Atlas Header Pipes

Hello Alan - The part of the pipe that exits from the cylinder head is usually 1 5/8 inches and the section where the silencer afixes is also 1 5/8 inches.  Some exhaust pipes were 1 3/8 inches for the remaining pipe length but some of the last Atlas's had all the pipe at 1 5/8 inches.  Some of the after-market offerings may have been 1 1/2 inch (especially the Cafe Racer swept-back pipes) but usually the silencers were 1 5/8 inches.   The 1 3/8 inch pipe came in when Nortons introduced the semi-downdraught cylinder heads for the 650 Models.  They claimed it performed better but quite frankly you will see if you try to adjust the position by bending them it is far easier with the narrow pipe.  It is also easier to get the steeper exhaust exit angle with a more malleable small pipe.  They are also easier to get them tucked-in under the engine, gearbox and primary cover.  Unsurprisingly the very fast Dunstall Atlas's used the larger swept -back pipes to get rid of the gases faster.   Good Luck, Howard

The 66-67-68 master parts list contains both large bore (p/n 23432+23433) and small bore (p/n 24020+24021). As the large bore parts number is lower, they are probably earlier.

Our 67 Atlas came to us with old Big bore pipes that appear original. They are difficult to work with and are so tight in the head nuts .They do not allow any "adjustment". Would not recommend them at all.

The introduction of the SS cylinder head, on most of the big post-1960 twin engines, messed up the run of the exhaust pipes down to the silencers. Causing them to stand away from the engine which in turn caused problems with fouling by the brake and kickstart levers and also the rev counter drive cable. The solution on the Manxman 650 and Mark 1 Atlas was to reduce the pipe diameter, after exiting the cylinder head, and also introduce compound bends into each pipe in order to bring the run back closer to the engine. This tucked the nearside pipe almost under the primary case and the off-side pipe under the timing cover. Unfortunately, manufacturing these pipes was time consuming and expensive and they were quickly superseded by waisted pipes minus the extra bends. Meanwhile the 'after-market' pipe makers were profitting out of selling replacement pipes with a uniform 1 5/8" diameter for a fraction of the price compared to the Norton items. Cranked kick-starts and a bend in the brake lever sorting out any conflicts with the pipe run and those parts.

Phil's knowledge is incredible!  I just pulled Dunstall's tuning book off the shelf, and he recommends the 1+3/8 for road use (to keep gas velocity up), and bigger bore pipes for more extreme race cams.  The VintageNorton site describes tuning the 16H, and the three big items are improving squish (with compression ratio and gas flow in the head), using a bigger carb and valves, and fitting smaller diameter exhaust pipes.  Big may look better but isn't necessarily quicker, and it looks as if smaller is certainly not inferior.  And from the above messages, it looks like both can equally well be described as 'original'.