<VV> Long> Dual Points-advantage

[BBRT]

>From a web site-http://www.mgaguru.com/mgtech/ignition/ig200.htm

Here is a longer and more detailed explanation about dual points.

Chuck S
BBRT

So if neither dwell nor open time is particularly important, you might ask 
why dual points may have any advantage. The answer is stability, durability, 
accuracy, and a certain ease or convenience of adjustment.

Contact points "wear" with use in various ways. The rubbing foot may wear 
down a bit, which brings the points closer together and increases dwell. The 
contacts may wear or burn away, which would make the points gap larger and 
decrease dwell. If the condenser is going bad or is the wrong capacitance 
value, points may transfer material from one to the other, leaving a pit in 
one side and a peak on the other side. This may increase or decrease dwell 
depending on the pattern of erosion and deposition. As dwell is increased 
the points open later and close earlier, and vice versa for less dwell. The 
closing time is not very important, but the opening time is, as this 
directly affects spark timing. In the interest of maintaining accurate spark 
timing, there is a vested interest to minimize changes of dwell, which leads 
to an interest in decreasing wear on the contact points.

With the dual points distributor, the first points set to close "makes" the 
circuit and will incur any ravages caused by a small amount of arcing when 
closing. The last points set to open "breaks" the circuit and will incur any 
ravages caused by a small amount of arcing when opening. Since the make and 
break functions are done by different contact sets, the two sets of points 
share the wear and each will therefore wear less with time, causing less 
variation of dwell and timing, and it can run longer between required 
adjustment or points replacement. Also the two contact sets can share the 
electrical current load most of the time, so there can be less heating of 
the contacts and/or rubbing foot. This reduction of running heat leads to 
less wear on the rubbing foot as well as the contacts. This may be even more 
important if you also install a high output ignition coil which draws more 
current, which may noticeably shorten the life of single points. If the 
points may last roughly twice as long in service (and they do), it may not 
be such an increased expense to replace two sets of points instead of one. 
The reduced labor or inconvenience may save considerably on cost and/or 
tinkering time.

There is another effect which makes adjustment easier. A change of points 
gap induces a change in spark timing along with change of dwell. With single 
points you set the contact gap first, then run the engine (or spin the 
distributor) to check dwell, then re-adjust the points (if necessary) to get 
the correct dwell. Then you may need to go back and check/reset the timing 
again, as the dwell adjustment could have changed the spark timing. With 
dual points you set the gap on both sets of points, then set the timing, 
which is controlled solely by the secondary points (which open last). Then 
if necessary you can change the dwell by adjusting the primary contact set, 
which will have no effect whatsoever on spark timing, so you don't have to 
reset timing after adjusting dwell.

The real reason for using dual points is for improved life, long term 
stability and reduction of service calls, but has naught to do with 
increasing dwell. For what it's worth, I commonly run more than 20,000 miles 
on the Mallory Dual Points distributor without touching the points. This is 
one robust distributor, and I am one happy customer (except for the prices 
on Mallory tune up parts).