right my current engine has an NC23 head on a 29 bottom end. I'm building my spare engine to a similar (or better) spec and starting to get parts together as the engineer is nearly finished with the crank and input shaft.

What i would liek to know is:
1. Are NC23 and NC29 valves/springs interchangeable? The manual only lists slightly different clearances.
2. If using NC29 cams in a 23 head, do I need to adjust the valve clearances from std?

Tah muchly. Hopefully soon I can resurrect the race engine thread with some action.

Hi there, The valve springs are interchangeable,though you prob better fitting new ones, & the cams do have the same lift etc so clearances wil be the same, but do you realise that the cam drive gears on the nc23 cams are differant to the NC29 the 23s are bigger & have differant teeth sizes.
Hope tthat helps.

Excellent.

I have a set of relatively new valve springs on my old NC29 head, and will be using NC29 cams with the NC23 head on a 29 bottom end, so issues with the cam sprockets being different doesn't apply.

The pile of rebuild bits is getting bigger. Fingers crossed i'll get this puppy together on the dyno around Chrissie.

How about fitting NC29 head to NC23 bottom ? Is it possible ?

Not sure, There would be no advantage doing it, the NC23 has a better head then the NC29 - it flows better from standard.

stupid question but why not just port the 29 head?surely thats gonna flow better than standard 23 head.i know the casting are rough as a ducks arse on the 29 but cant help think if its off then why not just have a go and lap the valves in?

You could just port the NC29 head, but...

The NC23 head flows better because the ports are actually smaller in diameter - particularly on the exhaust side - creating higher velocity gas flow into and out of the engine. Opening up the ports can be the wrong thing to do, back in the day tuners would just make the hole as big as possible, but that can actually slow the air down so much you get less air rather than more (volumetric inefficiency). Now they attach a digital flow meter and test the air flow at different velocity's (engine speeds) to create the best compromise across the engines rev range for your use (or at least the tuners who can afford digital flow benches do...). They can even measure 'dead spots' in your cylinder where the air movement is relatively still or exhaust isn't scavenged from etc.

From what i've seen and experienced with the NC29 and 23 heads, the 23 is a better starting point, then you sharpen (but don't knife edge) the inlet port divider, and straighten it slightly, making the diameter (and hence volume) more consistent from the inlet port (carb) to the divider. The exhaust port just gets a bit of a polish and is left small.

Having said that i've never seen inside one of the high horsepower english motors. The best 29 here would probably be Dans and be edging 70?