Allen’s back with THREE pages…

Classic Motorcycle Mechanics - - CONTENTS -

Iquite of­ten wake up at night with my mind buzzing in the mid­dle of work­ing out how to make a part, re­solve a prob­lem, or in the case of my RC374 en­gine, think­ing ‘what have I got my­self into this time?’ The en­gine I’m at­tempt­ing to make is based on a very frag­ile 1960s six-cylin­der race en­gine with a short and lim­ited life that was orig­i­nally made with space age ex­otic ma­te­ri­als and pre­ci­sion. These en­gines would barely run below 5000rpm but would top 17,000rpm at full chat and I’m try­ing to make a sim­i­lar en­gine from a cou­ple of old Yamaha en­gines in my shed with quite ba­sic tools and equip­ment. The hard­est and most com­pli­cated part of my en­gine was mak­ing the new pressed-up roller bear­ing crankshaft from scratch. I ex­plained the ini­tial stages last month; the next stage was to start mak­ing ac­tual parts. I had al­ready made a rough draw­ing based on the orig­i­nal Honda de­sign, not­ing down a few crit­i­cal di­men­sions re­quired for each part. I usu­ally make parts by feel from an im­age in my mind but this crankshaft was so com­pli­cated I found it vir­tu­ally im­pos­si­ble to vi­su­alise all the parts in de­tail while I was ma­chin­ing. To sim­plify the man­ual ma­chin­ing process I made a fix­ture that would en­sure the dis­tance be­tween the crankpin cen­tre line and main jour­nal was iden­ti­cal on all parts; this is crit­i­cal on a pressed-up crankshaft for it to run true once as­sem­bled. I set this di­men­sion to ex­actly 17mm (half the stroke of my en­gine) on my fix­ture us­ing a tool­mak­ers but­ton, slip gauges and a dial test in­di­ca­tor (DTI). This is an old pre CNC method of set­ting up parts where ac­cu­rate lo­ca­tion of cen­tres is re­quired.

The steel for the crankshaft webs was de­liv­ered in 12 pre-cut bil­lets; these were set up on my lathe in a four jaw chuck and ma­chined into six webs with in­te­gral big-end shafts and six webs with in­te­gral main shafts. About 80% of the ma­te­rial ended up as swarf! I then set up my ma­chin­ing fix­ture in the four jaw chuck with a DTI check­ing that the da­tum face and di­am­e­ter were run­ning true. The webs were then set up one at a time on the fix­ture, lo­cat­ing on the fin­ish ma­chined main jour­nal to ma­chine the big-end bore. By us­ing a re­duc­ing sleeve in the fix­ture bore the other six webs were set up lo­cat­ing on the big-end shaft to ma­chine the main shaft bore. The ma­chin­ing of the bores to match the di­am­e­ter of cor­re­spond­ing shafts was crit­i­cal to en­sure the cor­rect in­ter­fer­ence fit when the crankshaft was as­sem­bled. My ear­lier tri­als with the sin­gle test crankshaft had helped me es­tab­lish the di­men­sions of the counter bal­ance so these sizes were repli­cated on five pairs of webs dur­ing ma­chin­ing, but one web was un­der­weight due to ad­di­tional ma­chin­ing for the gear drive. This was re­solved by drilling two 12mm holes through the web that is next to the geared web and press­ing in two slugs of Tung­sten. Tung­sten metal is one of the heav­i­est el­e­ments, weigh­ing in at al­most 2.5 times heav­ier than steel and it was just enough to bring the web back into bal­ance. The last ma­chin­ing job on the webs was to drill oil feed holes in or­der to feed oil to the big-ends. The oil flow to the main bear­ings could be routed to the big-ends by drilling a small hole through the main jour­nal across the web into the big-end jour­nal. This was car­ried out on my drilling ma­chine with a sim­ple an­gle fix­ture to se­cure the webs in the cor­rect po­si­tion so that the hole would exit in the cor­rect place. All sharp edges were then re­moved and smoothed to a ra­dius with a fine file and emery cloth fol­lowed by a pol­ish on my buff­ing wheel. The webs were then cleaned with carb cleaner and an air­line to re­move all traces of buff­ing soap, swarf and metal filings. With all the parts ma­chined I was ready to as­sem­ble the crankshaft. I had been think­ing it through for sev­eral days prior, to work out in my mind the best as­sem­bly se­quence, and de­cided to start from the cen­tre and work out­wards. The cen­tre sec­tion was a straight shaft, the camshaft drive sprocket was pressed on to it and lo­cated in the cen­tre, then two caged nee­dle roller bear­ings com­plete with hard­ened in­ner rings were pressed onto the shaft, fol­lowed by the first two webs, one each side us­ing my hy­draulic press.

The first two webs were easy to as­sem­ble be­cause they were in-line for pis­tons three and four to rise and fall to­gether, this al­lowed me to lay them flat on a block of steel and tap the webs into per­fect align­ment with a cop­per mal­let. These two webs were then drilled and pinned to the cen­tral shaft. At each stage of as­sem­bly I checked that the grow­ing crankshaft fit­ted into the crankcases and would ro­tate freely, fet­tling the crankcases to re­move any high spots as re­quired. The next stage was to as­sem­ble the webs for cylin­ders two and five. These had to be aligned at 120º rel­a­tive to the cen­tral webs and in-line with each other. I mounted the part-com­pleted crankshaft in my lathe to set up the 120º an­gle with a de­gree disc, then press­ing the web on a lit­tle bit with the tail stock be­fore care­fully re­mov­ing the crankshaft for press­ing to­gether us­ing my hy­draulic press. The pro­cesses of set­ting up, press­ing, check­ing align­ment and pin­ning were re­peated un­til the whole crankshaft was as­sem­bled. Af­ter fi­nal checks for run-out the crankshaft was placed into the bot­tom crank­case and the up­per crank­case tight­ened down. Six bare pis­tons were at­tached to the con­nect­ing rods then the base gas­ket and bar­rels were low­ered down and tight­ened. I care­fully turned the crankshaft slowly with a span­ner and was pleased to see all six pis­tons ris­ing and fall­ing in pairs. I then set up a pair of DTIS both set to zero when a pair of pis­tons were at top dead cen­tre (TDC). When the crankshaft was ro­tated each pair of pis­tons rose and fell in uni­son reach­ing TDC at the same time in­di­cated by si­mul­ta­ne­ous zero read­ings on both DTI. I then mounted a de­gree disc on the end of the crankshaft and ro­tated the crankshaft to check that the three pairs of pis­tons were spaced 120º. I was pleased to see per­fect align­ment so with the crankshaft com­plete I was ready to carry out the first com­plete wet as­sem­bly of the en­gine to check oil flow, leaks and com­pres­sion. I will tell you how that went next time!

Pri­mary drive webs.

Fin­ished and pol­ished com­po­nents.

Drilling holes for Tung­sten slugs.

Ma­chin­ing a bil­let of steel into a crankshaft web.

Fin­ished crankshaft in crankcases.

Crankshaft as­sem­bly, check­ing align­ment.

Fin­ished crankshaft in my hand.

Drilling hole for lo­ca­tion pin.

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