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1935 Daimler Straight Eight Contact Breaker Gap

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ds8w
Posts: 22
Joined: Wed Mar 16, 2016 9:43 am

1935 Daimler Straight Eight Contact Breaker Gap

Post by ds8w »

Such a simple question but -

I spent hours with a meter trying to figure out how the two sets of breakers work with the four lobe cam to produce eight sparks !

The two points are in effect in parallel.

It wasn't obvious but basically both sets of points have to be open and they do this when they "overlap" on the cam. So as one is just opening the other is closing and that brief overlap is the open period. So vice versa for the extra sparks.

Does anyone know what the right gap is ?

By experimentation I think it is at least 15 thou.

Stan Thomas
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Re: 1935 Daimler Straight Eight Contact Breaker Gap

Post by Stan Thomas »

It is "15 thou" as you say.

ds8w
Posts: 22
Joined: Wed Mar 16, 2016 9:43 am

Re: 1935 Daimler Straight Eight Contact Breaker Gap

Post by ds8w »

Thanks Stan.

By great good fortune I have today received a Straight Eight Instruction Manual and this says 12 thou. I found it listed on a Spanish book site but it was posted to me from a dealer 5 miles away in Wokingham !

However the aforementioned book also says that each set of points controls four plug leads each which is not quite the whole truth.

The Distributor Doctor was very helpful and suggested that if in doubt adjust to get a 32 degree dwell angle and 15 thou for new points settling to 12 as a good guide.

kind regards

Richard

Stan Thomas
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Re: 1935 Daimler Straight Eight Contact Breaker Gap

Post by Stan Thomas »

Dwell angle is the proper way to do the job - but requires a meter, so as a more basic approach, the manufacturer interprets the dwell angle into a measurement between the points - usually "15 thou".

In simplistic terms, the dwell angle is the time the points are closed to allow the primary winding of the coil the "charge up". If the points gap is too large, the dwell angle is reduced - hence a poor spark. If the points gap is too small, the slight arcing as they break does not collapse the current quick enough, causing an intermittent spar and hence misfiring.

Always re-check the gap after fitting new points after a few hundred miles, as the heel will wear quickly to the contour of the cam lobe peek and close the gap down slightly.

ds8w
Posts: 22
Joined: Wed Mar 16, 2016 9:43 am

Re: 1935 Daimler Straight Eight Contact Breaker Gap

Post by ds8w »

Thanks Stan and others.

Last word and my thanks to Kerry Berrisford for pointing ( oops ! ) this one out -

Old, maybe very old, contact sets had a much thicker shoe shaped heel rather than the more pointed modern type. As a consequence they would open much sooner and close later as the shoe rode up the cam. So the gap needed to achieve the "Straight Eight" overlapping dwell angles with modern point sets is bigger.

16 thou got me the right dwell angle.

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