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Discussion Starter · #1 ·
Hey guys, very excited to say tonight I will Be picking up a 74 yamaha dt250, and well I had been doing some research on these bikes and already have a few questions 1.) first question (and a bigger deciding factor on the build). I've owned a little 2 stroke chopper when I was younger, only got to ride it a couple times before it got stolen but I was young nd dumb ne ways so probably better off lol

I've noticed that this is a popular cafe style? Racer and I like that look, but I'd like know exactly what potential she could have, can I install a more powerful, modern engine to the 5 speed trans, can it be done period? Lol

2.) I've found the fork conversion for the bearing size, but how exactly do iknow what can fit and what's the right length?

3.) is there any kind of modification that can be done to the rear swing arm? Aka modern conversion swap

4.) if by doing any of this can I get away from a tubeless tire withouthaving to tape the inside of the wheel, I'f this is all possible I'd hate to run tubes lol and if I can find a conversion to disc brakes would be greatly appreciated also
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| ---- and as far as the brakes go am I correct to understand that doing a fork conversion would at least gain me front disc? Thank you very much guys, happy riding.

Also im sorry if this has been asked 100000000 times lol i did search
 

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The friendly Ghost.
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1) I do not know of any modern engines that have an external trans that may fit the frame well. You may be able to remove both the engine and trans and work a modern 250CC 2 stroke dirty engine.

2) Fork swaps are relatively simple if you get the entire front end from another bike.

http://www.clubchopper.com/forums/m...-tube-sizes-neck-bearings-axle-diameters.html

This link has your bike's steering stem bearings listed as:
Upper: 25 x 43 x 11
Lower: 30 x 48 x 12

That means the steering stem is stepped from 30mm on the bottom to 25mm on top with the head stock diameters of 48mm on the bottom and 43mm on the top with 12mm and 11mm heights respectively.

This brings up a problem as the XXx43x11 and XXx48x12 bearings are not very common, so you are left with the choice of sticking with your same fork diameters or finding a steering stem of the right length and 30mm bottom and 25mm top. However, this opens up the possibility to many Suzukis and Yamahas that I can think of.

XJ600
FZR600
YZF600R
GSF600
GSF1200
SV650

There are many more on the list, just make sure the steering stem length is long enough to go through the head stock and short enough to allow the nut to provide clamping force. These will take your forks from 34mm (?) to 38mm, 38mm, 41mm, 41mm, 43mm, 41mm respectively. The FZR600 and YZF600R can use the YZF-R series conventional brakes.

3) I do not know of any that are close, most swingarm swaps require a bit more work, I would assume that this bike has a fairly narrow frame and therefore most modern swing arms and wheels will cause issues with chain alignment and frame contact...

4) Best way to upgrade to disc brakes and tubeless wheels is to swap the wheels out. You may be able to stuff an XJ600 rear wheel into the swingarm as it is a 3.5x17 and when looking up the stock DT250 tire size its showing 4.0x18, you will just have to worry about the chain clearing the sidewall of the tire, the width at the hub compared to the width of the swingarm, the rear wheel bearing sizes and the difference in chain alignment between the XJ600 sprocket and the DT250 sprocket.

(I can get you most measurements from an XJ600, some from the front end of an FZR600, all measurements from an SV650 and some measurements for a few other bikes I have used parts from: YZF600R, YZF-R6, YZF-R1, GS500 and GSF600.)

Hope this helps!
 

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Discussion Starter · #3 · (Edited)
Honestly, thank you for answering my newby questions lol, i would honestly just like to know on the motor to go to something newer, this thing is 40 years old lol, it is now the oldest vehicle i own (but only by 4 yrs :p)

And 2.) thank you for putting up with my newbersterness lol, i was just assuming to do diffrent/newer wheels i would have to change my swingarm for diffrent brands etc. i guess i just didnt realize you could interchange the wheel from the sprocket. and what your saying sounds exactly like what im gonna have to do, just find a diffrent wheel, and i work in a tire store so i realize that means i can go to a taller beefier 17, as long as i stay the same height overall correct? Any idea on a size that might be close?

