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Author Topic: Yamaha 250 gearbox swaps.  (Read 7386 times)

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Offline Nathan S

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Yamaha 250 gearbox swaps.
« on: August 01, 2013, 07:36:50 pm »
So... Imagine I had a YZ250K or L and wanted to fit a wider ratio gearset. Does anything fit?

YZ250G 6-speed?
IT250H/K?
IT490 5-speed?
IT250K/L?
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Offline ARN

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Re: Yamaha 250 gearbox swaps.
« Reply #1 on: August 01, 2013, 08:29:14 pm »
yz g and h main shaft (clutch) is shorter then 83 84 yz's ,I tried fitting 84 gear box to yz 250g with no luck. 
the cases are wider on the newer models by about 5mm, the it's with the same cases or 490's possibly work
cheers arno
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Offline rocketfrog

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Re: Yamaha 250 gearbox swaps.
« Reply #2 on: August 06, 2013, 08:00:32 am »
Arn, are you saying the 83 and 84 IT cases are wider than the 1980/81 YZ and the 1981/82 IT 250's?

It looked as though the complete gearbox would swap over if you use the matching clutch.

That is to say you could use a 1981/82 IT transmission in the 1983/84 cases if you also use the 1981/82 clutch assembley. (have not attempted to do so- just a casual observation)

The clutch used on the 1983 IT250 is a 23X number the same as YZ250/490K, differences in the splined end of the input shaft (as you have said) do not allow the clutches to interchange.
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Offline ARN

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Re: Yamaha 250 gearbox swaps.
« Reply #3 on: August 06, 2013, 08:34:15 pm »
not sure about IT's but the 84 yz  main shaft is longer, on the 80 81 cases the bearing opposite the clutch is not parallel with sprocket bearing ,I haven't got an 84 in pieces at the moment but I think they are parallel.
i think you can use the newer clutch ,they have a straight cut gear so you would have to use the primary drive gear of a 490 with the key way not the splines as in the 250's.
cheers arno
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Offline Nathan S

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Re: Yamaha 250 gearbox swaps.
« Reply #4 on: August 06, 2013, 09:28:01 pm »
You blokes are awesome - I thought it was only me that kept stuff like that in my head!
Keep it coming! :)
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Offline rocketfrog

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Re: Yamaha 250 gearbox swaps.
« Reply #5 on: August 12, 2013, 11:03:32 pm »


Top photo is 3R4 Case (RH) from YZ250G 1980

Bottom photo is 29V Case (RH) from IT250K 1983

Note the position of the crankcase breather fitting, adjacent the kick starter housing on the 3R4 and exiting the rear of the cylinder mounting flange on the 29V. Both parts carry the 3R400 Identifier code for the casting which is on the reverse side (see next photo).

Apart from this detail, the cases are identical in all dimensions and will have the the same form fit and function.
« Last Edit: August 12, 2013, 11:19:06 pm by rocketfrog »
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Simo63

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Re: Yamaha 250 gearbox swaps.
« Reply #6 on: August 12, 2013, 11:22:30 pm »
Quote from: rocketfrog on August 12, 2013, 11:03:32 pm


Top photo is 3R4 Case (RH) from YZ250G 1980

Bottom photo is 29V Case (RH) from IT250K 1983

Note the position of the crankcase breather fitting, adjacent the kick starter housing on the 3R4 and exiting the rear of the cylinder mounting flange on the 29V. Both parts carry the 3R400 Identifier code for the casting which is on the reverse side (see next photo).

Apart from this detail, the cases are identical in all dimensions and will have the the same form fit and function.

That is awesome Jason. I have a YZ250H and also a spare set of IT250K cases. I thought they looked the same but never went right into it but I now will keep those cases as spares for the YZ if I ever need them  8)
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Offline rocketfrog

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Re: Yamaha 250 gearbox swaps.
« Reply #7 on: August 12, 2013, 11:23:07 pm »


You can see the 3R400 Identifier above the Input Shaft Bearing Housing in the centre of the cases.
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Offline rocketfrog

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Re: Yamaha 250 gearbox swaps.
« Reply #8 on: August 12, 2013, 11:37:45 pm »


Once again, the top case is the 29V IT250K Case and the lower is the 3R4 YZ250G Case.

Notice the slight variation here, the IT Case features an additional sprocket cover mounting boss adjacent the output shaft bearing housing.

The timing scribe marks are placed differently for the two engines, if you look close you can see the scribe mark filled with chalk to highlight the position on the black 29V case at approximately the 12:30 oclock position.

The sandblasted 3R4 case is a little more difficult to see, but it is at the 3:00 oclock position (approximately).

As per the right cases, the casting carries the 3R400 Identifier and is the same dimensionally.
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Offline rocketfrog

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Re: Yamaha 250 gearbox swaps.
« Reply #9 on: August 12, 2013, 11:50:45 pm »


Shown here are the clutch spline ends of the input shafts. Compare the top photo (YZ250G 3R4) with the lower photo which happens to be a YZ250K (24Y) in this case.

Notice the length of the splines on the 3R4 Shaft are longer than the 24Y shaft. The spline count and the shaft diameter is the same, the length of the shaft from the first gear pinion face to the point where the shaft is reduced for the thread is the same but the actual threaded extension where the clutch retaining nut mounts is longer on the 3R4 shaft (threads are the same).

Actual dimensions and more on this topic coming soon. In the meantime if any one has an IT250K Input (pinion shaft) to compare the clutch end as per the examples shown I would be interested.
« Last Edit: August 12, 2013, 11:56:05 pm by rocketfrog »
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Offline Nathan S

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Re: Yamaha 250 gearbox swaps.
« Reply #10 on: August 13, 2013, 08:47:35 am »
To quote Dennis deNuto:
"forking Brilliant!!!".

:)
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Offline rocketfrog

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Re: Yamaha 250 gearbox swaps.
« Reply #11 on: August 16, 2013, 11:11:00 am »
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Offline rocketfrog

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Re: Yamaha 250 gearbox swaps.
« Reply #12 on: August 16, 2013, 11:11:49 am »
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Offline rocketfrog

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Re: Yamaha 250 gearbox swaps.
« Reply #13 on: August 16, 2013, 11:12:52 am »
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Offline rocketfrog

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Re: Yamaha 250 gearbox swaps.
« Reply #14 on: August 16, 2013, 11:13:40 am »
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