IN THE GROOVE

The sci­ence be­hind piston ring grooves

Chevy High Performance - - Contents - TEXT: Mike Magda | PHO­TOS: Evan Perkins

The sci­ence be­hind piston ring grooves

Piston ring grooves serve a larger pur­pose than sim­ply sup­port­ing the rings. They im­pact com­bus­tion seal­ing, oil con­trol, fric­tion, and many more en­gine at­tributes.

Wiseco is a world leader in piston tech­nol­ogy be­cause it fo­cuses on the minute de­tails of cylin­der seal­ing sci­ence. Among those de­tails, the type of piston rings used in rac­ing and high-per­for­mance en­gines are al­ways a hot topic. But the ring grooves in pis­tons also play a ma­jor role in seal­ing com­bus­tion pres­sure and con­trol­ling oil and blow-by.

That’s one area where Wiseco ex­tends the ex­tra en­gi­neer­ing ef­fort to en­sure max­i­mum per­for­mance from its pis­tons. While var­i­ous types of piston rings are more suit­able to spe­cific ap­pli­ca­tions, the ring grooves them­selves are of­ten over­looked in the pur­suit of op­ti­mum cylin­der seal­ing.

Many en­gine builders ex­pend a lot of ef­fort to file pre­ci­sion end gaps, but ring clear­ance in the groove fre­quently goes unchecked and the qual­ity of the ring groove seal­ing sur­face is rarely con­sid­ered. Clear­ance and ring groove qual­ity are fre­quently as­sumed. Wiseco goes to great lengths to make sure this is right, even if you don’t check it.

Decades ago, rac­ers like Bill “Grumpy” Jenk­ins used to pur­chase un­fin­ished pis­tons and cut their own ring grooves ex­actly where they wanted them and to the fin­ish and tol­er­ance they de­sired. To­day, that’s no longer nec­es­sary as man­u­fac­tur­ers pay con­sid­er­able at­ten­tion to ring groove de­sign.

The in­ter­face be­tween the ring face and the cylin­der wall is still widely dis­cussed among rac­ers and en­gine builders, but com­bus­tion seal­ing in the ring groove is equally im­por­tant. For the best re­sults, the ring groove must of­fer the ideal clear­ance and free­dom of move­ment for the ring and it must pro­vide a hard, flat seal­ing sur­face for the ring to bear against un­der high cylin­der pres­sure.

RING GROOVE NOMENCLATURE

Ax­ial Clear­ance: The ver­ti­cal clear­ance re­main­ing in the piston ring groove af­ter the ring is in­stalled. De­pend­ing on the ap­pli­ca­tion, the

ver­ti­cal clear­ance is typ­i­cally 0.0010.003-inch. With to­day’s thin rings, some rac­ers run ax­ial clear­ance as tight as 0.0004-0.0005-inch, us­ing gas ports to sup­ple­ment ring pres­sure.

Ra­dial Back Clear­ance: Ring groove space be­hind ring when the ring face is flush with the piston ring land. A back clear­ance of 0.008-0.012 inch is typ­i­cal for rac­ing and high­per­for­mance pis­tons.

Ra­dial Wall Thick­ness: Ring di­men­sion from the front face touch­ing the cylin­der wall to the back or in­side face of the ring. “D-wall” is the stan­dard au­to­mo­tive thick­ness (SAE stan­dard) and it’s cal­cu­lated by bore di­am­e­ter di­vided by 22. For ex­am­ple: 4.125-inch bore di­vided by 22 is 0.187-inch ra­dial wall thick­ness.

AX­IAL HEIGHT

The di­men­sion from one side face to the other, or ring thick­ness, usu­ally ex­pressed in frac­tional sizes like 1/16 inch, dec­i­mal sizes like 0.043 inch, or met­ric sizes like 1.5 mm. Smaller ring thick­ness means less mass and less in­er­tia to over­come when the ring re­v­erses di­rec­tion at TDC and BDC. A thin­ner ring also gen­er­ates less fric­tion but there is a re­la­tion­ship be­tween ring di­men­sions, seal­ing, and oil con­trol.

MEA­SUR­ING RING GROOVE CLEAR­ANCES

Ax­ial clear­ance is typ­i­cally checked by in­sert­ing a feeler gauge be­tween the ring and the top of the ring groove. A slight drag will in­di­cate the proper clear­ance. Start with the min­i­mum of 0.001-inch and work up. Don’t mea­sure be­tween the ring and the bot­tom of the ring groove as you might dam­age the seal­ing sur­face. Some builders mea­sure the groove it­self with a stack of feeler gauges and then com­pare it to mea­sur­ing the ring thick­ness with a mi­crom­e­ter. The first method is pre­ferred and it is good prac­tice to check it at sev­eral places around the perime­ter of the piston.

