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Twin spark heads

Puddle jumper

Well Known and Active Forum Website User
Non-VOC Member
Hi Gang,
Has any one got any advice or tips on modifying cylinder heads to twin sparks heads, from the outside they look as if they where cast with this in mind, and companys like Pazon offer ignition kits for vincents with twin spark plugs per head, so it must have been done before.
Also what is the best way to repair a stripped exhaust manifold thread, I was thinking of recutting the thread in the head and making an over sized bronze nut to suit, or is it best to insert the head and use a standard nut.

Cheers Paul
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Comet Rider

Well Known and Active Forum Website User
VOC Member
Sparks and exhaust ports

Hi Paul,
Firstly there is a member in Milton Keynes who offers a reclamation service for the exhaust port. He mahines out the damaged thread, and puts in a bronze insert, thus retaining a standard nut:D.
The guy to contact is Roger Forsythe, AKA ROFOR Engineering.

With regard to twin sparks I have a different view to most, :eek: which would be to have your standard plug thread welded up, and re-machined for a 12mm plug, then put another 12mm plug on the other side. This means that you have more meat between the plug and the valve seats.;)
Another point to note is that you will need to pull back your ignition timing as you will now have a much wider flame front in the combustion chamber. This can be as much as 10 deg's, but a rolling road will be able to help you fine tune this:)

Ken Tidswell

Well Known and Active Forum Website User
VOC Member
Twin plug heads

You will indeed be able to retard the ignition by about 10 degrees. A long distance rider well known in the club has removed his Pazon Ignitia and replaced it with a DKX2A Prince of Darkness distributor. Due to unreliability. So you have been warned. An equally expensive alternative is a new BT-H, i have heard only one report of bad starting due to miss matched coils, now rectified, Ken


Well Known and Active Forum Website User
VOC Member
I put 2 plugs in my heads I took them into conways and had them back 20 mins later, they can do the exhaust rethread insert as well.
Peter Allen

Puddle jumper

Well Known and Active Forum Website User
Non-VOC Member

Hi Peter,

Can you tell me what plugs you are using for your twin plug heads and what difference does it make to
performance etc

Cheers Paul
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twin plug headfs

For what it's worth, I've been running this set up for quite a few years, both on ordinary coil and now with a Lucas Rita system. With the coils I ran 10 degrees less than standard, found a lot more torque and a few less revs, but better fuel consumption. With the Rita set up, I had occasion to talk to John Carpenter at Mistral Engineering, who services this system, and he advised me that 10 degrees was a bit too much, saying that 6 degres was more like it. In passing he said that the only machine that they ran with 10 degrees less was a Kwacker 900. I suspect that Guzzis and Be-ems which run 10 degrees less on twin plugs, like the Vin, are not all that fussed about a few degrees, unless one is after the ultimate in performance, and then I guess it is off to a rolling road. I currently use BP7ES plugs. I have noticed that the original plug will retain it's colour, whilst the second plug will read weaker, and this has to be because the fuel mixture is directed at the original plug which then sweeps round to hit the other one, but in a diluted state if you see what I mean. Maybe there is a need for a diferent heat range for the second plug. I cannot give an answer for the setting of 6 degrees less as the blooming thing is once again apart awaiting reallignment of the flywheels, so watch this space.
Phil Primmer

roy the mechanic

Well Known and Active Forum Website User
VOC Member
twin spark

In my experience of twin spark ignition ,Mainly with ex-works Alfa's ,it is recommended to run one degree difference between the pair per cylinder! Fortunately the works"worked out the actual timing required. Overall, it was not that far off a stocker,with single spark,The only reason to retard must be connected to innefficent original combustion chamber design. The alfa is also a "hemi" head, like the vin{ok in miniature] but the principles must be similar? Roy.


Well Known and Active Forum Website User
VOC Member
Twin plugs work great in hemi heads.The piston can obscure the flame front.In the late 70's people started doing this on BMWs a lot because of their hi (9:1) pistons.They could retard the timing so much that they fired AFTER TDC on idle (full retard).
This was a trick used on Ducatis in the 60's and 70's for racing.

Franz X. Kreiner

New Forum Website User
Non-VOC Member
I am racing a modified Rapide since years and made a number of experiments with different ignition systems. In my opinion the best twin spark system is from Francois Grosset (adress in MPH Magazine). It has a truly reliable mechanic advance and an electronic ignition box.
I am running my Rapide with 24 degrees (full advance), Kawasaki coils (highly heat resistent - can be mounted under the magneto cover - and inexpensive) and NGK BP7ES spark plugs. The engine runs perfectly and has much less vibrations it had with single sparks.
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