Magneto points gap for 3 litre

Julian West · Joined: 27 Aug 2012 Posts: 29 Location: Bayern, Germany

In the Haynes Bentley handbook the magneto points gap is given as 19 thou, whereas in the Technical Facts it is only 12 thou. Could someone please tell me which of these two values is correct?

Nicholas Simons · Posted: Fri Jun 17, 2022 8:28 am Post subject:

The original instructions provided by Bentley for the 3 litre car state that the gap on the M L Magnetos should be set to 0.019" when fully open.
If the points are found to have previously been incorrectly set it would be prudent to check the timing. The instructions require that the timing should be set to 45 degrees before TDC when the timing is fully advanced.

Julian West · Joined: 27 Aug 2012 Posts: 29 Location: Bayern, Germany

Thank you for clarifying this for me.

John Murch · Posted: Thu Jun 23, 2022 5:37 am Post subject:

Over the last 45 years I have had to have a considerable number of ML mags rebuilt, I seem to have used all of the most respected names in magneto rebuilds. In the last few years Carl Kenney at Bicester has done all 4 of mine! The two fitted are giving excellent service. Carl advises me that the ML mag as fitted to the 3ltr and early 4.5 is marginal and at the limit of ability. Higher compressions, large plug gaps (and points gap?) all put extra stress on the mag. If your car gets a lot of use, then a mag rebuild every 5 years or so is to be expected.
So it would seem to me wise to err on the side of smaller plug and points gaps.

Nicholas Simons · Posted: Thu Jun 23, 2022 8:29 am Post subject:

John
Thank you for your input. I continue to be concerned that so few of the club's WO experts ( or any experts for that matter ) respond to questions on this message board. My response was based solely on the original documentation I received with my car and also that the engine was as originally manufactured. The Green Book mentions 12 thou but gives no justification for this value being smaller than what was originally specified by the designer. A small gap will give a longer dwell angle, which could have an effect on the spark if someone has increased the plug gap or increased the compression ratio, or chooses to run the engine at a very high speed. My engine is as built and I never need to rev it above 2,000 rpm so why change from what WO specified? Of course, an owner is free to choose a gap anywhere between the two figures, but should ensure his timing is correct.

Julian West · Joined: 27 Aug 2012 Posts: 29 Location: Bayern, Germany

Hi John,

Could you please let me know Carl's address and phone number. I'm looking forward to seeing you again at the TT 100.

John Murch · Posted: Thu Jun 23, 2022 11:00 am Post subject:

Nicholas, you are very kind to your car. Mine regularly goes to 3000rpm and cruises (overseas) if I have to use motorways, at 70+. It’s compression is raised and usually I do over 5000 miles a year.

John Murch · Posted: Thu Jun 23, 2022 11:02 am Post subject:

https://bicesterheritage.co.uk/specialists/vintage-magnetos

Carl can be difficult to get hold of! But if you are at Bicester and call in, he is kind and helpful.

My comments are solely based on my experience and I definitely do not hold myself out as an expert.

Nicholas Simons · Posted: Thu Jun 23, 2022 11:24 am Post subject:

John
My car has an overdrive fitted, and I find around 50mph is comfortable on the open road. Chassis 664, previously owned by Nigel Tucker.

Julian West · Joined: 27 Aug 2012 Posts: 29 Location: Bayern, Germany

Thanks John, for the tip about contacting Carl. I see you're not on the TT 100 list. Perhaps we could arrange to get together in London sometime?

John Murch · Posted: Fri Jun 24, 2022 4:12 pm Post subject:

Hi Julian, I’m not on the TT trip, would be pleased to meet up when you are about, most of the summer I’m based at RG88BX winter in London.
John

Robert Craven · Posted: Sat Jun 25, 2022 5:24 pm Post subject:

An Autocar article of 21st May 1926 warns against letting the points gap exceed 0.012". I wonder if they got their figure from Bentley.
There was apparently a separate Magneto Instruction Book but I've never seen one.

Nicholas Simons · Posted: Sat Jun 25, 2022 7:32 pm Post subject:

But why?

Andrew Moran · Joined: 01 Jan 1970 Posts: 6 Location: Victoria, Australia

Hi all - not an expert but:

From original instruction book.

John Murch · Posted: Thu Jul 07, 2022 7:15 am Post subject:

Thanks for posting that extract, is it from the magneto manual or from a Bentley manual. I’ve never actually seen a magneto manual, they must be rare!

Nicholas Simons · Posted: Thu Jul 07, 2022 8:20 am Post subject:

This is getting interesting. My original reply was based on an original Bentley 3 Litre handbook which stated the gap should be 0.019".

Andrew Moran · Joined: 01 Jan 1970 Posts: 6 Location: Victoria, Australia

I think there is possibly a difference between early and later Bentley manuals.

The first 1924 manual just refers to the instruction books supplied for various devices.