ClassicRacer 53
As we leftthe storyinthe last issue(at the end of the 1955 season)
BobMcIntyre had received offers to ride forboth MotoGuzziand
Gileraduringthe 1956 season.Whichofthe twonameswould he
sign for?Theanswer would be neither.Bob decidedto continueto
ride forJoe Pottsin1956.Words:YorkshireFerretImages:JoePotts ArchiveSS
owhy would Bobturn down these
works ridesand insteaddecide
to stay withJoe, workingat the
backofafuneralparlourin Bellshill,
Glassgow?Theanswer: Bobwas aman of his
worddand he had madeplanswith Joefor the
19566 season.Bob’sfriendshipwith Pottsand
hisbbelief in whattheycouldachieve
togeether meantthat he would continuewith
thesseplans.
During the 1955 seasonit wasapparent
that the JoePotts/Bob McIntyre teamwas
theoonly privateerable to competewith the
Worrks teams.With the reputationgainedby
bbootthhin 1955, Joewas able to start getting lots
of sparesout of the ‘backdoor’ofNorton’s.
This includedunmachinedcylinderheads,
crankcases, pistonsetc. Joehad effectively
takenoff and movedonfromwhere the
Works teamhad beenoperating. Norton
recognisedthis and as well as providingthe
aforementionedparts,it wasalso supplying
drawingsof partsand enginesfromthe Works
team.With the unmachinedcrankcases
Joewould nowbeable to start creatingthe
highlydevelopedspecialPottsenginesthat
were stampedwith JP enginenumbers.
Hugeamountsof sponsorshipmoneystarted
floodingin to the team,the largestportion
comingfrom Shell.
So whatwere thenew plansbetween Bob
and Joe? It wasPotts Manxmachines,but
mostinterestinglyis that therewould be an
additional machine to the stable;the Potts
250cc.The Potts250 startedlifeasa
short-stroke350ccManxcrankcase,cambox,
barrel and unmachinedcylinderhead. Oneinterestingaspectof the Potts250 is that it
wasbasedon adrawingprovidedto Joeby
Norton,of asimilarengineto the one JoeCraig
had envisaged.Thebore and stroke dimensions
forthe Potts250 waschosento be 70mmby
64mm.Thenew crankshaftassemblywas
machinedin the workshop at NorthRoad to
allowfor thereduction in engine stroke.The
newcrankwas aone-pieceunit that wasfitted
with an outsideflywheelto allowthe use of
an ultra shortconnectingrod.Theconnecting
rod wasmachinedby Joe and wasmade from
aluminiumand it featured asplit big-endeye,
so aplain shell-bearing couldbe used.The
white metal bearingcame froman Austin A35,
somethingthat Joewould have beenable to
get readily,ashewas an Austin car dealer.
Theplain bearing big-end requiredhigher
oil pressureforlubrication than aconventional
rollerbig-end. In ordertoincrease oilpressure
to 60lb/sq in, asynthetic rubber oil seal was
fitted to wherethe big-end feed jet enteredthe
crankshaft.Tosuit the reductionin bore,Joe
machined araw Nortoncylinderhead casting.
Thehead wasfitted withaslightlysmaller
inlet valveand an increaseddowndraftfor
thecarburettor.
Thepistonusedforthe 250 wasablank
forging from along-stroke350ccManx, which
againwas machined at Bellshill.Thecylinder
barrel wassleevedto70mmand reducedin
length, so it had only threefins.The250 had
special cams,whichJoe machinedto his own
specification.Manychanges would be madeto
theignitionsystem of the 250, whichincluded
twin-spark(for better spread of power) andcoil
ignition(so the enginecouldrunathigherrpm).052 PottsMcIntyrePart two_SEP.indd 53 31/07/201709:35:03