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Yamaha DT1 tranny
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Posted by: Succubus
It is the worst case scenario. But I guess it means I get to check everything out since I will have to completely disassemble my motor. After fixing the clutch on my 1969 Yamaha DT1 I determined that it wasn't the clutch at all that was making first gear slip. My tranny is shot. Bummer. Symptoms- the bike slips badly in first. It really never engages completely. I can come of the line in second and all other gears grab well. I haven't split the cases yet so I don't know which part is bad. A few questions: Will I need a special tool to split the cases? Can anyone tell me if the trannys from any other Yamahas will fit my bike? And, other than the Bay of E, does anyone know where I can get a DT1 tranny or tranny parts?
At least my '74 Suzuki TS250 is running very well now.....
Posted by: originaldirt
The only special tools required will be a flywheel puller (if you decide to pull the crankshaft and do new case bearings and seals [recommended]) and a clutch holder tool to remove the clutch hub (if you don't have access to air impact tools). Other than that the cases split with little effort. The best way is to leave the tranny and crank in the L.H. case to disassemble, and reassemble that way. You'll have to use a soft hammer to rap on the r.h. crank and tranny shaft to get the cases to move. Reassembly is easier if the r.h. case is pre-heated in an oven or on a kero heater.
The DT-? series trannys are mostly interchangeable, but unknown from there. Certainly not from JT's, GT's, LT's AT's, and CT's, probably not form RT's or YZ's. Some new tranny parts are still avail NOS from vintage Yamaha dealer specialists like Speed & Sport, Bloomsburg, PA (www.yamatopdog.com) and The Cycle Center in Culpepper, VA. They both have large NOS stocks.
OriginalDirt
Posted by: Succubus
Thanks, OD. I actually have a flywheel puller I acquired during the "why won't this stupid thing spark" chapter of this rebuild and I have a compressor and air tools, so clutches are no problem. When I read the manual, though, it was wanting a special crankcase splitter too. All my manuals from all my bikes want a crankcase puller, but I've never needed one. I've never split a Yamaha dirt bike case, however, and I didn't want to run into a surprise that slows the rebuild to a screeching halt. So will later model DT trannies work with this '69 DT1? Or should I restrict myself to only 68-70 DTs or Dt2s and DT3s? DT250s, what with there being many more of them, are more common. I just don't want to add to my stock of unusable, wrong model parts.
The bike actually runs very well and really kicks it in second through fifth gears. Maybe I ought to slap a huge rear sprocket on this sucker and forget about first gear completely.
Thanks for your input.
Posted by: originaldirt
I'm not sure how many years basically interchange, up to when they changed the case design I think.
OD
Posted by: mickd
1974 was the year of the major changes with the DT250A, the cases do not look the same as earlier model DT1's. The '74 also had a new frame, I think it is very similar to the earlier MX250.
Posted by: fatboy570
Theres a guy named pryor that posts in this section. He has a bunch of late 60s Yamahas and may be able to help you. I bought some chain adjusters and a few other parts from him and got some real good stuff
Posted by: zoommx
Case splitter definately makes the job easier. I made one myself from .25" x 1" steel. Milled slots for the bolts that thread into the case, and welded a coupling nut for the bolt that drives against the crank. re-assembly is easier, just use heat and cold; I use the freezer and oven.
Roger--491
Posted by: pryor
I have any dt1 trans parts you may need. 
Posted by: firehawkmph
You will probably not need a whole trans assy. More than likely you'll find a bent shift fork or first gear with the dogs worn. Does it go into first and pop out when you gas it? That would indicate worn shift dogs. If first gear is a steady slipping feeling, sounds more like a clutch problem. A manual or parts blowup is nice for reference to make sure all shims go back in the proper places, etc.
Mike
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