Norton 750 Atlas Cylinder Barrel & Pistons

Whist they may not have been manufactured for very long they were designed to keep the compression ratio down in an attempt to reduce the vibes everyone knew woukld occur with the new 750 motor. However in those days the majority of owners changed them for high comp pistons so as to go faster (AND increase the vibes) so I bet there are a few of these pistons layig around with very little wear on them in garages all over the free World!!
 
There is that story that won't go away how the man contracted to do the crank balancing for AMC didn't do such a good job.
And yet some engines were said to be quite smooth, as are some that have had a (modern) dynamic balance.
Evidently, not all parallel twins are created equal....
 
Rohan said:
There is that story that won't go away how the man contracted to do the crank balancing for AMC didn't do such a good job.
And yet some engines were said to be quite smooth, as are some that have had a (modern) dynamic balance.
Evidently, not all parallel twins are created equal....
Click to expand...

I have also found, after a recent magneto rebuild, that if you get the cylinders firing spot on 360 crank degrees, the vibes are considerably less. Perhaps some engines suffered from the crappy cam rings that were said to come from WW2 worn out machinery.

Slick
 
Crappy cam rings??? After Lucas stopped manufacture and a friend bought 'a few new' competition mags / armatures etc etc from Lucas new made only God knows where cam rings eventually 'appeared' and there was only one word to describe them...it begins in S and ends in T. They have appeared at auto jumbles etc for decades. Timing up on one cyliider meant the other was a few degrees out. One simply tried to equalise the error although one friend sometimes regrinds new cam rings for his customers to correct them but I bet he does not charge anything like what it should cost......He will never be rich...as I often remind him.....
Then I often wonder how many cranks have been taken apart and shoved back together with the flywheel the wrong way round so the bits removed to balance the crank are in the wrong place.....
Add to that is how the cranks were DESIGNED to go together. Due to a total lack of modern machinary Mr Hopwood had to employ a 3 piece crank design and for central alignment borrowed a much earlier Vauxhall or somones idea of the central dowell to align the shafts but to ensure that both big ends were exactly in line one of the two top bolts was designed to be a tight fit in both the webs and flywheel (Mr Hopwood wrote to me on the subject a long long time ago). A few years ago a friend rebuilt my ex police MK2a crank.... when he put it up between centres one big end was 0.010inch out to the other....... Many years ago in a friends shop I took all the crank fixing kits out of stock and ran a micrometer over all the fixings...all were undersize. Are things any better these days? Do you checxk such things or simply make the serious error of assuming they must be correct? My Mk2A crank was reassembled and with everything CORRECTLY aligned several fixings were reamed and oversixe fixings employed.....The friend often does this for his customers....along with removing the stress raiser inside the drive side crank half.....
Add to all this main bearing internal clearances about which so few apparently know ANYTHING...... I have seen mains removed from Dommy etc motors which were 4 dot (C4) beaings which had an internal clearance of, when new ,between 0.0009 amd 0.0016 inch but according to the olde Ransom and Marles drawing for the original so called superblend beaing (6MRJA30) they were, if memory is correct, 2 dot (C2) internal clearance which was between 0.0000 and 0.00042 inch. The new 6MRJA30 bearings I once had a few of when I measured them had internal clearances making them CN fit bearings with 0.0002 to 0.0008 internal clearance. Oe friend assembles engines with C2 fit bearings and feels that this reduces vibes considearably as the cranks no longer bounce up and down in the bearings BUT his cranks will be correctly aligned to start with......ands he has to heat up the cases a touch before the crank rotates easily...
Over the deades I have heard of 'magic' factors to use of anything from 65% to 80%...but have NEVEr ridden a solid mounted engine Norton twin that does not vibrate badly somewhere in the rev range and all vibrated at the 6-7 thousand rpm I used much of the time in my youth some more than others ........
 
[quote="texasSlick] Perhaps some engines suffered from the crappy cam rings that were said to come from WW2 worn out machinery.[/quote]

I'd doubt that K2F cam rings were made "on worn out WW2 machinery", since the K2F appeared after the war and nothing during the war used a K2F - the magdyno all having face type cams.

Didn't many/most Atlas's use the points ignition thingies ?

I'd also comment that most Lucas stuff must have been made to quite a reasonable standard, when you look at surviving Lucas bits - some of which are still giving good service even today. That's none too shabby, at all.....

The Atlas didn't have a reputation as a problem crank either, did it ?
(In fact it solved the problems with the earlier tuned 99's.)

But we diverge, slightly.
 
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