And as far as the forks go i guess im kind of lost lol i do understand im probably going to have to use a longer stem, i read on another link with these measurements, that makes this alot harder, but i guess i dont understand how there measured, but i do understand that if these arent correct the frame wont fit the forks, because that is my plan to just swap a diffrent front end under it,now do these measurements also give me the rake angle?

Thank you very much :)
 

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Is my bike ok?
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You can always swap your existing steering stem in to whatever front end you decide to use. You wont be on the hunt for bearings that way.

There's pros and cons to both ways (stem swap vs bearing conversion), but I like stem swaps.

Also, you wont be interchanging a new wheel to your existing sprocket. If you do a rim swap you will use the cushion drive / sprocket carrier for that rim. You'll just swap that whole wheel unit in place of yours. So when you go shopping keep in mind which side of your bike your chain is on now. That's not to say that you can't run a rim backwards, but personally, I don't like to reverse cushion drives
They are somewhat directional. But on a light, lowish powered ( no offense) bike like yours, it probably wouldn't be a big deal.

A member on here ( NeverEnough) has a CRF swingarm for sale in the classifieds. Since it came off a dirtbike it will have a narrow pivot like your bike probably does. Kinda long, though.
 

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Discussion Starter · #5 · (Edited)
Shinyribs - thank you for the info, as far as keeping my older stem, thats a great point and ill look into that, bit ive also read all over the place that the bearings for older bikes like this are harder to find, and in the shape the bikes in now the forks will need rebuilt anyways, just had it rolling yesterday and hit the brakes and it plumitted like 12 inches lol (figures as much) so if i coyld i would really just like to swap to a diffrent/ more modern fork set anyways, he had an extra set of triple trees with the bike, could i just hunt for the rest of the parts or would just doing a swap be easier(going to rebuild the newer forks when i find what exactly i can use)

and on the wheel swap, i undertandcompletely about the drive side, but does that mean that most rear wheels/sprockets have the same amount of teeth/same mounting dimensions? and are the axles the same (as in from wheel to wheel key being side of drive? Ex ford vs chevy have different bolt patterns?)
 

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Is my bike ok?
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Yeah, it definitely sounds like your fork springs are shot. :D

There's a bunch of different bolt circles ( motorcycle guys tend to call it PCD : pitch circle diameter) when it comes to sprockets. Not all use the same tooth count and not all use the same tooth itself. And its a good thing you thought about that, because you may be limited to whatever will match up to your primary, or engine mounted, sprocket. Your front sprocket is probably a 530 or a 630. Check and see what is available for your bike, you may have to hunt for a wheel that can accept the same pitch (520,530,630 etc) sprocket as what you're using now.

Did that make sense at all?
 

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Discussion Starter · #7 ·
Yeah, it definitely sounds like your fork springs are shot. :D

There's a bunch of different bolt circles ( motorcycle guys tend to call it PCD : pitch circle diameter) when it comes to sprockets. Not all use the same tooth count and not all use the same tooth itself. And its a good thing you thought about that, because you may be limited to whatever will match up to your primary, or engine mounted, sprocket. Your front sprocket is probably a 530 or a 630. Check and see what is available for your bike, you may have to hunt for a wheel that can accept the same pitch (520,530,630 etc) sprocket as what you're using now.

Did that make sense at all?
ABSOLUTELY lol and yea it does, almost the same as the offsets, whats the best way to measure the sprocket teeth? And obviously the wheel sprocket is going to be bigger, im no good at gearing lol would just a simple google search tell me thge information i need?
 

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FNG
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chain size is measured in 1/8 inch increments. 530= 5/8" c-c of rollers, 3/8" between inner link plates. if you are just measuring sprockets, tooth to tooth and sprocket width. along with wheel and tire choice, any change in gearing is going to affect acceleration and top speed. the old steel frame dirt bikes like the DT are pretty much wide open to just about anything you want to do to them. there are options for tubeless spoked wheels that you may be able to lace to your hubs, though probably not cheap.
http://caferacerspecial.blogspot.com/2011/07/yamaha-dt250-bobber-by-gravel-crew.html
good luck and have fun, them little smokers can be a blast, or a giant PITA.
bb7
 
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