Author Topic: Magneto transplant! Fitting a BTH  (Read 4763 times)

Online chaterlea25

  • Wise & Enlightened
  • *
  • Join Date: Jun 2009
  • Posts: 3050
  • Karma: 45
Magneto transplant! Fitting a BTH
« on: 31.07. 2009 23:34 »
Hi All,
Having my Super Rocket on the road now since 2002 and having had several problems with the rebuilt Lucas magneto, the bike was always difficult to start, sometimes hot sometimes cold,
A fraction of movement on the advance lever would mean kicking back or no start!!!

So sometime ago I decided to source a BTH manual magneto and have it rebuilt and fit it to the SR,
I bought a mag for rebuild on ebay but the thread where the advance cable adjuster boss fits into the brass end was wrecked, this piece is threaded in and then pinned.
Eventually I sourced a replacement end and all the other bits to complete the mag
CMES in Bristol have a good reputation so I duly handed the mag to Sean for rewind etc.
Last week I decided to fit the mag before a vintage run on Saturday
Lucas out in no time,
Try and slot in the BTH! No go???
Remove the 2 top mag studs from the case, mag went in further but was fouling the crankcase!!
Comparing the two mags the Lucas is chamfered on one side, and the advance housing is not as proud.
I put the mag in the milling vice and machined the chamfer as on the Lucas
It took several attempts to sort out the advance housing, including relieving the crankcase a little.
More hassle on fitting the BTH  as the flange is thicker so the studs were too short
and there is no room for full size nuts on the top studs
Late on Friday night I eventually finished, and was happy when it fired up easily even though it had not been started in several weeks.

Saturday morning I got the bike out, and my wife and I headed off, 
It never went as well as this before???  even though it always went well
I?m delighted with the results, easy starting, better tickover, acceleration and definitely more power

Later Saturday night a couple of friends and I flew to the UK for Founders Day Rally on Sunday, I bought some bits and bobs for my bikes at the autojumble and enjoyed the day thoroughly
On Monday we went to the Imperial war Museum at Duxford, and I will have to definitely go back there again with more time.
Looking at all the aero engines there, a question came to me,
Why are the aero engines I saw there fitted with BTH magnetos ???
Not a Lucas in sight!!!!

Regards
John O R


1961 Super Rocket
1963 RGS (ongoing)

Offline rocket man

  • Valued Contributor
  • ****
  • Join Date: Mar 2009
  • Posts: 466
  • Karma: 2
Re: Magneto transplant! Fitting a BTH
« Reply #1 on: 01.08. 2009 00:30 »
nice mod mate well done *smile*

Offline a10gf

  • Wise & Enlightened
  • *
  • Join Date: Sep 2006
  • Posts: 2632
  • Karma: 42
  • West Coast, Norway & Alpes Maritimes, France
    • A10 GF
Re: Magneto transplant! Fitting a BTH
« Reply #2 on: 01.08. 2009 15:34 »
Compliments for a great job, and an interesting topic.

A10 GF '53 My A10 website
"Success only gets you a ticket to a much more difficult task"

Offline Lannis

  • Moving Up
  • **
  • Join Date: Jan 2009
  • Posts: 90
  • Karma: 0
Re: Magneto transplant! Fitting a BTH
« Reply #3 on: 03.08. 2009 14:49 »
That looks like a good and useful upgrade.

It's true, I think, that the BTH magneto does not have the spinning armature with the buried condenser in it, and supposedly lasts longer and works better on bikes where it is original?

Lannis
1961 A10 Golden Flash
1969 A65 Firebird Scrambler
1955 M21 Commodore
1935 Matchless Model X Project
1990 Moto Guzzi California III
1983 Moto Guzzi 1000SP
1986 Yamaha TT225 trail bike
1966 Morgan 4/4

Online Triton Thrasher

  • Scotland
  • Wise & Enlightened
  • *
  • Join Date: Feb 2009
  • Posts: 1412
  • Karma: 20
Re: Magneto transplant! Fitting a BTH
« Reply #4 on: 03.08. 2009 15:35 »

 the BTH magneto does not have the spinning armature with the buried condenser in it,


I think it does have.

Offline beezalex

  • North Carolina, USA
  • Valued Contributor
  • ****
  • Join Date: Jan 2008
  • Posts: 374
  • Karma: 4
Re: Magneto transplant! Fitting a BTH
« Reply #5 on: 03.08. 2009 17:30 »

 the BTH magneto does not have the spinning armature with the buried condenser in it,


I think it does have.

Yup. Messed up rotating armature design is not uniquely the domain of the prince of darkness...
Alex

Too many BSA's


Online Triton Thrasher

  • Scotland
  • Wise & Enlightened
  • *
  • Join Date: Feb 2009
  • Posts: 1412
  • Karma: 20
Re: Magneto transplant! Fitting a BTH
« Reply #6 on: 03.08. 2009 17:34 »
rotating armature design ..

It made sense when magnets were very big.

Online chaterlea25

  • Wise & Enlightened
  • *
  • Join Date: Jun 2009
  • Posts: 3050
  • Karma: 45
Re: Magneto transplant! Fitting a BTH
« Reply #7 on: 03.08. 2009 23:20 »
Hi All,
Yes the BTH has rotating windings and condenser (capacitor)
Theres another thread on at the moment about sorting out Lucas issues!!!
BTH magnetos Are built to closer tolerances and any BTH part will fit another magneto
CMES (Sean Hawker) has hand made capacitors (that he imports from USA?)
The other capacitor that is recommended is a panasonic product
I cannot find the part number at the moment!!!!
This is recommended by Charles Falco, head of Physics in University of Arizona
Charles is an avid vintagent, and has done a huge amount of research in magneto/magnetism
He is coming over to the Irish National vintage Assembly later this month and I will ask him the number again!

He recommends two .2 micro farad capacitors in parallel

I have BTH's fitted to several of my bikes (as original) and have always been impressed by their reliability

Regards
John O R
1961 Super Rocket
1963 RGS (ongoing)