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Swing arm for my 1974 MX250

01 Sep 2016 09:55
SlowPoke
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Swing arm for my 1974 MX250 #1
How do you guys weigh in on installing a DT250 swing arm on my MX. I have a leftover one from a 1973 DT3 that is about 1" longer. Is there any evidence that it will noticeably improve the handling? If so do I have to install longer shocks? Or is this pointless............

"Creativity without control is merely chaos"
1970 AT1M
1971 DT250 Motocross
1976 TY175
2018 WR250R
Hammond Organ
Fender Bass

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11 Oct 2016 20:23
MotoDave0
Sportsman
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1 enduro and 3 vmx. All Yamaha's!
Swing arm for my 1974 MX250 #2
I am not sure if this helps answer your question, but.
I am putting a 74 MX250 in a Profab frame that I am close to finishing.
And after reading your question I walked over and measured the Profab's and stock MX250 swing arms.
I measured from the center of swing arm bolt to the front and back of the opening for the rear axle bolt.
Here is what I got: Stock 16.25" to 18.25" - Profab 17.25" to 19"
As far as handling not sure have not rode the Profab yet.
I can say this though, my Trackmaster framed AT1 has a 1.5" longer swing arm than stock and handling is much beter.
You can use the same shock if the mount is in the same place as the stock one is.
Hope this helps

74 MX250 in a Profab frame
70 AT1 in a Trackmaster frame
72 RT2MX
72 DT2MX
74 MX360
70 CAT 400x Mini bike

Future rebuilds:
73 AT3
74 YZ80
71 JT1L 60

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12 Oct 2016 08:02
SlowPoke
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Swing arm for my 1974 MX250 #3
Can you post some pictures of your project? Sounds really interesting! My measurements show a DT250 swingarm to be +- 1.25" longer than the MX250 swingarm. I had an old DT chassis kicking around and thought it would be interesting to try the swap but sadly the frame sold. I was just curious to see if anyone on the site had actually ridden a bike that had been modified in this way. I'm VERY interested in what you are doing.
Bruce

"Creativity without control is merely chaos"
1970 AT1M
1971 DT250 Motocross
1976 TY175
2018 WR250R
Hammond Organ
Fender Bass

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20 Apr 2018 07:34 20 Apr 2018 08:00 by Taps.
Taps
Sportsman
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Swing arm for my 1974 MX250 #4
I'm also curious about this! Been wondering about doing something similar with my mx360. I had read somewhere that 1" longer provides a bit of an improvement without downsides of going too long.

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20 Apr 2018 08:59
Scootern29
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Swing arm for my 1974 MX250 #5
Going with the longer swing arm will help reduce the Yama "hop" and make the bike more stable. It should help a little with cornering as it will move a little weight forward in theory. 1 inch shouldn't effect the shocks. Try it, if you don't like it swap it back.

Question authority, think for yourself!
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20 Apr 2018 09:05 20 Apr 2018 11:08 by Taps.
Taps
Sportsman
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Swing arm for my 1974 MX250 #6
I have a dt swingarm on its way, but since you have it sitting there... Is the shock mount the same distance from the pivot on both arms?

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26 Apr 2018 21:20 27 Apr 2018 07:41 by Makotosun.
Taps
Sportsman
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Swing arm for my 1974 MX250 #7
I received my '74 dt250 swingarm today, and it is...the same length as my '74 mx360 arm: 16.25" to whatever. On the upside, I can run passenger pegs on my mx now.

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26 Apr 2018 21:29
Taps
Sportsman
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Swing arm for my 1974 MX250 #8
Just checked - '74+ DT uses the same length as MX. Apparently have to use '73 arm. The more you know!

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11 May 2018 23:11
Taps
Sportsman
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Swing arm for my 1974 MX250 #9
The saga continues. I have a pre '74 swing arm on the way. If nothing else, I can compare the pre '74 dt, '74+ dt, and the 73-74 MX swing arms since I will have them all here.

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14 May 2018 12:22 14 May 2018 14:21 by Taps.
Taps
Sportsman
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Swing arm for my 1974 MX250 #10
I know I'm the only one playing along here, but for those that come along later, yes, the pre-1974 DT swing arm is longer than the MX arm. Shock mounts are also moved back. I'm particularly happy about this because I went with Noleen's recommendation of 14.75" shocks. General wisdom on the internet is 14" shocks and xl500 progressive springs up front. I've also heard of 14" with an 1" longer swing arm. And then 14.5+ with 1-1.5" longer arm. I'll post more pics/measurements later.

Pictured are the '74 and '73 dt arms. '74 on top. Swing arm bolt holes are lined up.

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