the armature shaft itself, so could not easily be replace with gentler ones. However, both of these magnetos are for twins, so perhaps the ones for singles have gentler cams(?).

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#468158 - 12/17/12 4:33 pm Re: Lucas SR1 Mag [Re: Magnetoman]
Ryan C. Offline
BritBike Forum member

Registered: 08/05/01
Posts: 60
Loc: Vancouver, BC
The FM mag uses a single points assembly. On a twin the FM mags fire every rev. On a single would be every 2 revs as long as the correct singles shaft is used. Hunt made mags for the Gold Star back in the 50's & 60's for the flat track race guys and they had no problems reving to 7000 plus.

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#468281 - 12/18/12 1:17 pm Re: Lucas SR1 Mag [Re: Ryan C.]
Magnetoman Offline
BritBike Forum member

Registered: 11/05/11
Posts: 1856
Loc: U.S.
Originally Posted By: Ryan C.
Hunt made mags for the Gold Star back in the 50's & 60's for the flat track race guys and they had no problems reving to 7000 plus.
I had meant my post as a caution to be aware of a possible problem because of the aggressive cam if adapting a twin FM (with a wasted spark) for use on a high reving single. Other than that possible problem, there's nothing else about the FM that would keep it from being used at high rpms.

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#468306 - 12/18/12 4:57 pm Re: Lucas SR1 Mag [Re: Magnetoman]
Ryan C. Offline
BritBike Forum member

Registered: 08/05/01
Posts: 60
Loc: Vancouver, BC
Just a note of interest. The Harley XR-750 racer used the FM mag and was rated at 82hp @ 7700 rpm.

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#468312 - 12/18/12 5:14 pm Re: Lucas SR1 Mag [Re: Ryan C.]
Magnetoman Offline
BritBike Forum member

Registered: 11/05/11
Posts: 1856
Loc: U.S.
Originally Posted By: Ryan C.
The Harley XR-750 racer used the FM mag and was rated at 82hp @ 7700 rpm.
Right. But was the mag straight out of the Sportster parts bin, or was it modified?

p.s. I found a pdf of the XR-750 manual on line. In the section on the magneto it says "The XR-750 is fitted with a new type of Magneto. The unit is manufactured, as in the past, by FairbanksMorse and differs from previous units in construction details and operation."


Edited by Magnetoman (12/18/12 5:20 pm)
Edit Reason: added p.s.

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#468317 - 12/18/12 5:30 pm Re: Lucas SR1 Mag [Re: Magnetoman]
Ryan C. Offline
BritBike Forum member

Registered: 08/05/01
Posts: 60
Loc: Vancouver, BC
My understanding is FM used a standard mag modified with gear drive for the front mount position on the racers. They used standard points, condenser and coil. Maybe some of the old racers could provide aditional info.

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#468320 - 12/18/12 5:47 pm Re: Lucas SR1 Mag [Re: Ryan C.]
Magnetoman Offline
BritBike Forum member

Registered: 11/05/11
Posts: 1856
Loc: U.S.
Originally Posted By: Ryan C.
My understanding is FM used a standard mag modified with gear drive for the front mount position on the racers. They used standard points, condenser and coil. Maybe some of the old racers could provide aditional info.
Additionally from the manual:

This new magneto fires each
cylinder independent of the other cylinder. The condensor
is now mounted on the outside of the body casting
and is held in place with two straps. The magneto cap is
of a different design to accommodate the new
distributor rotor and two spark plug leads.


That sounds like there were more differences than just a gear drive. I can't tell for sure from the text, but the clause about firing each cylinder independently is consistent with there being two sets of points that could be independently adjusted. But, that's reading a lot into a few words. I'd have to see one of those magnetos myself to know how it differed from a standard FM mag.

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#468324 - 12/18/12 6:09 pm Re: Lucas SR1 Mag [Re: Magnetoman]
Ryan C. Offline
BritBike Forum member

Registered: 08/05/01
Posts: 60
Loc: Vancouver, BC
The later mags Post 77 used a distributor to eliminate the wasted spark. The parts list show the same points, coil condensor for all the xr750 mags. ARD used the distributor on their Triumph twin mags.

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#468325 - 12/18/12 6:17 pm Re: Lucas SR1 Mag [Re: Ryan C.]
Magnetoman Offline
BritBike Forum member

Registered: 11/05/11
Posts: 1856
Loc: U.S.
Originally Posted By: Ryan C.
The parts list show the same points, coil condensor for all the xr750 mags.
Do those components have the same parts numbers as for a standard FM mag of the time?

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#468326 - 12/18/12 6:27 pm Re: Lucas SR1 Mag [Re: Magnetoman]
Ryan C. Offline
BritBike Forum member

Registered: 08/05/01
Posts: 60
Loc: Vancouver, BC
Oops. I missed the difference in the coil. The early used the coil with 2 plug connections, the later a single connection with the distributor.

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#468328 - 12/18/12 6:30 pm Re: Lucas SR1 Mag [Re: Ryan C.]
Magnetoman Offline
BritBike Forum member

Registered: 11/05/11
Posts: 1856
Loc: U.S.
Originally Posted By: Ryan C.
Oops. I missed the difference in the coil.
What about the armatures? Were they the same part number on the XR750 mags as on standard FM mags for street bikes? The cam is ground into the end of the armature, and is what looks pretty aggressive on the standard FM mag I have.

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#468329 - 12/18/12 6:35 pm Re: Lucas SR1 Mag [Re: Magnetoman]
Ryan C. Offline
BritBike Forum member

Registered: 08/05/01
Posts: 60
Loc: Vancouver, BC
The armatures are different since one has a spline section to drive the rotor for the distributor. Not sure about the cam. I have seen the cam on the Triumph ARD and it appears to be the same as a stock FM...but I have no drawings to confirm. Back to you Magnetoman....

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#468337 - 12/18/12 7:30 pm Re: Lucas SR1 Mag [Re: Ryan C.]
Magnetoman Offline
BritBike Forum member

Registered: 11/05/11
Posts: 1856
Loc: U.S.
Originally Posted By: Ryan C.
Back to you Magnetoman....
To summarize what we've discussed: a standard FM magneto can be adapted to physically fit on a Gold Star. However, whether it will operate up to the max. rpm of a Gold Star without the points floating or the rubbing block breaking depends on the mass of the points assembly and the aggressiveness of the cam in a standard FM.

The FM "new type of magneto" on a Harley XR750 operates to the same high rpm, indicating at a minimum that the spring is strong enough to keep the points from floating. However, the two remaining pieces of the puzzle are whether FM's racing mags used lighter points than standard, and if the profile of the cam in the XR750's mag is gentler than in a stock FM.

Back to you Ryan...

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#468446 - 12/19/12 3:55 pm Re: Lucas SR1 Mag [Re: Magnetoman]
Ryan C. Offline
BritBike Forum member

Registered: 08/05/01
Posts: 60
Loc: Vancouver, BC
Don't know about the factory racers....but let's look at the standard FMX modified mags used on the Tri, BSA and Norton twins which reved to 6000+ rpm. They used all standard FM parts, adapted for different mounting and drives. Both the 1 cyl and 2 cyl FMX mags used the same points assembly for the same rotation. Also remember the cam profiles are different, the 2 cyl fired each rev and the single, every 2 revs. The points assemblies had no problem at 6000+ for the twin, so should be an easy job at 3750 (7500 engine rpm) for the singles.
Over and out....

Merry Xmas

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