ClassicMotorcycleMechanicsJune2020

cmm
Allen MMillyardd
Award-winning mootorcycleengineer!

Using G-clamps to partially engage web on to main journal.

Trial fitofpart-built crankshaft into upper crankcase.

Pressing on web after setting position

on milling machine.

Set up on milling machine to align crank-pin five with crank-

pin two.

Onceall partswere strippedIcleaned

off th etarnish and hardened oil stains

with afine wire brushand brak ecleaner,

theninspected all the parts for wear or

damage. TheZ1crankshaft has

substantial,purpo se-made roller bearings,

and overyears of use they have proved to

be extremely reliableandrarelyneed

attention. My two donor crankshafts had

no signsofc ondensation damage or wear

and theroller bearing tracks stillhad the

original cross hatched lapping marks on

the bearing surfaces, indicatingagentle

lifewithgoodoil. Looking at all theparts,

Icould seethatthe central four setsof

websand outer leftpair could be made

with unmodified original parts,but the

websfor the right-handside wouldneed

somemodificationto maintain alignment

with main bearing journalsand balance

as asix-cylinderengine, whichIwill

explainlater on.

Ihad decidedto configuremynew

crankshaftsot hatthe pistons will rise and

fallinp airs at 120-degree intervals, giving

a1-4-2-6-3-5 firing orderfor smooth

running. The first thingIdid was to

mountthecentral part of thecrankshaft

with the centralconnectingrods three and

four on to my lathe ina3-jawchuck. Then

Iset connecting rod fourontoapin held

in the tool post to prevent thecrankshaft

from rotatingwiththe bi gend at the six

o’clock position .Ithen setupt he degree

disk andset apointertozero. Ithen

carefully traversed thesaddle backto

disengaging the location pin from

connecting rod four and engaging it with

connecting rod five, which wouldlocate

the web in the correctrotational

orientation on the mainjournal with slight

pressurefrom thelathe tailstock

temporarily holdingitinplace.

The central portion of crankshaft was

then rotated 120 degrees, while the new

web was held stillby connecti ng rod five

that was attached to the pinlocated inthe

tool post. Alighttap with ahammer

engaged theweb sufficiently on to the

main shaft so thatIcould transfer the

part-built crankshaft to my press to press

the new web fully home.Ireturned the

part-builtcrankshaftto

my lathe and

re-checked thatrotational alignment

between crank-pin four and five was still at

120 degr ees and nothing had slipped.

The next job was to align crank-pin five

with crank-pin two, which had to be

aligned togethersot hatpistons twoand

five wouldrise and fall togetherat120

degrees to pistons three and four.

To do thisIused aset of slip gauges

and my flatcastiron milling machine table

to se tupthe crankshaft with two of its

main bearings resting on identical size slip

gaugessot hatthe webs were freeto

rotate, then clamped crank-pin five on to a

two-inchslip gauge and lightly clamped it

in place.Ithenappliedlight pressureto

the crankshaft to hold it inplace while I

set upcrank-pin two by resting it on an

identical two-inchslipgauge and lightly

clamping it inplace. With crank-pin two

held in thecorrect alignment with

crank-pin five,Iused acouple of G-clamps

to carefully engagethe webon to th emain

shaft sufficiently tohold it inplace while I

transferred the crankshafttomypress.

Once pressed together,Ireturned the

crankshaft to the milling machine and

re-checked withthe slip gauges as

detailed above to ensurecrank-pins two

and fivewere aligned perfectly before

movingon to the next stage.cmm

54 /classicmotorcyclemechanics