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/125685
left is the light hub from the KR clutch. Notice the thickness of the area where the studs are mounted on the stock hub on the ri ght. It is far too heavy. again more than 7000 times a minute, Left is the replacement Sportster piston. Right is the stock Offenhauser part that is too heavy even after radical lightening. Much of the Sportster's ski rt wi" be removed. The machine -owner cooperating with us drilled metal out from under the crowns of his pistons. He drilled until his tuner judged them lightened ,beyond any liqlit of reliability and he still considered them entirely too heavy. The solution these people are trying begins as a Sportster .plston. Right out of the box It Is a half ounce lighter than the radically drilled XR piston. We can't say what a stock XR piston weighs, but the lightened one we saw tips the scale at 12-3/4 ounces. On the other hand, the stock Sportster piston weighs 12-1/4 ounches, and almostafull inch of its skirt will be removed, and it too will be drilled before It Is mounted on the XR rods. That's going to result In a mass that Is several ounces lighter than stock pistons, and when several ounces are started, stopped, and started rr < "0 sacrifice rellablity for performance) but some can and must be accomplished. One very important change must be made in the pistons. The Offenhauser slugs that come in the machine are wholly unsatisfactory, and piston failure on the track bears .this out (Remember Daytona ~d Kent, Washington?), They are such massive, heavy. chunks of metal, it is not likely that any amoqnt of lightening will improve them. they amount to a great deal of inertial weight. Besides, compared to the Sportster piston, the cast alum,inum Offenhauser item is a sadly executed chunk of ore, and rather carelessly finished. To hole! our lightened, forgedSportster pistons on the rods, we fish Into ali old KR and swipe Its lightened wrist pins. These little numbers - believe it or not - each weigh a full ounce less than the stock XR 'Items. By now we've removed nearly a quarter pound of upper-end reciprocating weight. Continu1llg on toward the lower end, our tuner next decided to scrap his XR tappets In favor of the KR parts. PorPElrly prepared KR tappets are lightened, their tops are radiused to encourage 011 drainage, and they have a flattened side that acts as an oil groove. However, here Is one part where the XR versionlspotentially superior. We discovered that the stock XR tappet weighs exactly the same as the lightened KR tappet. TherefQre, when it Is properly drilled and machined for oil flow, the result should be a part muCh 'lighter than the KR counterpart. Next, a major modUication Is carried out on the r~ Intake cam gear and shaft. This does not concern lightening, but it is a modUication that has proven its necessity In the KR motor.• In the stock XR all camshaft ride In balI bearings ,'II.rl~. or' .:rr' e cept e rear intalCe cam, This rides in a bushing, and for a racing motor lbe bushing should be removed and the shaft turned down to accept a bearing. Now we move. to the left side of the motor where literally pounds of iron can be removed from rotating parts that limit the rate of acceleration. Here, however, our tuner considers the modification necessary rather 'than ideal. The clutch hub and clutch drum that come in the XR are massive chunks of metal, They alone would make the motor sluggish, but again a solution may be lifted from the KR. XR hub and drum may be replaced with KR parts, and Immediately several pounds of rapidly spinning mass are eliminated. However, the KR drum holds a five-plate clutch, and there is some doubt that it will be an adequate substitute for the XR' s seven-plate clutch. This Is why we say the solution is not Ideal, Better performance should result, but so should a higher rate of clutch destruction. The ideal solution would be a new and lighter hub anddrum made from aluminum for. the seven-plate XR clutch; but at present none Is avallable. When you get your primary drive all put together, you can remove a full ounce from the primary sprocket nut alone. The stock nut weighs2-l/40unces. When cut down to 1-1/4 ounces, there still Is plenty of area for your wrench jaws to grab. Above is the lightened K R tappet. Below is the stock tappet from the XR. Notice the radiused top, flat sic;le, lightening holes, and radiused rotrer on the modified part. That takes care of some of the reciprocating weights, so as you assemble the motor, you take the grinders to the parts that increase overall weight. You may remove a half inch from each cylindP.r stud witho,ut doing any damage. All of the heavy iron bolts on the frame should be replaced with aircrafttype aluminum bolts. These can be as much as 70% lighter than the bolts that come on the XR. Finally, now that your bike is lighter and running freer, you must modify the transmission to get that newly-found power to the ground. The stock Sportster Left is the rear intake cam shaft minus the drive gear. The thick area where a bushi ng rides wi" be turned down and have a bearing fitted like the KR part on the right. cogs that come in the XR simply don't provide the proper ratios for half mile racing. This box - and the pistons - are areas where Milwaukee can be accused of cutting the corners too sharply In order - to reach the 1970 season with the required ' number of bikes. Again, simply do what they should have done: rob your KR. That piece of factory literature we mentioned at the first of this article also states that the xR Is" •••destined to rival the KR flathead in frame." We don't think this Is mere advertising bull. In fact, we don't doubt it. However, let's reflect upon the number of hours that went Into the m, That fabulous racer wasaresult of hundreds of people working over a period of 15 years. We should not expect to deny the XR the same patience and attention• . AOHS S dOlDlnates lDotocrossr" ENTRY FEE S8.58 Trophies - 20% 01 elllry POll Enlri': S50," an, All-American Championship Motocross -' 8/2/70 Bay Mare, Calif. 125 PRO , lst Gene Cannady - SACHS MOTOCROSS - 8/2/70 Champion Speedway' Brl sbane, Calif. 125 SENIOR lst Carl Cranke - SAC H5 U.. standa'd A,M.A. Itntf)' bl.n~ Impart,d by: Dn",'nl for .'-rUne po,ltlon. P3rU.'II.' or P'Y'",.nt. MaUro: Gary Rlicll.back 119.. Du...n 51, R... da, Calif, 91335 A....A. Dl.t. 31 Ph. 0 ••" .,Scr.mblu • Motocross· ONLY RACE ENTERED BIKES ALLOWED ON GROUNDS! RACING TIM!: bt. ':00 to 11:00, 11-100 Noy. 1:00 to 3:00. 101·2S0 Now. HERCULES DISTRIBUTING, LTD. 1112 Colorado Avtnue Santa Monlcl, t'llfor"l. M)404 LIMITED ENTRIES fnt,)' apen - 7·1·70 ClolU -1·27·70 TAKE HI-WAY 14 TO ROSAMOND TURN LEFT ON ROSAMOND BLVD" FOLLOW ROSAMONO TO THE TRACK MCING TI ..E SlIn. ':00 to 11:00, 2SI·Up Nov. 12:00 to 2:00, o-Up Am. & f_p. 3:30 to 4;30, Sld,iuIck So.mething For Everyonel Free Admission' To Alii' .... ....