I've been away from this conversation for a while, busy days. My experience with Norton isn't huge, or rather wasn't, as I've mainly worked on Triumphs for the last 45 years. However, this has changed a lot lately, as so many of the trusty old Norton hands packs it in due to old age, poor health, or the purchase of an RV...
I'm fascinated by the reasoning behind the difference between RH4 vs the RH10 heads, why did they do it? I learnt from this forum and Comnoz in particular, that the smaller port heads gives the engine a substantial increase in midrange power, which is where most people spend their riding time. As the experience quoted above didn't include running the engine with twin Amals on 32mm ports, it wasn't really conclusive to me.
Now I find myself with 3 Commandos in my garage, a Mk2A 850 (a general work-over), a Mk3 (cam change...) and an early ca 1970 Roadster 750 (tossed the single Mikuni for twin 930 Concentrics).
Riding these bikes back to back these last few days have left me with the following "seat of the pants"impressions: The Mk2A stomps away from the Mk3 in a roll on from almost any speed, in fact the Mk3 needs around 5000 rpm before it matches the other.
The 750 is stronger than the Mk3 at any speed, though a little softer than the Mk2A below say 4000 rpm. Beyond that it feels faster.
If I was to choose between them I'd have the 750, light as a Bonnie, fast, and smoother than either 850. (with the Mikuni my mother could have outrun it)
Back to the sleeve trick, I learnt a great lesson some 3 decades ago when I built a super quick Triumph TR7 1973. I used the standard inlet cam, a 1/2 race Hyde exhaust cam, standard pistons and carb. Apart from some cleaning up of the really rough ports, I left them at 1 1/16" or 27mm. Acceleration was almost brutal, and so much harder than any T140 I had tried similar tricks on. It was a bit peaky though, useful power from 4000, it needed 5000 before the party started, and by 6500 the party was over. Anyone who has ridden a T140 will recognize the shape of the power curve, only this one gave so much more.
I've since tried a similar trick on a 66 Bonnie, using Norton inlet sleeves, taking the ports from 1 1/8" to 1 1/16", or just under. It gave the engine a useful boost in the midrange, and I think the top end was at least equal.
Fitting a dual carb set-up to a 1971 Triumph TR6 (small ports) for a customer nailed it for me. I've had many Bonnies including 71/72 model 650s, and they were no match for the twin carb TR6 at any speed.
Without a dyno it's easy to dismiss this as wishful thinking, and it would be foolish of me to claim they're anything other than MY impressions.
Still, they ARE my impressions, and fueled by my "success" I made up a triplet of sleeves to take my Trident inlets from 1 1/16" to 1". Any standard Trident will be found a bit lacking in oomph below 5000 rpm, long cams, big ports and valves see to that. Short of a re-design, different cams, or a big bore kit, there isn't a lot to be done about this, but a set of sleeves is relatively easy to make. Again, no dyno runs (yet) but my impression is more power through the rev range, and if there's any loss up top it can't be much. Very similar results to the RH4/RH10 conversion.
I never radius or blend the sleeves in, I leave them stepped. I try to determine how far in they need to go and notch them for the valve guides. Very unscientific and "eyeball" engineering, but also simple to do, and to my mind, well worth it. After all, where I live and ride it's all about turns, bends and hills, a solid midrange is what you want, and by the time your mate on a 130 bhp 600 finds the right gear, you're through the bend, and long gone.
SR