Back clear­ance is checked by in­sert­ing the edge of a ring into a groove and press­ing it all the way to the back of the groove to make cer­tain it does not pro­trude past the face of the ring land. You can de­ter­mine the back clear­ance by mea­sur­ing the depth of the ring groove with the depth scale on a set of dial calipers and then com­pare it to the mea­sured ra­dial thick­ness. If the ring groove is too thin to mea­sure with a caliper depth scale, a ma­chin­ist’s scale is an­other op­tion. An­other way is to use mod­el­ing clay or soft wax; press

it into the groove then re­move it and mea­sure with a caliper.

RING GROOVE QUAL­ITY

Ring grooves must be per­fectly per­pen­dic­u­lar to the cylin­der wall so cylin­der pres­sure can press the ring down against the land and out against the cylin­der wall for op­ti­mum seal­ing. Dur­ing the com­bus­tion event, cylin­der pres­sure fol­lows a path down the crevice vol­ume and into the ring groove above the ring. It then fills the ring back spac­ing to press the ring against the cylin­der wall. Ax­ial clear­ance in the ring groove pro­vides the fill­ing path to the back of the com­pres­sion ring so the ring is pres­sur­ized against the cylin­der wall and the bot­tom of the ring groove.

The back space di­men­sion is crit­i­cal be­cause it con­trols the ring’s re­sponse time. Too much back spac­ing makes the ring slow to re­spond, but there must be some clear­ance so the ring can move and con­form dur­ing all dy­namic con­di­tions. The main pur­pose for ax­ial clear­ance is to al­low the ring to spin. The cross hatch in the cylin­der walls in­duces ro­ta­tion of the rings. Ver­ti­cal and hor­i­zon­tal gas ports in pis­tons are also an ac­cepted way of rout­ing cylin­der pres­sure to the back of the ring.

MICROWELDING

One of the most crit­i­cal prob­lems ac­com­pa­ny­ing se­vere rac­ing and high-per­for­mance piston ap­pli­ca­tions is the po­ten­tial for “microwelding.” Microwelding dam­ages the piston ring or ring groove seal­ing sur­face when lo­cal­ized fric­tion weld­ing causes the trans­fer­ence of ma­te­rial from the ring land to the ring side, most of­ten the bot­tom ring side.

High tem­per­a­ture and ex­ces­sive move­ment are the root causes. It fre­quently ac­com­pa­nies high tem­per­a­ture en­durance and/or su­per­charged ap­pli­ca­tions where ring place­ment is too high on the piston and too close to el­e­vated com­bus­tion tem­per­a­tures, or in en­durance ap­pli­ca­tions where se­vere con­di­tions are on­go­ing. Microwelding up­sets ring-seal­ing qual­ity and can even jam the ring in the groove. It should be noted that ring grooves (es­pe­cially the top groove) do not ex­pand evenly since ma­te­rial thick­ness is not uni­form due to dome pro­files, dish pro­files, and valve re­liefs.

RING GROOVE PLACE­MENT

For years, the trend has grav­i­tated

to­ward tighter and higher ring packs on nat­u­rally as­pi­rated ap­pli­ca­tions. It pro­motes sta­bil­ity by spread­ing the con­tact points be­tween the rings and the skirt and re­duces the crevice vol­ume, which re­sists det­o­na­tion and pro­motes a more con­sis­tent burn, mak­ing the cylin­der more ac­tive. Rac­ers in the Su­per Stock classes al­ways lo­cate the top ring as high as pos­si­ble to en­sure sta­bil­ity of the ring pack and to pro­mote a more ac­tive and thor­ough com­bus­tion event. It also lets them use shorter and lighter pis­tons.

Forced-in­duc­tion and ni­trous ap­pli­ca­tions that are sub­jected to ex­treme ther­mal and pres­sure shock loads typ­i­cally re­quire mov­ing the top ring down from the piston top to about 0.300-inch. In many cases this is also dic­tated by valve size and po­si­tion­ing, valve pocket re­quire­ments, the ra­dial width of the top ring, and the piston pin lo­ca­tion.

Some­times the ring pack is moved down well over 0.400 inch to ac­com­mo­date th­ese con­cerns. In­take valve pocket specs gen­er­ally con­trol top ring po­si­tion be­cause the in­take valve is al­ways larger with more prox­im­ity to the edge of the piston crown. Thin­ner rings and smaller ring grooves of­fer more lee­way for op­ti­mum ring place­ment be­cause they re­quire less space, but they are at risk in se­vere ap­pli­ca­tions.

Moder­ate ni­trous ap­pli­ca­tions in the 250hp range will work with the ring pack ap­prox­i­mately 0.250-inch down. Above that, more is al­ways bet­ter as ni­trous air/fuel ra­tios are al­ways chaotic and un­pre­dictable. In that case, 0.450-inch or more is not un­rea­son­able.

GAS PORTS AND RING FLUTTER

Ul­tra-thin, low-ten­sion rings need com­bus­tion pres­sure pro­vided by gas port­ing to achieve op­ti­mum seal­ing. Oval track and road rac­ing pis­tons use hor­i­zon­tal gas ports at the top of the ring groove to re­sist car­bon buildup while shorter du­ra­tion drag rac­ing en­gines use more ef­fec­tive ver­ti­cal gas ports.

