Cycle News is a weekly magazine that covers all aspects of motorcycling including Supercross, Motocross and MotoGP as well as new motorcycles
Issue link: https://magazine.cyclenews.com/i/127409
WORKBENCH . By Michael R oss WHENWINNING Taking the Guesswork Out of a Good Fit heth er you 're an off-road racer , a motocrosser or just a weekend warrior, sooner or later the piston and cylinder of your two-stroke will require service. With a few tips th e average home mechan ic ca n save hundreds of dollars over th e life of a bike by doing his or her own top en d mainten ance. Big exhaust ports, gaping intake ports , a n d es pecia lly a poorlyserv iced a ir cleaner will wear out pi stons and cylinde rs in sho rt order. It's a good idea to service th e piston on a rac e bike after every 10 hours of operation. If yo u find that the eng ine is hard to sta rt or idl es poorly, it may be a signal that it's time to check th e top end. The tools nee ded to do th e job are minimal, basica lly a 12mm socket and 14mm end wrench will remove the head and cylinder on 90%of roday's bikes. With the head an d cylinder re moved, clea n th e cy li n de r a nd pis to n in hot wa ter and detergent, drying th em qu ickl y befor e they ru st. Rem ove th e rin gs from th e piston and note if there are bl ack deposi ts below the rings. Carbo nized oil mea ns blow-by and the rings are history. If no blow-by is detected, slide the ring into the barrel using the piston to square it up, then ho ld it up to a bare light bulb. You shou ldn't see any light between the p isto n ri ng and the cylinder wal l. Excessive end gap (over .015") means tha t the rin g is worn out. Repl ace the rings if either co ndi tion exists. T he next step is to check th e pis ton to cy linder clearance. This is crucia l to engine performance but very easy to check. Use a lon g .002" feeler gauge a nd oil up the cyli nder with WD-40. Turn the cylinder upsid e down and lay th e feeler ga uge on the inta ke side of the cylinde r wall and push th e piston into the bore so the bottom of the skirt is fI ush with the bottom of th e cylinde r. Draw th e feeler gauge ou t. A perfect .002 fi t will give a pull of six pounds. Nea rly all 125s, 250s and 360s are set up for a .002 to .0025" fit. If the pi ston falls through , it's time for a rebore to th e nex t oversize. (usually in .010" increments to .040") If you determine that you r cylinder is out of standard, get it bored out a nd repeat the above check when yo u get it back from the machine shop. If it is too loos e or too tight, tak e it back for an adjustment. If the bore is okay and just rings ar e needed, use a ball hone and an electr ic drill to put a 45-degree cro sshatch on the cyli nder to seat th e new rings. Remember to thoroughly clean the cylinder in hot water and strong laundry soap. Wipe th e inside of the bore dow n with co ntact cleaner until black deposits no lo nger appear on your clean white rag. Assemble dry. If the cy linde r does n't sli de back on easi ly, back up and try agai n, ma kin g sure th at th e ri ng en d ga p li nes up with th e ring loca tor p ins. Sometim es if you twist the cylinder as you sl ide it down , th e p iston rin g will hop over th e locat or p in and you won't be able to push th e cylinde r down any farther or pu ll it off . Don 't panic. Take o ff th e reed cage and eyeball which ring is misalig ned. T wist th e cylinder until th e rin g is back in position ar o und th e loca to r p in and press it back in place with a sma ll scre wdriver as you pull off the barrel. Retorque th e cyli n der and head bolts after th e fir st test ride 01 and you 're ready to race. .IS THE NAME OF THE G AME... B EL-RAY IS THENAMEOF THEOIL W Congratulations Jim Filice and Team L.A. Motor Works 1991 AMA 250cc Grand Prix Champions Street Legal Knobbies METZELER introduces the NEW MCE rear knobby for agressive Dual Sport offroad riding. Pair this up with our world famous MX front tire - now DOT approvedI This set of DOT knobbies are intended for 10% street and 90% offroad application. Choose your Dual Sport METZELER also offers the Enduro 3 sahara for 50% street - 50% offroad and Enduro for 80% street 20% offroad. The tires that took the Gold at the 1991 ISDE 6-days. MCE 120/90 MC~ 140/80 *MCE 120/80 * MCE 130/80 X X X X 18 18 19 19 •Available early 1992 For more information contact: METZELER @ FEEL THE DIFFERENCE 4520 107thStSW,Mukilteo , WA 98275 (206) 348-4000