classic motorcycle forum

Some clarity needed here
He is talking heads and you are talking cylinders 

Might be a slip of the pen.
But that photo would help ...

That's interesting. I think we might need a photo.

Most Francis Barnetts used the 172cc Super Sport engine, which has prefix BZ. The hot Brooklands version has prefix Y.

KZ is the prefix for for a 196cc Brooklands engine, but I'm not familiar with KZB... Maybe it's some variant on the Brooklands/Super Sports.

The drilling in the cylinder between the exhaust ports is for an oil feed. The Brooklands head has a boss for a decompressor, but it is usually not fitted. Perhaps super-sporty riders/racers made do without a decompressor. If it's a huge problem, maybe you could use a thin aluminium gasket under the cylinder (not too thick or it will change the port timing) or a thicker head gasket to lower the compression a little.

Leon

I'm set !
A Bantam ? tin bracket and later (?) Lucas lamp seem made for the job.
And that aforementioned small 6v battery.

Have a few bicycle ones. Decided there was no mileage in them = dry cell batteries again.
A better quality of rechargeables might work, but have proved elusive.
And look notably modern.

I'd still be curious what the model of this light switch is ??

they originally gave but wangling a replacement in and out of that headlight clip every month or so would drive me nuts.

You and everyone else, so after the one BSA supplied (if they ever did) went up the swanee doubtless they were never replaced. Just park off-road or where a copper isn't going to visit.
If someone was poor enough to be riding a little two-stroke in the 1950s, wasting money on dry-cells would have been well down the list of necessary expenditures.

I understand your lack of concern over lighting. Who does expect to go riding one of these beastliest at night anyway BUT, (there’s always a but) you do occasionally get caught out. Country town rides and rallies have caught me out a few times, somehow you end up at a local riders home or a different pub and bingo, it’s got dark quicker than you expected. I also like to have a brake/stop light fitted. I have little faith in the road users    following behind. Many only react to brake lights and never think beyond them.
You can fit a battery powered total loss system but then you’re always faffing around charging up batteries. Fit a basic direct lighting set up and you always have no hassle lighting but the brake light set up is iffy. There’s never an easy and simple 100% satisfactory solution is there.

I smiled at the 4.5v battery chatter. They can be sheer frustration. I don’t know exactly when they went off the market and I have mucked around making them up from two ‘D’ cells but eventually I got fed up and stopped bothering. I don’t know how many hours light they originally gave but wangling a replacement in and out of that headlight clip every month or so would drive me nuts.