Once you’ve got your old bike’s en­gine run­ning per­fectly, the next step is to trans­fer all that im­pe­tus to the rear wheel. A quick get­away and a quiet ride start with smooth trans­mis­sion. Neil Cairns re­builds the drive train on a typ­i­cal Bri­tish sin­gle…

Real Classic - - Contents - More old bikes on­line: Real-Clas­sic.co.uk Pho­tos by Neil Cairns

Once you’ve got your old bike’s en­gine run­ning per­fectly, the next step is to trans­fer all that im­pe­tus to the rear wheel. A quick get­away and a quiet ride start with smooth trans­mis­sion. Neil Cairns re­builds the drive train on a typ­i­cal Bri­tish sin­gle…

Wr­e­fit­ted it and pulled the chain up as tight as I could (re­mem­ber, it was a rigid frame). I could not af­ford new bits on ap­pren­tice’s pay. To this day I still au­to­mat­i­cally look at rear sprock­ets on bikes.

It’s now 2020 and I have new gear­box and rear sprock­ets, and new chains on my bench ready to fit to my 1952 AJS 18S solo. I’ve owned this bike for a decade and it’s done a the chain jump­ing the rear sprocket. It was so worn that the worn chain sim­ply jumped the hooked teeth un­less you were gen­tle on the clutch.

Be­ing a bit of an en­gi­neer­ing twit back then, I re­moved the sprocket and took it to work to file the teeth deeper. A tool­maker pointed out I was ac­tu­ally re­duc­ing its di­am­e­ter and mak­ing things worse. So I hen I was a teenager in the early 1960s, my first road-le­gal mo­tor­cy­cle was a 1946 BSA 250cc C11 with a rigid rear end and a dy­namo that did not work. It was a wreck, but to me manna from heaven. If I tried to do a fast get­away to im­press my mates, all I got when I dropped the clutch was a loud rasp­ing noise and no go. It was

lot of miles as my main ma­chine. It ate its last chain very quickly, which points to fool­ishly putting a new chain on worn sprock­ets – and this proved to be true.

On my Hinck­ley Bon­neville, fit­ting new sprock­ets and the rear chain is very easy, but the AJS re­quires a great deal of strip­ping down. The first thing was to clean the bike as much as pos­si­ble. Any­thing near ‘open’ rear chains gets well and truly cov­ered in oily-greasy muck. This is good for preser­va­tion but bad for fin­ger­nails. The rear part of the mud­guard has to be re­moved, then the rear chain (I should have left this in place, I found out later). The speedome­ter cable needs dis­con­nect­ing along with the brake rod, and the brake drum torque arm’s stud must be re­moved from the swing­ing arm. Only then can the wheel be pulled out. Lots of bits to undo, un­like the G12 that has a QD rear wheel.

Once the wheel was on the floor, com­par­ing the new and old sprock­ets con­firmed that ten years of all-round use had done its bit. The sprocket is part and par­cel of the rear brake drum. When I needed a new rear sprocket on my 1961 Pan­ther M120, I had the old one ma­chined off the rear drum and pressed on the new one in my vice. I then drilled three holes on the joint, fit­ting peined-over dow­els for se­cu­rity. Hands up those who have never heard of pein­ing? It is what the ball-pein round bit of your ham­mer is for, ba­si­cally riv­et­ing.

The AJS drum is held on to the spool by five bolts which are locked in­side the drum by nuts, them­selves locked by tab-wash­ers. The wash­ers had ob­vi­ously been used be­fore so, once I had it apart, I made five new ones from some 22swg sheet steel from my stock (ac­tu­ally a side of an old twin-tub wash­ing ma­chine. Never throw things away as they come handy even­tu­ally).

The ‘spool’ is the cen­tre of the wheel. I re­built this very wheel some years ago with new spokes and a stain­less steel rim with a kit from an ad­vert in RC, putting the wheel in the vice to true it up. I needed to re-tighten the spoke nuts af­ter about a month.

Fit­ting the new sprocket / drum was easy, and bend­ing over the newly made tab-wash­ers was done with a handy set of mole-grips. The new brake drum re­quired a good clean as it had preser­va­tive on it. The wheel was then put back into the swing­ing arm loose, as it will be re­quired when I come to un­do­ing the gear­box sprocket’s big nut (yes, I’ve done this be­fore).

Next came the re­moval of the pri­mary drive to get ac­cess to that gear­box sprocket. It was a filthy job as ev­ery­thing was cov­ered in thick, black muddy greasy road dirt. It all looked in ex­cel­lent con­di­tion, not sur­pris­ing as the pre­vi­ous owner had given me some re­ceipts for new clutch bits and chains. A large box-span­ner I pur­chased in 1979 for the Pan­ther once again be­came use­ful. It fit­ted the big nut that locked the sprocket to the gear­box shaft. The tab wash­ers on the clutch bas­ket nut and the gear­box sprocket nut looked like new and were re­us­able as there were ex­tra un­used tabs.

