YAMAHA YDS1

We have the motor on the bench ready to split the cases. Yes, our Project Yamaha YDS1 continues!

Scoop is about to split the cases!

In our obsessive world of old bikes, many of us are magpies for the peripheral ephemera that go with the machines we love.

Some go for original tool kits, others hunt down factory accessories such as racks and screens, whilst far too many of us spend serious dosh on machine brochures. Back in mid-1959 few would have thought such material would have been desirable, let alone valuable. Our opening shot clearly details Yamaha’s pride at what was then its new flagship model, or rather the over-thecounter race bike version they knew would be a huge seller. Pretty much everything we CMM fans take for granted on our stroker twins now was perfectly showcased here for the very first time. And now we’re going deeper into the guts of the engine.

Ok, so to recap our investigations to date. We’ve taken off both side engine cases and discovered the oil has turned to what looks and feels like varnish. We have the odd-ball clutch off the end of the crank and to everyone’s relief there are no signs of damage. The cylinder heads have been liberated and barrels lifted off the pistons; again thankfully there are no obvious issues with either. If this was a younger Yamaha twin we’d be flipping the short motor over and undoing a hatful of bolts ready to separate the crankcase halves, but not here. With the engine being vertically split there are some more hoops to jump through, one or two hurdles to get over and some specialist kit required to the engine down to the point where we can liberate the crankshaft assembly. So far it’s all gone rather better than expected, which might very well imply we’ll be hitting a snag any time soon!

We’re very close now to getting this super-rare engine apart ready for a good look-see. What’s very apparent is just how well the motor was engineered in comparison to many of the period. Yamaha’s engineers obviously studied the Alder MD250 on which the YDS unit is based and added some considered refinements. Amazingly, the Iwata factory had the foresight to recognise that the bike could and would be used in competition, so designed a decent level of robustness into the motive unit. The vertically split YDS engine would remain as work in progress right up until 1969 with the final iteration sold as the YDS6. It’s probably no surprise that the various TD 250 race bikes owed much to the YDS road bikes. England’s Rod Gould won the 1970 250 cc World Championship on a TD2 with Australian, Kel Carruthers, coming in second on an almost identical machine – both were based on the YDS6.

4/ Not a job for the faint-hearted: a series of packers, wedges and a metal chisel are used to very gradually free the pair from the crank. Working from all angles, a few taps at a time evinces minute amounts of movement… slowly, slowly, catchy monkey, etc. We’re working against a parallel shaft with an interference fit so it’s a painfully slow process, but we can’t afford to take any risks on something this rare. 5/ Eventually our patience is rewarded. The gear set comes off to reveal a remarkably clean shaft with just a trace of corrosion present on the inner face of the components. We are definitely making progress. 6/ Before we can think about splitting the cases the sump plate needs to come off as it bridges the vertical joint. It’s an easy win, but what’s inside is neither pretty nor factory approved. This is what 50-year-old gear oil looks like folks – solid, like shellac on a 1920s’ magneto, only several millimetres thick! 7/ Peeking into the gearbox, we see the underside of the shift mechanism and, to be honest, it looks relatively clean,, but…

1/ No roller bearing little-ends here then. The YDS1 worked perfectly well with what looks like phosphor bronze bearings and slotted rods machined and drilled to supply and retain lubricant. Other than witness marks they look good, but they’ll be carefully measured before the rebuild. The marked lack of corrosion on the studs further supports our belief that this bike has been stored correctly. 2/ There’s more varnish in this engine than on the Antiques Roadshow! It’s either very old pre-mix or leaking primary drive-cum-gearbox oil. Whatever’s happened, it looks like it’s preserved the engine internals remarkably well. 3/ Ok, so now we have to attend to the elephant in the room. With the primary drive gear and clutch inner on the left, there’s two schools of thought here. One says they should come off when the crank is pulled through the case, whilst the other says they may potentially stymie that very process because they’ll be stuck fast.

8/ …When removed and inverted we have a much better idea as to why the gears wouldn’t select. The ancient oil has set like glue and has bound all the selectors, forks and pivots to their shafts almost as effectively as thread locker. 9/ Well, we’re almost ready to split the cases so every cross-head screw needs to be removed and carefully stored; they’ll all be JIS threads and not the more modern ISO ones. We know some are likely to be coated in that evil lacquer so it’s out with impact driver and fresh bit – we’re taking no chances here!