Yamy RT2 with a 250 crank [why] i dunno, because i had some old bits!!

[OverTheHill]

Back on to my 'long parked' project using a 250 crank in an RT2-360 [using a pile of way past their useby--old parts]. She'll qualify as a "Roach of the Week" bike so far as looks go but more interested in how it's going to run mechanically at this stage--6mm off top of cylinder etc. Only have an RT1 [no holes] Rocky Brand 3rd over piston kit that fits--it'll work for testing, rod has 311 on it, not sure what yam that's from. Only 250 crank wheels i could find were YZ250A ones [sacrilege i know--standing in the naughty boy corner] Reason for this project [who needs a reason] is mainly due to already having done a 250 DT1 using a 360 crank so wanted to try the reverse configuration & just using worn out old [best of the rest] parts picked over many times. I'll report back in a week or so & let ya know if it was [another] wasted effort!!. Thanks.

[JC]

You should be able to get it working pretty well, but depending on yr rod/piston combo you may need a spacer under the cyl to get it lined up right at TDC/BDC if you took 6mm off the top.

Just as a guide for port height/timing, for 64mm stroke, 36mm from top of cyl/stroke to top of exhaust is about 180deg duration; 51mm for transfer is about 120deg duration. 

If you started w a std RT2 cyl, exh port is about 48mm & transfer 58/59mm. With 6mm off the top, transfer timing is reasonable, but exh timing would be very mild.

Should be about 330cc w 3rd o/s piston

Be interesting to see how you go - I've nearly done it myself.

[OverTheHill]

Hi, i'll read your comment in depth tomorrow [too tired now] but is nearly ready to fire up, just have to [borrow] a pipe & magneto off one of my others [cant find the mag--i know i have somewhere!!]. Basically what i did was--6mm off top of cylinder due to the 6mm less stroke. sorted through the rods to find one that came up with piston close to top [needed some 1.5mm gasket paper under cylinder[repco] to get 1.5mm squish at head just to be safe. Then ummm only had an RT1 piston port piston [same as RT2 gudgeon height] unlike the difference in DT1 250 & DT2 250 where height in the two pistons is different]. Wasn't going to worry about the reed holes initially but it got the better of me & cut a 'big' hole high up in the skirt but wish i hadn't now as might fall to bits but is 'new' & a good fit. Originally eyeballed the exhaust duration just sitting a mag flywheel on & going [looks like half a turn almost spot on] & looks like 180 degrees duration fully uncovered so thought transfers would be ok too. Inlet is fully uncovers at tdc [& now have a window--make that a 'door']. Might have gearbox issue yet but just damn well wasn't going to pull apart till it's fired up at least. Found when revolving the clutch center it feels like a dodgy tooth on either first on mainshaft or second also on main as is every turn in same place & doesn't alter changing through gears on bench--something like that. Can see it all ending in tears [gears] Thanks for your advice [i'll digest it tomorrow] Watch this space--Cheers.

[OverTheHill]

to JC, just thinking back 12 months, think i may have simply found the rod [yamaha 311 rod] to get the ports as near to fully uncovered at bdc then lifted cylinder a little to fully uncover [hence the 1.5 base gasket required]. Think i then went to tdc & marked the cylinder using the piston as the guide. The 6mm might be the figure i had in mind because it's 64 instead of 70 stroke. Never mind, we'll find out the outcome soon enough. Way oversquare but so was the sc500 at about 90x70 by memory although it had porting to match. Sort of thinking of mine as a big bore 250. Think i wrote somewhere a while back that i'd done the reverse [360 crank in a 250] which went very well. Thanks.

[Lozza]

The 6mm difference in stroke isn't just at the top it's 3mm higher at TDC and 3mm lower at BDC 

[JC]

Yeh, that's why I said it'll need a spacer under the cyl, but if you read how he did it, he's on top of all that.

[OverTheHill]

just read what 'Lozza wrote which it right of course--just me not thinking sorry [geriatric--or yamaheimers]. But wait we have lift off, 2nd kick & runs absolutely perfect, doesn't vibrate [yz250a crank webs] i'm sure it's a 311 rod. Have a homemade chamber off another 360 which it doesn't work very well on but on 'this one' it feels torquee but comes on stronger as it climbs in revs--feels a 'willing motor' Early days but promising at least. Clutch won't disengage as made up out of odd bits, possibly dt250/400 centre with splined pressure plate & sits further out & damn it the clutch won't lift [couldn't think why] till i saw the clutch cover bulging when i tried real hard to pull it in. Must be 10 thou between cover & pressure plate initially. Anyway, no worries--live n learn. Just rolled it to a fast walk then into gear & away up the road.
Had our NZSBK superbikes qualifying on out in the shed, my boy [well,39 or something] qualed second to 'your' Superfast Robbie Bugden, hope craig can keep him in sight to maybe learn a few tricks. One seconds a big gap at that speed though. There's a few guys close on time but Bugden seems to always have that bit extra when needed which is of course the mark of a champion. Had the video going mainly for the sound & 'the sound' of those 1000's wheel spinning out of the corner where the mike must've been was just mind blowing.
Now, back to the important things!!, clutch off next. Also, if it proves 'not' to be a disaster i might bore it & use a YZ400F 82mm piston kit [1979 only long stroke 400] as have a couple of new ones. Thanks for your helpful advise JC plus Lozza for putting me right too--all good. Takes me back to 1974 & our w/shop foreman said to me Morley--you go to extreme lengths to make a F!@#up, which prettywell sums me up sometimes but i had the last laugh shortly after when he was re-assembling an MX250A, all set to go pretty much & he went to slide the clutch pushrod in from the left but DAMN IT [worse] rod has to go into main shaft before crankcases go together as is super large by memory, fat at both ends & skinny in the middle [or was something else!! i'll ask the wife]. I never laughed at his mistake [till he wasn't looking. Cheers Morley Shirriffs. ps, must learn how to load pics up here sometime.