This was when I found I needed to re­fit the chain, so I could lock up the rear wheel to be able to undo that big nut. The sprocket it­self was not go­ing to play fair, so I at­tacked it with a strong puller. I won. Note the oil tank’s vent pipe dis­charg­ing onto the chain.

Re­assem­bling the pri­mary drive was straight­for­ward, apart from hav­ing to clean ev­ery­thing. While it was in bits I fit­ted a new chain, which I’d orig­i­nally bought as a spare for the G12 so it was one link too long. I ground off the rivet head and pressed it out, to shorten the new chain. The photo ac­tu­ally shows the orig­i­nal chain that I fit­ted first, with the spring-links the wrong way around. The clip’s open end should face away from the di­rec­tion of run­ning. The new chain only has one link, fit­ted cor­rectly.

I then had to re-ad­just the free play on the new chains. The pri­mary chain is done by the gear­box ad­juster un­der the oil tank: easy. The rear chain is done by turn­ing the snail-cam ad­justers for the wheel axle with a re­ally big span­ner. And al­though the dy­namo drive chain is ac­tu­ally driv­ing an Al­ton al­ter­na­tor, it too re­quired a small ad­just­ment. Oddly enough, its lock­ing clamp has the only A/F headed nut on the bike, ½” across the flats, an Amer­i­can con­fec­tion, noth­ing to do with the thread. I won­der why?

Fit­ting the outer cover to the rear of the pri­mary chain-case re­quires a bit of care, as it uses a fid­dly rub­ber seal. The rub­ber seal I used was a late one with a lip be­tween the two faces, with a T-shaped cross-

sec­tion. I took some time over this to try to get an oil-tight joint, not some­thing these AMC bikes (and Nor­tons) are fa­mous for. It is a bad de­sign be­cause all the ‘clamp­ing’ is only done at the rear edge where the screw joins the two ends of the al­loy strap. I hate us­ing sealants and even with sil­i­con I’ve never suc­cess­fully sealed my other 18S chain-case. This time I filled it up with oil and – so far – no leaks. It will wait un­til I’ve left, and then leak onto the garage floor…

The fi­nal thing was to start the bike. It is usu­ally a first or second kick ma­chine as long as you set the tim­ing to fully re­tarded, the choke lever to closed and flood the car­bu­ret­tor. Not this time, alas. I kicked and kicked for ages, just get­ting the odd ‘woooof’ from the ex­haust. At least it was try­ing. When I cleaned the AJS I had used ‘Muck­Off’ which works well. Too well in fact as it had di­luted the Vase­line in the mag­neto HT wire Bake­lite slip-ring con­nec­tor.

AMC in their wis­dom fit­ted the mag­neto in front of the en­gine, and even though it has a splash-guard the front wheel can some­times del­uge it and swamp the spark. So I seal it with Vase­line as I’ve been do­ing to my bikes since the dawn of time. I un­screwed it and dried it all out, re­fit­ting with fresh Vase­line. The en­gine burst into life at the next kick and sat there tick­ing over as if noth­ing had hap­pened.

Back when I had that BSA C11 I would have done this job in a day. 58 years later it took me two whole days, and my knees are sore. I’ve some kneel­ing pads some­where. Some­where…

These bolts hold the drum to the wheel’s spool

The five nuts that lock the se­cur­ing bolts. The drum is threaded as well

New sprocket / drum fit­ted with new DIY lock­ing tabs. No one wants a loose nut in their brakes

OldOld andd new rear sprock­ets.kt OOn the­seth ma­chi­neshi theth sprock­et­skt are part of the brake drum

It does get rather filthy around the gear­box sprocket

The pri­mary drive needs to be re­moved for ac­cess to the gear­box sprocket

Neil had re­fit­ted the old chain be­fore re­mem­ber­ing he was fit­ting a new chain. Just as well as he’d fit­ted the spring clip the wrong way round

This huge boxbox-spanspan­ner­was bboughtht foraM120fM Pan­ther yearsago,used­now to undo the gear­box sprocket nut

The sprocket it­self was firmly fixed, so a puller was em­ployed to shift it

The new pri­mary chain was too long, so a link was re­moved

A care­fully assem­bled pri­mary drive cover to dis­suade oil leaks, hope­fully

Ad­just­ing the Al­ton dy­namo chain

Ad­just­ing the rear chain, us­ing the snail-cams on the bike’s rear axle

All re­assem­bled and ready to start, but it wouldn’t!

Suc­cess! Job done

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