Useless assorted Norton musings.....

Re: 100+ horsepower Norton 750???

beng said:

worntorn said:

According to Mick Walker, as compared with the SS, the original Atlas came with a mild cam, low compression ratio and single carb. This was at Doug Heles direction because he knew the 750 size would create vibration problems. After Hele left Norton, the Atlas Compression ratios were gradually increased to that of the 650 and twin carbs were added. Eventually the Atlas also came with the SS cam too. Hele was correct, the vibration problem got out of hand.

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Mick Walker either never said any of that, or he was simply wrong.

The Atlas road and scramblers always had 7.5-7.6:1 compression pistons through it's production run, and it always had the same cam as the 650ss. The first year the US models came with a single 376 carb, then the next year they came out with a MkII Atlas with dual carbs. Norton did not go back to a flat-top piston and 650ss compression except for the Commando.

From 1962 onwards Norton used the same cam timing and profile for every street bike right through the end of the Commando production, just altered it for the points and tach drive the Commando had after a spell. The last standard cams were used up in 1961 on the standard model Dominators.

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From Mick Walkers' Dominator book under the heading "Even bigger"

"As we have seen ,the Berliners had been responsible for strectching the six-hundred twin to a six-fifty. Not long afterwards the Norton engine size was increased again, this time to a seven-fifty. When originally conceived by development engineer Doug Hele, the Atlas engine ran on the realtively low 7.6:1 compression ratio, with mild camshaft lift and a single Amal Monobloc carburettor. However,even though Hele strongly advised against it, the American iimporters demanded (and were eventually given) ever higher compression ratios and fiercer cam lifts, plus twin carbs. All of this, of course, gave greater performance and acceleration, but at the cost of increased vibration levels and mechanical stress"

I am sure Mick Walker was a nice guy, but he published a lot of books on a lot of different subjects, just like Roy Bacon, and neither they nor anyone else is the last word on what they cover.

Apparently Mick Walker even contradicts himself, for in his book on the Manx, which has a whole chapter dedicated to the Domiracer, he states "A cylinder head from the 650 Manxman was employed because it featured a steeper inlet port downdraught and wider splaying......"

You can probably read several different books and find several different descriptions of the Domiracer development and engine, and also the Norton 750 Atlas specifications. I always go with the facts first, factory literature, manuals and parts books and supplements. It is interesting to read magazines and books, but unless they have a first-hand quote by Doug Hele about the Domiracer, I would not take it as gospel. So in the absence of anything like that I will not say which came first.

As far as the Atlas goes, I have the 1962 Atlas owners manual supplement introducing the model, and it says that either single or twin carbs were available. It states the differences between the Atlas and the 650 Manxman, and it says nothing about the cam. I have the specs for the 1962 Atlas listed in a letter from Norton dated April 1962 which lists the cam timing;, at .013" checking clearance it was intake 50/74, exhaust 82/42, to be run with the same clearance as the Norton SS at .006&.008. I have personally dismantled early spigotted and late Norton Atlas engines from the later 60's that were on standard bore with their original pistons and I have never seen anything but the low compression dished pistons.

It is easy to find books and magazines and internet forums stating anything you can imagine about Norton motorcycle specifications, I would take them all with a grain of salt and stick to all the factory literature first. Another good source are period photos that are dated. For instance I just looked at a photo of an Atlas on display at a motorcycle show in Daytona Florida in March 1963. The bike has twin carbs, high bars and the large tank with the fuel cap on the right.

It seems like a lot of information on early Norton bikes is based on pre-production specs and ideas. You can find more than one article that says the Norton Manxman had lower compression than the 650ss but they have the same head, cylinder and pistons, even in the parts books. So in interviews there is a chance that Doug Hele or other development people would say certain things that would not make it into the production bikes. I have a photo of the prototype Manxman up on the Facebook page for Nortons that shows it with a pre-downdraught head that has been machined to hold the carbs at an angle, a real oddball setup...

Getting back to the 650 bikes, I like all Norton featherbed bikes, I just find the history and bikes of the original Norton Works more interesting than what AMC, etc. did with the marque.

Norton cared more, turning the bikes out with more expensive parts and processes in a variety of colors and participating in racing the Manx and Domi bikes, where after 1962 all the art went in the shit-can.

Hey! worntorn, I used to have a 82 Maico 490 moto-crosser back in the eighties when I was immortal, first year for the monoshock rear and last year for the chain primary drive if I remember right. Not as nice in the slow stuff as my cousin's YZ490 with the reed valves, but it would edge him out on the top-end at high revs like you would expect. Mine had an aftermarket alloy swingarm and no compression release, and was supposed to have been sort of an ex-team bike. I still know where it is, but a later owner screwed the frame up trying to weld a Honda shock into it or something....

