BTH 'TT' Mag on 1950s 350 Enfield Trials Bike

Voyager

Well-known member
UKGSer Subscriber
Joined
Nov 11, 2006
Messages
2,157
Reaction score
0
Location
Kirkliston, Scotland
I currently have a neighbour's 350 Bullet Trials bike in my workshop for fettling :cool:.

Long story short - the neighbour (who has owned the bike on and off since the early 70s) handed it to a friend for sorting . . but the friend is going through a bad patch and let him down - so I got the (unpaid) job.

The bike was constructed in the late 1950s from 'Factory Special' parts by 'Bell & Small' an Edinburgh MC Dealers. It competed in various trials over the years until it was ridden in the 1962 SSDT by Eric MacNamara under No. 190. Eric represented the 'Edinburgh & District motor Club'. This is a Special machine and differs from the 350 'bullet' of the time.

The bike has a BTH 'TT' magneto, with a broken advance/retard cable. The reason the cable broke (I assume) is that the slip ring was seized. I have stripped the contact breaker assembly down.

I have been searching the net for details of this Mag, but haven't found much apart from some pictures of complete units and sites offering Magneto Refurbishment.

The problem I have is that there is a steel plug with a small steel peg that fits through the side of the bronze housing. The peg fits a slot in the slip ring and limits the ring's rotation. The peg is offset and may possibly be rotated to provide fine adjustment to maximum advance or retard. It has been abused by someone in the past and is fixed solid.

I need to remove this plug in order to 'lap' the slip ring into the bronze housing to restore easy, shake-free movement. Heating and tapping with a drift hasn't worked and I don't want to be brutal (yet). This Mag is over 60 years old.

I need an exploded diagram . . before I find the exploded bits in my hand :blast.

Does anyone have details of this 'TT' mag, or a link to such ??

Any clues gratefully received.

Bob.
 
Thanks Jazbee, I'll have a look at the Britbike Forum.

I have Thorspark solid state ignition on my 1958 Ariel 'VH' 500 and have three Lucas MO1 magnetos that I have converted to Brightspark condensers - they 'spark' very well :thumb.

I'm not intending to re-build the BTH 'TT' Mag, its just that the slipring was corroded and stuck. Removing the steel plug/peg will make the lapping easier. The mag works fine but the advance/retard doesn't :rolleyes:. I'd like to be sure about the plug before I attack it - is it pressed or threaded :confused:.

I have two BTH M.1-F2 Mags for my 1931 Panther M50 . . Getting them refurbished by "The Magneto Guys" cost me £880 and took nearly a year (should have done them myself) !

The restorations (3) aren't progressing very fast - just too many projects :blast

Bob.
 
Do you still spend most of your time hauling horses about the countryside? That would seriously slow down your projects. :D
 
Give Paul a ring at APL magnetos in Shaftesbury and ask him for some advice.
 
Do you still spend most of your time hauling horses about the countryside? That would seriously slow down your projects. :D

I don't do much horse hauling these days . . The Horse Daughter can do that herself since she passed her trailer test (she is a Police Traffic Officer and just passed the Advanced Driving course so she drives the fast, stripy cars - not someone you want as a passenger :blast).

The problem is rust and I have three 4x4s off the road at present while I replace inner sills, wheel arches etc. I'm not that good with tin bashing and don't have much enthusiasm for crawling under old trucks with a MIG or TIG welder. I need to get on with it and get the Toyota Amazon back on the road as 'PC Daughter' is chewing my ears about not having a towing vehicle at present. Fortunately, the Covid restrictions mean that horse competitions are 'off' at present. They have a 'virtual competition' which entails younger daughter filming her sister on horseback and submitting the video for judging :D.

Our old horse reached the end in November and had to be 'put down' - he was 31 and we had him for 25 years. He leaves a hole in our lives but my bank balance may recover a bit (if I don't go mad with bike restorations). The local Vet Practice will be laying off staff now :D.

Not having a decrepit horse means less need of a horse tug - which doesn't help the winter car repair task, particularly as I have been landed with an interesting Royal Enfield to fix.

Daughter's remaining horse is only 14 and quite fit so I feel the heat is off a bit :thumb.

Are you still working, Kevan, or has retirement caught up with you ??

Bob.
 
Well, I'll answer my own question:

The steel plug that incorporates an offset pin that limits the slipring motion is screwed-in.

About three hours on the Milling Machine later I managed to mill out the 'stake' that someone had mashed into it, bored the centre down, put two holes just inboard of the thread and, with a couple of steel pins and a Mole wrench. . unscrewed it.

The thread is 26 tpi (BSCY) and I have a bolt that fits it so I'll make a new plug on the small lathe. The 4-jaw independent chuck will allow me to centre the bolt stem on the offset pin.

Next problem is to clean-out the slightly distorted bronze housing to allow the slipring to 'slip'. I'll use the rotary table under the mill with a 4mm polishing stick to start with. If that doesn't work I'll have to lightly mill it.

These simple little jobs turn out to be anything but and take days. The trick is not to rush it and have a good think about the process before going near 60-year-old castings with power tools. Putting bronze back on is a lot harder than taking it off :D.

Bob.
 


Back
Top Bottom