9 THINGS YOU NEED TO KNOW…

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Motorcycle News (UK) - - Garage -

MA­TE­RIAL

In terms of weight, cost and ease of pro­duc­tion, alu­minium is the ma­te­rial of choice, bar none. The alu­minium used is al­loyed with other ma­te­ri­als to en­hance its suit­abil­ity for use in such an ex­treme en­vi­ron­ment. Cop­per, nickel and mag­ne­sium are used in small amounts (less than 1%) to im­prove cast­ing and machin­abil­ity but the key al­loy­ing agent is sil­i­con. The sil­i­con con­tent may vary from as lit­tle 2% to around 20%. Higher sil­i­con con­tent re­duces the rate at which the pis­ton expands. This is im­por­tant in mod­ern pro­duc­tion en­gines which re­quire the pis­ton to run in its bore with min­i­mal clear­ance to re­duce emis­sions by bet­ter con­trol­ling oil con­sump­tion. A low sil­i­con con­tent al­loy will dis­si­pate heat more ef­fi­ciently and is less brit­tle and there­fore less likely to break in event of det­o­na­tion oc­cur­ring, mak­ing low sil­i­con al­loys the pre­ferred choice for rac­ing.

MAN­U­FAC­TUR­ING

Pis­tons can ei­ther be cast or forged. The ma­jor­ity of pro­duc­tion pis­tons are cast by pour­ing the moul­ten ma­te­rial into a mould. With forg­ing the ad­di­tion of huge pres­sure to the process squeezes the moul­ten metal pro­duc­ing a finer mi­crostruc­ture, en­hanc­ing its in­tegrity. Both types of pro­duc­tion re­quire fin­ish ma­chin­ing but cast­ing al­lows for more in­tri­cate shapes to be pro­duced, re­duc­ing the amount of ma­chine time re­quired. Con­se­quently forged pis­tons are sig­nif­i­cantly more ex­pen­sive to pro­duce.

SHAPE

Pis­tons are not the sim­ple cylin­dri­cal shape that they ap­pear to be at first glance. Most ta­per from top to bot­tom to al­low for the greater ex­pan­sion at the solid pis­ton crown and higher tem­per­a­tures in this area. Sim­i­larly, due to the pres­ence of ma­te­rial across the pis­ton around the gud­geon pin, they may not be round ei­ther, re­quir­ing an el­lip­ti­cal shape from cold to en­sure near per­fect round­ness and best pos­si­ble fit in the cylin­der at op­er­at­ing tem­per­a­ture.

CROWN

The top of the pis­ton, pro­filed to achieve op­ti­mum com­pres­sion com­bus­tion cham­ber shape, of­ten fea­tures re­cesses for valve clear­ance.

RING LANDS

Im­me­di­ately be­low the crown, ma­chined to ac­cept the com­pres­sion and oil con­trol rings and oil drain holes. This area of the pis­ton is crit­i­cal for good pis­ton-to­bore seal­ing.

GUD­GEON PIN

Con­nects the pis­ton to the small end of the con­rod. Usu­ally runs as a plain bear­ing in the pis­ton. Lives its life on the very edge of de­struc­tion.

SKIRT

The area be­low the ring lands, usu­ally cut away to the sides, ie ad­ja­cent to the pis­ton pin boss, to re­duce weight. The thrust faces to the front and rear pre­vent the pis­ton rock­ing in its bores and trans­fer heat to the cylin­der wall.

WEBS

Or ribs, re­in­force the pis­ton and help con­trol ex­pan­sion and shape.

LIM­ITS

Pis­ton speed is gen­er­ally the lim­it­ing fac­tor in the quest for more revs and power. All the cur­rent crop of top-end sports­bikes have a mean pis­ton speed of around 22 me­tres per sec­ond at peak revs. They will over­rev to around 25 me­tres per sec­ond – the ac­cepted safe limit for pro­duc­tion en­gines. More ex­treme pa­ram­e­ters can be set, but re­li­a­bil­ity and ser­vice in­ter­vals would not be suit­able for pro­duc­tion en­gines.

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