Gas ports de­liver di­rect cylin­der pres­sure be­hind the ring to seal the ring against the bot­tom sur­face of the ring land and to force it out­ward against the cylin­der wall. The di­am­e­ter and num­ber of gas ports is largely based on ap­pli­ca­tion and piston di­am­e­ter. The gas pres­sure must be evenly ap­plied to the ring to pro­mote a good seal and to pre­vent detri­men­tal ring flutter.

CON­TACT RE­DUC­TION GROOVES

Th­ese grooves are ma­chined into the top ring land above the top ring to min­i­mize con­tact drag when the piston rocks over upon re­ver­sal. They add min­i­mal vol­ume to the crevice vol­ume, and they also help re­sist det­o­na­tion by dis­rupt­ing flame travel into the crevice vol­ume where pres­sure spikes might un­seat the ring.

ACCUMULATOR GROOVE

The accumulator groove is ma­chined into the piston be­tween the top (com­pres­sion) ring and the sec­ond (scraper) ring. Its pur­pose is to pro­vide ad­di­tional re­lief space for pres­sure es­cap­ing past the top ring to build up be­fore it at­tempts to pass the sec­ond ring. It sup­ports top ring seal­ing by re­liev­ing pres­sure and it helps re­duce ring flutter due to pres­sure changes. Accumulator grooves have proved most ef­fec­tive and they are a com­mon fea­ture on many, if not most, high­per­for­mance and rac­ing pis­tons.

The qual­ity and place­ment of the ring grooves on your pis­tons is just as im­por­tant as your cam specs. Proper ring groove place­ment and ul­ti­mate seal­ing qual­ity are the keys to more power and dura­bil­ity un­der any se­vere-duty ap­pli­ca­tions. Hence it is im­por­tant that you use the rings spec­i­fied by your piston man­u­fac­turer or be pre­pared to share your ring pack in­for­ma­tion if you are pro­vid­ing your own rings. CHP

Hor­i­zon­tal gas ports around the top of the ring groove pro­vide an ad­di­tional path for com­bus­tion pres­sure to reach the back of the ring groove. They are used in en­durance ap­pli­ca­tions be­cause they don’t car­bon up as much as ver­ti­cal ports.

The strong­est piston in the world is ren­dered use­less with­out ring grooves de­signed for the ap­pli­ca­tion at hand.

Microwelding is the ab­nor­mal trans­fer­ence, or weld­ing, of ring land ma­te­rial to the piston ring due to ex­ces­sive heat. It pre­vents the ring from mov­ing prop­erly and, if se­vere, it can seize the ring in the groove.

Most pis­tons use drilled oil re­liefs at the back of the oil ring groove. Some have slot­ted holes that open up when the groove is cut. They are equally ef­fec­tive but some de­sign­ers feel they al­low more flex in the ring land.

Ax­ial clear­ance is the ver­ti­cal clear­ance left in the groove above the ring af­ter it has been in­stalled.

Po­si­tion­ing of the ring pack is ap­pli­ca­tion spe­cific and is dic­tated by the piston’s com­pres­sion height, the size and depth of the valve notches, and the di­men­sions of the ring pack­age. Many OEM pis­tons have the top ring 0.300-0.400 inches down from the piston top. En­durance ap­pli­ca­tions place the top ring 0.125-0.150 inches down. In heav­ily mod­i­fied nat­u­rally as­pi­rated race en­gines, the top ring lo­ca­tion can vary from 0.060-0.100 inches down from the crown.

Pis­tons for boosted ap­pli­ca­tions (right) have the en­tire ring pack moved well down the piston to move it away from higher tem­per­a­tures. Rings are placed higher on a max-ef­fort, nat­u­rally as­pi­rated pis­tons (left) to min­i­mize crevice vol­ume, ac­com­mo­date longer rod lengths, and spread the con­tact points to sta­bi­lize the piston on rock over. This is of­ten done on Su­per Stock and mod­i­fied drag rac­ing pis­tons.

Com­bus­tion pres­sure aids in seal­ing the ring to the cylin­der wall. Cylin­der pres­sure en­ters the ring groove from the crevice vol­ume, trav­el­ing across the top of the ring and down be­hind it to force it out against the cylin­der wall. The pres­sure above the ring seals it against the bot­tom of the groove.

The accumulator groove pro­vides ad­di­tional vol­ume be­low the top ring so pres­sure doesn’t build up and at­tempt to un­seat the top ring.

Ver­ti­cal gas ports in­ter­sect the very back of the ring groove to ex­ert di­rect pres­sure against the back of the ring. They are used in drag rac­ing ap­pli­ca­tions where car­bon buildup is not a fac­tor due to fre­quent re­builds.

Con­tact re­duc­tion grooves re­duce fric­tion by min­i­miz­ing piston ma­te­rial in con­tact with the cylin­der wall above the top ring. They also dis­rupt pres­sure spikes caused by det­o­na­tion.

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