Magneto repairs

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ROB. THOMASSON
Posts: 96
Joined: Mon May 14, 2012 5:36 pm

Magneto repairs

Post by ROB. THOMASSON » Thu Sep 02, 2021 7:44 am

Last year I had problems with the BTH magneto on my Gipsy major. I sent it to a company called Ribblesdale automotive on the recommendation of a friend. It took them 9 weeks to do the job and on its return the points came loose on the first start. I rectified this and flew the aircraft briefly before the lockdown. This year it failed again and I returned it. This came back within 3 days with a note saying the points had come loose(again)
However on checking it I found the impulse no longer worked. It clacked as normal but there was no spark. As result I sent it to a local engineer to look at. He found the following.

1) The impulse unit had been fitted one hole out of place, so the “TOP” label wasn’t at the top. Resetting this corrected the no sparks problem when in impulse mode.
2) The lock tab holding the impulse mechanism on the shaft was useless, as the tab into the slot for the woodruff key was missing.
3) Several of the 4BA bolts holding the impulse mechanism to the main housing were bent.
4) The brass locating tongue (almost a woodruff key) on the points was useless despite the low grade epoxy that had been used to keep it in the raised position.
5) The engineer fabricated a locking mechanism to prevent further point problems but then when run up on test by using a lathe as a drive source, the magneto would periodically fail.
6) There was brass shrapnel in the rotor probably causing the intermittent failures.
7) After rectifying these faults when tested on the aircraft, the output was insufficient to provide a smooth running engine.
8) The painted marks on the low speed and high speed wheels were in the wrong place causing the distributor timing to be incorrect. As a result the rotor arm was not pointing to the contacts in the distributor cap when the spark was generated.
9) The condenser mounted on the rotor, was loose.
10) Further investigation showed 2 of the 4 locating pins on the high speed wheel had been filed off. The armature head shows what appears to be 10 holes. However, two are dents due to a past attempt to assemble it when four new holes were drilled. It was a pity two were not in the right place and thus I presume someone took it into their head to file off the misaligning pins. These extra holes would perhaps explain the brass particles found in the rotor. My assumption from this is someone did not realise the 4 pins are deliberately not in a square to ensure correct assembly.
I wrote to Ribblesdale electrics two weeks ago asking for their explanation of these facts but as yet have not received a reply.
Thankfully I was able to borrow a magneto to keep flying.
So has anyone else had problems with them?
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ROB. THOMASSON
Posts: 96
Joined: Mon May 14, 2012 5:36 pm

Re: Magneto repairs

Post by ROB. THOMASSON » Tue Oct 05, 2021 3:40 pm

Further to the above it later turned out the external magnet was half strength and the internal timing was out by 26 degrees If you don't know the BTH it is unusual in that there is an external timing arm that swings the timing cam ring. Once this was adjusted things were a lot better.
I did contact my credit card provider to get my money back but since Ribblesdale don't supply a receipt or invoice this was unsuccessful.
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Terry Lee
Posts: 41
Joined: Thu Jan 28, 2010 3:44 pm

Re: Magneto repairs

Post by Terry Lee » Sat Nov 20, 2021 10:57 am

I had one supposedly overhauled by them years ago. There was one guy there, can't remember his name. Who I believed specialised in BTH mags, so I was lucky. If he did them they were usually fine. He retired quite a few years ago. All said I know one other friend who had problems with them.
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