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[apparently lucky eddie]

The standard low comp. bullet pistons are very slightly domed, so little it is barely significant. The high comp. items are, as you'd expect, significantly stepped with large recesses for the open valves. This is probably not a good shape for either perfect mixing of air/fuel of for an even burn hence the twin plug set-up giving a boost in performance. There are plenty of pistons in the accessories pages here at Itchcocks.

[Phil Ashbrook]

The Enfield head is a Hemi and the 535 piston is very close to a Jag E type piston , our hosts did a good thing with that 535 piston ..maybe the flat section of the crown was a bit too big but they were trying to please a lot of folk with different demands , my big end has gone for sure now so the bike is off the road till I get the 612 crank , I'm gonna comp it up and will do the putty valve clearence test , 9.5:1 here I come .
I should get the big inlet valve for the stroked engine so it can breathe better , I might run a drill on the crank case breather port but I have done all mods now bar the crank . God knows what the postage of a crank could be for Australia and I might take it to my man to V block mic test ..any big bump might knock it out .

[NicoV]

Phil, how did you adapt the opening for the second spark plug ? Did you cut out part of it to accomodate for a short-reach spark plug ?

[Revband]

There have been many threads on here regarding twin plugs and the consensus of opinion amongst the more knowledgeable is that it is a waste of time and money. In a previous time a long time ago I used to race tune British singles and like Bullet Whisper I found it gave no advantage, but if you do feel the need to waste your time and money, don't forget that you need to retard the ignition timing around 3 degrees or it will be running over advanced, any perceived improvement on one of these engines is only apparent in you mind because you spent the time and money.

[Bullet Whisperer]

If you really want twin plugs, for whatever reason, I have a new way of doing it with minimum outlay [and no gains I have ever noticed]. If you get a 14mm long reach plug, as used in some Vauxhall Astras, they have a slimmer body and smaller hexagon and can be had in 7 and 8 grades [BC prefix]. You will need to make a collar of about 4mm depth, to 'shorten' the plug to the right length [short reach will be too short IF available] and you can tap a thread into the decompressor tunnel to the exhaust port and block it with a grubscrew. No machining of the head required and can be done in situ in very little time, but it won't make the bike go any better, from my experiences, at any rate. Regards, Paul.

[ChrisD]

Hi Phil

…………..And then you can mount the decompressor in the centre of the head as shown in the images and sketch below.












I can confirm the regular tickover. I did this not for more oomph (although any is worth having) but to get a faster burn with the 8.5:1 forged high crested piston (Accralite) and to aid in cooling because sitting in traffic here (Cape Town) at 40+degC is hell on a bike.
Cheers, ChrisD

[NicoV]

Chris D, wonderful job !

Paul, thanks. I found an NGK spark plugs that fits in perfectly without any machining and doesn't need a collar: BP6EFS.