Other than the ongoing issue of congealed and solidified oil, our Yamaha YDS1 engine strip is going rather well. We’ve been hoping and praying nothing fundamental will come out damaged and to date it’s looking good.
However, should we find serious wear or damage issues it’s good to know that Britain’s artisan engineering specialists can, potentially, get all of us out of the deepest of self-dug holes! There are a few firms that can cast us new connecting rods if we need them, damaged cases once cleaned could be welded and, heaven forbid, if necessary we could even commission a new set of gears. And it’s the last item that would give us the biggest cause for concern. Yamaha machined most of the early transmissions in-house. There are some great Youtube videos of the process, but they also played fast and loose with gear-set clearances between models and often part-way through model runs. But perhaps we’re getting ahead of ourselves hear and overthinking potential challenges. Let’s get back to the workbench for the final push.
Rather like the BBC’S original television mandate the engine has informed, educated and entertained. Other than a few oddities it’s proved remarkably conventional in design and build. Anyone who has worked on any of the firm’s 60s, 70s or 80s sub 250cc twins would feel pretty much at home here, which only goes to show just how right the original design of the TD series was way back in the mid-1950s. From this point on the engine will be scrupulously cleaned, then inspected for any signs of wear or damage. We already have gaskets, pistons, seals and main bearings in stock so the rebuild should be relatively plain sailing providing the con-rods and big-ends are within specification. If they aren’t the parts are out there, but they won’t be cheap, that much is guaranteed. The crank rebuild is almost identical to our Yamaha CS3C street scrambler feature, so check back to that restoration if you want to know more about these vertically split twins. Hopefully, the next time the YDS1 appears in CMM we’ll be riding it for a full appraisal via a road test. I can’t wait!
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3/ Using the correct puller/pusher tools is vital for this job. The top case has two heavy threaded castings in which the legs are fitted. The centre screw’s tapered end fits snuggly into the end of the crank and is gradually wound down. 4/ Ever so slowly the two halves begin to separate, but it’s paramount that the ever-increasing gap remains parallel. If there’s any suggestion of the pair either not moving evenly or parting at an angle, we’ve either missed a screw or one of the dowels isn’t freeing off completely. Thankfully, all’s well and the centre seal block of the crank has made an appearance at the join. 5/ Success! Well, partial success at least. The right crankcase half has been completely extracted off and over the crankshaft assembly, which now sits on the outer left-hand main bearing. Amazingly, there is some vaguely mobile oil on the crank, but it does whiff a bit (alright it smells a lot) like tramp’s trousers! (Dare we ask how you know, Scoop? Bertie.)
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1/ Another oddity or foible of the early two-stroke twins is a seal holder that then screws to a machined gasketed face to ensure no loss of primary compression. Just a few years later the seal would be pressed straight into machined registers that were cast integrally with the crankcases, thereby reducing build costs and assembly time. 2/ The crankcase halves locate in each other via a series of large diameter, hollow, metal dowels and these are known to be prone to galvanic corrosion with the aluminium. Some thermal input at the various sites will dramatically aid getting the two case halves apart.
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6 6/ Before we race ahead to remove the other crankcase half there’s the gearbox’s giblets to sort out. Everything is taken out as a single, homogenous, assembly and stored in a lidded plastic box until it takes its turn in the review, refurbishment and replacement queue. We really don’t want to be dropping rare parts off this little lot on to the workshop floor. 7/ And now for the final push… literally. You can now clearly see the two heavily engineered bosses built into the cases for this very job. Gradual winding of the centre screw pushed the crankshaft out through the main bearing and into a waiting hand that’s ensuring the conrod doesn’t get snagged and potentially bent on the case’s mouth. 8/ And there’s the culmination of a day’s work. One period late 1950s Yamaha crankshaft freed ready for dissection at a later date. The centre alloy block contains the crank seals and two inner main bearings; unlike some, Yamaha have always fitted a pair of bearings per cylinder to all of their stroker twins, thereby increasing reliability.
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