I think it weighed about 230# and had 52hp? It was really something going 100mph. When you chopped the throttle at 80-90mph the front end on mine would shake as the front went down, incentive to keep it pinned.....

I'll definitely agree about the low compression dished pistons in the Atlas motor. I've personally dismantled several Atlas motors from 1962, 1966 and 1967 and I found that all that were on standard bore with their original pistons, did indeed have the low compression dished pistons. I've never seen an Atlas motor with flat top pistons except where rebuilt at a later point in time. And I've been around several others that belonged to friends, and seen the same pistons in those motors as well.

I had thought it was pretty well known that the low compression dished pistons were used as standard on the Atlas since the factory hoped that it would help reduce the vibration created with the larger displacement 750 motor.

I only speak from personal experience on motors that I've owned or personally seen.

I expect Mick Walker is correct.
The Haynes shop manual concurs with the information in his book.While a single compression ratio is listed for all years of 650ss, compression ratio for the Atlas is given as 7.5:1, then beside in brackets (various).

As Mick Walker explained, the factory wasnt fully in charge, particularly after Doug Hele left. The increase to 745cc was done at the insistance of Berliners, as were the other changes to twin carbs, ss cam and increases to comp over the years.

Glen

worntorn said:

I expect Mick Walker is correct.Glen

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Why? That is what you came up with, the word "various" in a Haynes manual? Haynes that puts out manuals on a hundred thousand different vehicles for back-yard mechanics is the last word for you?

I find it amazing that you have no wish to look for actual information. In addition to the 1962 Atlas literature, I also have the Norton parts book for 1964 and the 66 through 68 parts book, all of which only lists one piston at a time for all Atlas road and scrambler bikes with no optional compression ratios.

Some comments in a few non-Norton publications don't mean shit and it is ridiculous to argue that they are facts or the last word on anything. The Atlas was a common and popular bike in the 1960's and I am betting there is a lot more factory literature that I am going to be able to come up with from old dealers and other sources to back up that pre-Commando 750 Nortons never got high compression.

I WOULD be thrilled to find real information showing flat-top pistons in those old 750's but I would not put a dime on that bet.

The Norton piston listed in the parts books for 1966-68 for the Atlas road bikes and Scramblers is part# 25389/25390 RH and LH, and it is 7.5:1. Strangely the earlier Atlas piston is listed at 7.6:1 compression, so the later Atlas had even lower compression than the later ones by a hair, which is the exact opposite of WornTorn's weakly supported assertions.

Later Norton Dominators did go with the 30mm concentric after the monobloc was axed from the production lines of every British bike marque. So the later Dominators may have had more top-end with that larger carb. But I am sure that it was not a carefully executed plan to increase the performance of the bikes, but more likely that the only choices they had for carburetors at that point was between the 930 or 926 Concentric, and it was a cheap way to go to buy and bolt the same carb onto everything they had going out their doors.

beng said:

The Norton piston listed in the parts books for 1966-68 for the Atlas road bikes and Scramblers is part# 25389/25390 RH and LH, and it is 7.5:1. Strangely the earlier Atlas piston is listed at 7.6:1 compression,

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Maybe they measured it again in 1966 and got a different answer. 0.1 of a ratio isn't much combustion chamber volume.

That is as good a guess as any.
AMC did get rid of the cylinders with the spigot on top along with it's matching recess into the cylinder head for the 1966 model year, which is a lot of work in the parts that can affect Compression ratio.

Also at one point in the mid-sixties Norton changed to a new type of piston ring called something like "Duoflex" . At one point they put holes in the rods to oil the cylinders because they were having pistons seize, then they doubled the speed of the oil pump and had oil control problems! So then they tried to fix that with different rings.
Heinz Kegler turned his bearings shells around to block the oil squirting out of the rods onto the cylinder wall. In Dunstall's famous tuning book, he used the old low capacity pumps from the 50's with the double-speed gears and larger scavenging gears machined in which he said gave the engine enough oil to race with, and kept the sump cleaner.

So there were some strange things going on with the Atlas while AMC was turning it out. Because I was lucky enough to get onto a 650 Norton first, which had enough vibration already, I never wanted an Atlas, especially knowing that they had no more power and vibrated more, who would want one? But that can all be fixed now, that is if you have the $1300 to spend on long Carrillo rods and light pistons like Jim Schmidt sells.

Maybe "wilkey113" can chime in with the specs of his Atlas which he enjoys riding which I think has standard rods. Before Jim Schmidt became busy promoting his long rods and matching pistons, he had a few suggestions for making the standard Atlas comfortable, like gearing it up so it had low revs on the highway, drilling holes in the pistons for weight, and keeping the balance factor low on a bike used for touring and using a high balance factor on high-rpm race bikes.

