Scoop goes all cranky…

Classic Motorcycle Mechanics - - CONTENTS -

This in­stal­ment’s mis­sive is all about the en­gine and the faults, the is­sues and the sub­se­quent re­build. Those read­ers who haven’t yet lost the will to live may re­call the chaos that was the top-end of the orig­i­nal mo­tor and the spare pair of cylin­ders. There was a hope that they’d only need a light hone, re­mem­ber? And then we found out that the wretched things were con­sid­er­ably worn. So worn in fact, that they needed to go out to max­i­mum over­size at 1.00mm. Well, I’d like to re­port back to say that ev­ery­thing went ac­cord­ing to plan but, of course, any­thing in­volv­ing a ‘Scoop Scoot’ gen­er­ally throws its owner a curve ball. And some things don’t change: the bar­rels pre­sented Rob Pem­ber­ton of SPA Mo­tor­cy­cles a chal­lenge. They ap­pear to have come from a sim­i­larly ill-treated CS3 that, amaz­ingly, has seen even more abuse than mine. Ap­par­ently, the sus­pi­cion is that the crank that part­nered my now re-bored bar­rels must have been spec­tac­u­larly worn with con-rods that were al­most flap­ping from side to side. Huge wear to the thrust wash­ers and/or big-ends must have seen pis­tons float­ing out side­ways. This pre­sented its own unique chal­lenge and meant Rob had to re-bore five-thou off cen­tre. Ac­cord­ing to the Guru of the Gauges: “Yes, I’ve seen sim­i­lar be­fore but it’s all in a day’s work for us.” Quite why he tol­er­ates me and my bikes I’ve yet to fully grasp! As re­ported in part 4 it was al­ways a cer­tainty that the crank would need a full re­build. It’s not a cheap job, but given that

it’s the beat­ing heart of the en­gine, there’s no room for skimp­ing. While some of the parts are still avail­able from Yamaha, a lot of them aren’t, so a big hur­rah for Yam­bits, who stock vir­tu­ally ev­ery­thing needed other than the cen­tre labyrinth seal. On most re­builds this vi­tal seal can be reused, but given just how much abuse this sub-7000-mile ma­chine ap­pears to have en­dured, we’re tak­ing no chances – ebay even­tu­ally throws up a new one but it’s a lot more ex­pen­sive than the last one I bought. Once again my good mate Dave of­fers to re­build the crank, which saves the project a sub­stan­tial wedge of cash. DIY crank re­builds are not nor­mally rec­om­mended but Dave has all the right kit and, more im­por­tantly, knows how to use it. First up the outer fly­wheels have to come off so the crank goes un­der the press. This leaves us with the cen­tre sec­tion in one piece still plus the rods, bear­ings, thrust wash­ers and outer fly­wheels. Ex­am­in­ing the lit­tle end eyes shows ev­i­dence of heavy wear, but cru­cially, there’s also cor­ro­sion to the bear­ing sur­faces, so these are scrap! More time on the press has the cen­tre block apart and all that re­mains is to heat this al­loy coat­ing up, gen­tly tap out the cen­tre bear­ings and then evict the labyrinth seal. It’s a straight­for­ward job if you have the tools… but most of us don’t, which is why we send cranks away to spe­cial­ists. As the glibly worded say­ing goes: “Assem­bly is the re­ver­sal of dis­as­sem­bly” and some­time later, but the same month, Dave has the

crank re­built with all new parts. What never ceases to amaze me is that the main bear­ings are still authen­tic Ja­pa­nese Koyo brand but at a frac­tion of the cost of iden­ti­cal bear­ings from Yamaha, which I guess only goes to show the mark-up on bought-in parts. From here, as Dave sim­plis­ti­cally re­marks, we just need to throw ev­ery­thing back into the cleaned crankcases. The in­ner main bear­ings are al­ready homed in the al­loy block that lives in the mid­dle of the crank­shaft, but the outer pair have been in Dave’s freezer for a week. Car­ry­ing on the kitchen theme here the crank cases have been mel­low­ing nicely in his oven for an hour – ex­pan­sion and con­trac­tion are our friends here. Suit­ably ther­mally mas­saged, the mains al­most fall into the crankcases, which means we don’t need to ap­ply any un­due force. The crank is set­tled in the right half crank case us­ing a puller and the gear set in­tro­duced. Be­cause it’s a ver­ti­cally split en­gine, we can’t sim­ply build ev­ery­thing into the lower case and then put on the lid. Ev­ery­thing needs to be in place and meshed prop­erly be­fore we fit the left-hand en­gine case, re­mem­ber­ing to add sol­vent cure case ad­he­sive first to the cen­tre joint. When the gas­ket goo is tack dry, the other half of the crank case is added, cen­tralised over the crank and gear set. Var­i­ously tapped, pulled and ca­joled into place, we check and dou­ble check ev­ery­thing is prop­erly lo­cated and then add the all-im­por­tant Philips screws that keep the two halves to­gether. As is of­ten the case, the crank will be im­per­cep­ti­bly out of kil­ter, hav­ing been ex­posed to var­i­ous forces dur­ing its re­hom­ing. A swift, sure blow on the end with a large plas­tic mal­let jolts it back to where it should be. I wince ev­ery time this is done but it is part and par­cel of re­build­ing an en­gine with ver­ti­cally split cases. Onto the fi­nal stages of the en­gine build now and the kick-starter pin­ion, pri­mary drive gear and se­lec­tor mech­a­nism all go in the left case, ready for the clutch bas­ket. And that’s pretty much it for the most of the en­gine stuff of Project Yamaha CS3C. Next up I need to fit the rings to the pis­tons, the pis­tons to the rods and, fi­nally, in­tro­duce the freshly re­bored and honed cylin­ders. After that the en­gine can go back into the rolling chas­sis ready for the next part of the ren­o­va­tion pro­gramme which is the wiring… oh what ut­ter joy I can look for­ward to!

Crank on the press.

Here’s the crank bro­ken down into its var­i­ous parts.

The cen­tre-block in pieces.

All to­gether now.

New parts: yum, yum...

The crank pins.

Check­ing the lit­tle end eye.

Time to re­build...

A gen­tle tap on the crank...

And now the clutch goes on.

Bear­ing goes in the case.

Cases now half-built...

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