NORTONATLAS
Subscribe and save:www.Real-Classic.co.uk/subs AUGUST 2019 I73
carburettor,ratherthanthetwoondad’s 650.
Published powerfigureswere roughly the
sameforboth 650 and 750, but thesevaried
depending on whereyoufind them and may
be misleading.Theyrange from 46bhpto
60bhpat 65 00rpmforthe 750, buttheAtlas
enginewastunedforstronger mid-range
muscle than the 650.
Themachine seen herecametomy
workshop withreports ofexcessiverattling
and vibration.Myfirstthoughtwas thatthere
mightbenothing wrong with it.Don’t they
all do that, sir?Theearlystages ofatestride
appearedto confirmthattheremaynot be
much wrong,although the blurredvision
Iexperiencedatlowspeeds suggested
something untoward.But afterafew miles
when the engine and oil becamewarmer,
Ibegantowonder ifwe’d make it home
without mechanical incident. Therewas
something inside the engine saying itwanted
to come out...
Intotheworkshop and out with the
spanners,then.Irarelyworkon big Norton
twins; perhaps duetogeographyorperhaps
therearefewerofthemaround?Perhaps
they aremorereliable and longer lasting
than other makes? Discuss.ThisAtlaswas
supposedlyalowmileage original,showing
about 22,000 miles on the Chronometric
speedometer,which shared spacewitharev
counterofthe samekind.Nice.It soon became evidentthatmanyother
British motorcycles areeasiertoworkonthan
theAtlas,especially when stripping thetop
end of the engine and thenextracting the
engine with thetopendalreadyremoved.
Infact, fortheclosing stages of therebuild I
made sureIfitted thecylinder head beforeI
replaced the engine intothe frame,inorder
toavoidapotential nightmareintryingtofit
the head with the engine in the frame.But
beforethen...
Thestripdownrevealed thatthe displayed
mileagewasprobably incorrectbyasmidgen,
judgingbythe huge gaps in the standard
pistonringsand theenormous playinthe big
ends,whose journalswerealreadyground
downto-0.020”.Therewasdefinitely more
than 22,000 milesworthofwear on the bores,
although these and the pistonswereOKfor
manymorethousands of miles.Deglazing,
cleaning,new pistonrings and circlipswere
all thatwererequired here, then.Allvalves
and guideswere replaced but the crank had
tobegroundto-0.040”,asthewearwastoo
badtoclean outat-0.030”!With the crank
reground and new main bearingsatthe
ready,itwas timetorebuild,but the camshaft
lookedabit tiredsoanewonewasfitted.
Everythingwaschecked,eventheoil
pumpwasdismantled andfoundtobe OK.I
hadabit of fun getting therightinletvalves
forthehead,and oilscraperringsfor the
GPMpistonsseemedtobe elusive. These
runinshallowergrooves in these pistons
than some othertypes and the three-piece
oilrings, whichcame with the new set of
rings, wouldn’t close down enoughtoenterOnefettledAtlas,revellingin its newsidestand
In caseyoueverwondered, this iswhere the framenumber is on featherbedNortons.Weaimtobehelpful