Anyway it was a lot of fiddling around that I thought I would stay away from, sticking with the 500-650cc Dominators which had pistons as light as Jim Schmidts right from the factory. They might be down on power from the larger engine, but they still have enough power to do the ton and go with traffic, which is all you need for a 50 year old bike if you are not racing.

If I had an Atlas now, I think the long rods and light pistons would be the best thing for it and would make it into a really great bike.

My 1966 Atlas still still has it's original rods. I've owned the bike for several years and bought it as nothing but a roller and then built it back up into a complete motorcycle. Up until last year, when I rebuilt the top end, the motor was 100% stock original. Including the original low compression dished pistons. The only thing I replaced was the pistons (NOS Hepolite standard size), rings, valves and valve guides. Everything else was inspected and serviced and then reassembled since it was found to still be in great condition. I prefer to keep them as original as possible. It's a motorcycle that I ride and enjoy nearly everyday (weather permitting), and for a road machine, I think that it runs just fine. If money were no object, then maybe I would have put the JS lightweight rods and pistons in it.

I own a few different versions of the Haynes manual covering the Dominator and Atlas. I don't know what the term "various" would refer to in the Haynes manual, so I won't speculate what that could mean.

I'd love to hear from anyone that's personally seen an Atlas with standard bore and it's original pistons that's found anything other than the low compression dished pistons. I'm not saying that it does or doesn't exist, I'm just saying that I've never seen or heard of an original Atlas that was built with anything other than the low compression dished piston.

Beng, for now Ill go with the info in the books I have here. What percentage of the total Atlas production do you think you have inpected? Maybe .001%?
Of this tiny sample, did you actually measure the compression ratio of any? More likely you saw a dished piston and assumed the number you have scorched into your brain.
Walker is saying the compression ratio was gradually increased, not suddenly changed to 8.9 or 9 to one. If he is correct, the dish would be a bit less in later bikes. The Haynes manual seems to agree.
Obviously I cant say for absolute certain he is correct, nor can you be at all certain he is incorrect.

Steve Wilson, in his book "Norton Motorcycles" also mentions the increase in compression for the later Atlas, so that is now three publications on Nortons which all refer to the increase. Might there be something to it?

Your knowledge is likely not as great as you feel it is, otherwise why the need to turn into this insulting blowhard on thread after thread?

Im going to self moderate here
Done

worntorn said:

Walker is saying the compression ratio was gradually increased, not suddenly changed to 8.9 or 9 to one. If he is correct, the dish would be a bit less in later bikes. The Haynes manual seems to agree.

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This is great. Now out of thin air, besides having one dished 750 piston, we have a number of different ones. If the compression ratio gradually increased, I wonder when it happened? Because if the 1962 bikes were 7.6:1, and so according to the parts books so were the 1964 750s, and 7.5:1 for 1966,1967 and 1968 Atlas, then according to worntorn these are the "gradual" Atlas specs:

1962 - 7.6:1
1963 - ?
1964 - 7.6:1
1965 - back up this year to worntorns "gradually" higher pistons of what compression ratio?
1966 - 7.5:1 according to the parts book.
1967 - 7.5:1 "
1968 - 7.5:1 "

Or, Worntorn is asking us to believe that Norton sold high compression Atlas bikes out of the dealer showrooms, but then did not list or sell pistons for them.....yea that happened.

Another amusing thing, is that all of Roy Bacon's books agree that the early Atlas Road and Scrambler engines were 7.6:1 and all the later ones 7.5:1, but for some reason he is not as credible as Haynes or Mick Walker?

So except for some rambling by Mick Walker, and the amazing word "(various)" printed in a Haynes manual which Worntorn has somehow interpreted as backing up the existence of an entire range of factory Norton pistons that do not appear in any parts books, he does not have a single thing.

I never claimed to know anything, and have nothing "scorched in my brain". I am simply looking at five years of Norton Atlas factory literature that lists no compression higher than 7.6:1, and asking someone else to come up with anything else. I am suspecting when I get my hands on literature for the 1963 and 65' model years which I do not presently have, they will also show nothing different.

Worntorn, you can say that I am insulting you by wanting to wait for actual hard facts, but actually your assertion, if not outright fabrication of Norton Atlas history is insulting to yourself. What gaps in your knowledge of Norton motorcycles will you make up for us next? Guess we will never know since you are really done right? Bye bye.......

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 said:

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

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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 said:

Dances with Shrapnel said:

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

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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.

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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.