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This is an old 18"x24" Berliner service poster I received from an old Norton dealer in my area. It is from 1968, the first year of the Commando, and last for the Atlas. Berliner was the Norton Distributor for the 27 eastern United States from late 1960 onwards.....

Note how not only is the compression ratio on the Atlas models much lower than the Commando, and lower than the 1962 spec of 7.6:1, that the ignition advance is also different between the Commando and Atlas to suit the different compression ratios....

according to the Norton '63 to '68 parts book, all 750's between those years share the same pistons, crank, con rods, cylinders and cylinder heads, so Triton Thrasher is most likely accurate in his proposal that they measured c.r. again and got a different result, or maybe someone just 'fat fingered' a keyboard/ pencil/ whatever they were using then.

Well the early Dominators including the Atlas through 1965 had the cylinder and head with the spigot, and afterwards the cylinder and head did not. After Norton quit making the heads with spigots in them, in the parts book they offered rings to press into the recess in the older cylinder heads of 500cc-750cc Dominators so they could be used with later non-spigotted cylinders.

So in the later 60's if the owner of an early Dominator ruined his top-end and it had to be fixed with current parts, then it would be up to the dealer or repairman to figure out what was useable, and what parts had to be ordered to make the new parts work with the old style. If someone had a cylinder with spigots and could not source a recessed cylinder head, then they might have to either get a new-style cylinder or have a machine shop knock the spigot of the top of theirs so it would work with the new style head.

All pretty much common sense.

Interesting stuff.

In line with the different ignition advance to suit the different compression ratios, it stands to reason (although speculative on my part) that maybe Norton thought they could squeeze a bit more performance out of their engines with the higher compression ratio in the Commandos since they had an isolastic system in place.

Right, the 650ss had about 9:1 compression ratio. When the Atlas 750 came out they put in the low compression. Why did they go to low compression? Norton raved about the new engines flexibility in advertising and of course said nothing about the bike having low compression to combat vibration, but it must have helped with the problem.

When the Commando chassis came out and solved some of the vibration problem, then Norton went back to the 650ss compression levels. So the low compression may have been a band-aid for vibration just like the Commando chassis was.

I've never thought low compression could be much help against serious high rpm vibration, except by making you go slower.

Primary vibrations comes from throwing pistons up and down.

Yes, low compression can contribute to a smoother engine.

From classical physics, Force = mass X acceleration . With a higher compression both the power stroke and compression strokes experience greater acceleration/deceleration.

Keep in mind that Norton went more or less balls to the walls with the balance factor on the Commandos since it had the isolastics. My gut feel is that an approximate 50% balance factor is the best mechanical trade off for the engine. With a solid connection as that on the Featherbeds, the manufacturer had to compromise by raising the balance factor for operator comfort. It's a logical conclusion.

The hi octane becoming available at gas stations in that era also had a part in Norton upping the CR. As all the solid mounted vertical twin use BF rather higher than 50% I hold that Norton picked the BF that used the least costly metal in cranks yet could still be isolated by the amount of rubber area supporting the mass. In other words likely a higher BF would be better for over all road performance and nicer sooner isolation til red line. i have a crank set up to experiment with this. My real interest in BF ain't isolation which Peel already had a disappearing act set up, but what it does to the tire traction va power pulses through the isolastics inline with drive thrust. Its like a second horizontal suspension system to me.

Dances with Shrapnel wrote:
From classical physics, Force = mass X acceleration . With a higher compression both the power stroke and compression strokes experience greater acceleration/deceleration.
.

I'd say that, at a constant 5000 rpm, differences in primary vibration, due to different compression ratios, are trivial.

Power pulses are stronger with higher compression and they cause one sort of vibration, but I wouldn't say it's very serious or destructive.

Triton Thrasher wrote:

Dances with Shrapnel wrote:
From classical physics, Force = mass X acceleration . With a higher compression both the power stroke and compression strokes experience greater acceleration/deceleration.
.

I'd say that, at a constant 5000 rpm, differences in primary vibration, due to different compression ratios, are trivial.

Power pulses are stronger with higher compression and they cause one sort of vibration, but I wouldn't say it's very serious or destructive.

Never said anything about vibrations as a result of higher compression being serious or destructive. I believe this was an opportunity for Norton to up the performance (mid range torque) through higher compression since the new isolastic system "masked" the nuisance vibration.

Although it may vary, the increased vibration due to higher compression is throughout the rpm range but agree that other factors become more significant as the rpm increases. This is all a compromise in design. I doubt Norton targeted 5,000 rpm or any other specific rpm as "the" vibration tuning point but rather looked at the whole spectrum. At the time it may have been done more by the seat of the pants - does it feel more comfortable or less comfortable. I just don't know.

hobot wrote:As all the solid mounted vertical twin use BF rather higher than 50% I hold that Norton picked the BF that used the least costly metal in cranks yet could still be isolated by the amount of rubber area supporting the mass.

Care to explain this? Are you suggesting Norton changed the balance factor when transitioning to the Commandos to save material costs?

You know we are only talking about maybe a few ounces of cast iron flywheel material. Any cost offset as a result of reducing the flywheel bob weight mass would certainly be more than off set by the energy and other costs of making that adjustment (machining, grinding and/or drilling).

Well I was done with this thread then my friend Tony returned my Norton Dominator Master Parts list, 63- 68. It is split into two separate sections, 63-65 and 66-68. The left and right pistons listed for the 63-65 Atlas are #24246 and #24247.

The 66 to 68 Atlas has a different piston, left and right for it are the 25389 and 25390.
I confirmed with Les Emery of Norvil that these latter pistons are flat top, 8.9- 9 to one compression and that the later Atlas bikes had them from factory.The earlier pistons are the dished 7.5 to 7.6.
Authors Mick Walker and Steve Wilson are correct in their statements that the later Atlas had higher compression.

The later pistons are those that are famous for losing the heads. As to why someone might find a later Atlas on standard bore fitted with the dished pistons, Les suggested the pistons may have been changed out by owners or dealers looking to reduce the vibes of the high comp setup. A change from high back to low could also have been year or two down the road to avoid the piston top problem, which was becoming quite well known.

He also told me that much of the problem with vibration in the Atlas came from the balance factor. It was done piece work by a Brit who did not care about his work. This fellow would drill a few token holes in the flywheels then send them back to Nortons with a bill for balancing. Les has found Atlas cranks ranging anywhere fro 40% balance all the way to 92%, all done by the same very low cost subcontractor!
He said the correct number is 84% which is what he uses for all new Atlas builds. At this number he tells me they are quite smooth even at 9 to one.

I don't know what Bengs ad is other than there is no featherbed Atlas shown on the part that is visible, but there is a Commando S , which is a model that first appeared in 1969.

While we're talking about vibration; when I first put my Triton on the road, I really did lose fillings from my teeth.

It occurred to me that the fellow driiling all those inexpensive but haphazard holes in the Norton flywheels might have secretly been on the Triumph payroll.

Triumph had the vibes too, didnt they?

Glen

It occurred to me that WornTorn put up three pages of a Norton parts book with no compression ratio specifications printed in them. Les Emery's opinion is just that, an opinion, not a factory document, which again you have failed to come up with to support your Opinion.

Maybe you simply do not understand the difference between opinion and actual documentation?

Norton piston part#s 25389 & 25390 are for 7.5:1 pistons, and you will not find any real documentation otherwise because there is none.

Anyone can find opinions saying anything at all, and we are simply not interested in them, get it?