Stan Stephens on how to make In­dian-bought RD bar­rels work!

Classic Motorcycle Mechanics - - CONTENTS - cmm

New gen­uine parts for your clas­sic are get­ting harder to find, so many parts are now not avail­able. Own­ers and re­stor­ers have to turn to pat­tern parts of which some are use­less but many are of fairly good qual­ity and let’s face it if there is no al­ter­na­tive they can save the day! These pat­tern parts come from a va­ri­ety of places, some are made in Ja­pan and are usu­ally ex­actly the same as gen­uine parts. For years many pat­tern parts were made in Tai­wan and their qual­ity was dread­ful but nowa­days the qual­ity has greatly im­proved. China has now taken Tai­wan’s pre­vi­ous man­tle in this... Re­cently when try­ing to source a set of pre-war girder forks for my Brook­lands Special I found new copies of old Bri­tish BSA, Nor­ton and Ariel forks avail­able and be­ing made in In­dia. As a last re­sort I or­dered some; they were rub­bish but when I thought about it at least I had a set of girder forks! This is when you have to look at things a lit­tle dif­fer­ently. I had the ba­sis of a set of forks and with some fairly sim­ple rema­chin­ing they were quite ac­cept­able. As you know, en­gines are my trade so most of my con­tact with pat­tern parts has been with en­gine parts. You can buy pat­tern in­di­ca­tors or footrest rub­bers and they can be poor but us­able. En­gine parts, how­ever, have to be right. Re­cently a cus­tomer brought in a pair of new Yamaha RD350 bar­rels and pis­tons, made in In­dia! The cus­tomer, who wasn’t a me­chanic or an en­gi­neer, said would I ma­chine the ports the same as in the orig­i­nal Yamaha bar­rels be­cause even he could see the ports were all over the place. The In­dian bar­rels looked great but ev­ery­thing I mea­sured was badly wrong. The ports didn’t match the lin­ers and were all dif­fer­ent heights and widths but the worst part was that the pis­tons would not go in the bores. The cus­tomer as­sured me that if you pushed them hard enough they would go in. I told you he wasn’t a me­chanic! Think­ing back to the In­dian forks, here was a set of new bar­rels for a 45-year-old bike; where else would you get them? Again, some ma­chin­ing would put them right and an­other clas­sic would be saved. No doubt there are or will be other ‘Made in In­dia’ bar­rels for sale in fu­ture so the work I had to do on these RD350 bar­rels will give CMM read­ers a few clues on what to look out for. The pis­ton kits were good qual­ity Ja­panese Mi­taka. The prob­lem was the cylin­der bores: they mea­sured 63.90mm but the pis­tons mea­sured 63.97mm. The pis­tons were 0.07mm larger than the bores. The clear­ance needed to be 0.05mm so the bores were 0.12mm too tight. To bore out such a small amount on a bor­ing bar ev­ery­thing would have to be per­fect in set­ting it up. My bor­ing bar is very ac­cu­rate but if you can imag­ine bor­ing an ap­prox­i­mately 100mm long liner it would only have to be the tini­est bit out at the bot­tom to com­pletely miss one side at the top. The cylin­der bar­rel sits on the bor­ing bar the same way it would do on the crankcases so the bore has to be at a right-an­gle to the cases, which also means the bore has to be at ex­actly a right-an­gle to the bot­tom gas­ket face of the bar­rel. I checked and the gas­ket face was not square to the bore, not by a lot but enough that the bore cut­ter would have missed by the time it came to the top of the bore. The amount that it was out would not have mat­tered if I was do­ing a re­bore but to take out such a small amount it had to be put right. I put the bar­rel onto the lathe

and cen­tralised the bore in the chuck and skimmed the min­i­mum amount off the bar­rel and then took the same amount off the other side. It was then time to bore the bar­rels. As the bor­ing bar goes through the bar­rel you can hear the con­tin­u­ous sound of the cut­ter bor­ing its way through. I held my breath, hop­ing that it did not be­come the in­ter­mit­tent sound it would make if it was miss­ing one side. All was okay and when I checked the clear­ance that was as it should be. An­other bad point of the bar­rels was that the ports had not been cham­fered but I would do that af­ter port­ing them. The cus­tomer had sup­plied a port­ing map di­a­gram for a stan­dard RD350 which he had down­loaded for me to ma­chine the ports to. Luck­ily all the ports in the bar­rels were too low and/or too nar­row so it was a matter of port­ing them to the cor­rect di­men­sions, I say luck­ily be­cause you can’t put metal back on. Here’s a tip. When faced with ports in the bar­rel not match­ing the ports in the liner, don’t just match one to the other as quite of­ten they don’t match but you don’t know which is right and which is wrong, the bar­rel or the liner! Frus­trat­ing! With the ports now cor­rect I cham­fered the ports and honed the bores. The cus­tomer now had a set of new bar­rels and pis­tons which were ready to go.

This pic­ture shows the pis­ton tight in the bore.

At last the trans­fers are fin­ished!

Now we’re skim­ming the bot­toms of the bar­rels.

The bar­rel mounted on the bor­ing bore.

The bar­rel now has the cor­rect clear­ance.

The ex­haust port mis­match be­tween bar­rel and liner is seen here!

This is the fin­ished ex­haust port. You do have to do some graft here!

My good­ness! Ev­ery­thing is wrong!

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