How well do you like your KX250F/K5 conversion

[sandblaster]

I recently acquired a 07 KX250F donor (2 hours ago). I already have a spare 99 K5.
I read all the articles in this forum on builds that have been done.
I also spoke with another member that seems to like his.
However, before I dig in and go for it I was wondering if any others have any input on this.
I'm not asking how to do it as there is plenty of info here on that.
What I would like is a more long term analysis of the pros and cons on handing, power to weight ratio, longevity, the type of riding being done, is there anything on the build that you would do differently, ect.
I do not race on MX tracks any more but I want to ride this Desert/Dunes so it has to be able to handle speeds 90+.
Any info would be greatly appreciated.


I just purchased a 17MM Drill bit.
Does that mean I'm now committed? 

[don46]

I have not used a 250f for the conversion, I used a 450f for my 500 conversion, however, I recently did a 250 2t into a 250f frame and felt that the small 2 stroke fit fairly snugly and wondered how the 500 would fit. My thoughts on this were as follows, use frame rails that JFAB manuf. they would be more suited to the 500 motor than modding the 250f frame rails. it seems there is plenty of room under the tank or at least that was the case with the 250 and the 500 isn't that much taller.

So now get ahold of Jerry and get them frame rails coming and start cutting.

[sandblaster]

Yeah, it's going to be a very close fit but it is doable.
I was going to order those rails but I am having a time getting in contact with anybody.
I guess with the holidays he must be very busy.
I know we are swamped up to our eyeballs 

[Good]

Besides, the 250f is what ServiceHonda uses in their conversions.  It should work fine, but you may need one of those stubby spark plugs.

[jfabmotorsports.com]

Sandblaster.....What did you decide to do?

JFAB

[sandblaster]

Bye bye Donor bike 

It's being disassembled as I write 

Nooooooooooooooo!!!!!!!!!!!!!!



Please, someone stop me......



Too late 

[sandblaster]

12-30-10
I went ahead and purchased some brand new engine cases for my engine build.
This left me with a nice set of blanks for my Mock Up Engine.
Installed a back up cylinder and head .
Now, it's time for a big drill....

[sandblaster]

Now you see it.....



Now you don't.....



I used my Sawsall with a fine metal cutting blade.
Then, after I removed the mount I used a hand grinder and then a orbital sander to smooth it out.
I know it's not necessary as nobody will see it.
Yet for some reason I will feel better 

[sandblaster]

Today I received my Mock up engine back from the machine shop I had drill my 17MM swing arm bolt hole.
As a side note, I bolted the case halves together with a factory gasket.
I had the machine shop set the cases on the clutch side with the clutch cover off on a set of parallel blocks and then drilled it to maintain perpendicularity.
Drilling was very easy.
Before I tried to fit the engine I did a test fit with the swing arm bolt.
I had to use a rubber mallet and hit it hard to get the dadnabit thing in.
So, I wire wheeled the pin and de-burred the hole.
It was still too tight.
I grabbed my digital veneer calipers and see that the holes through the frame are 17.05MM.
I guess they were allowing for manufacturing tolerances.
Also, I put a straight edge on my pin and it was nice and straight.
Since I do not have a ream of the correct size I went to the local auto parts store and picked up a wheel cylinder ream.

After about 15 minutes of oiling and reaming it finally fits snug but can be put in by hand.
So, the results for today are as shown below:


If I was going to order another drill, I would not use the 17mm.
I would use a 43/64" as it is 0.672" or 17.06 mm.
This is only .01mm or 0.0004" larger than the holes in the frame.
Now that I see how it fits, it's time to cut some more on the frame.
I better order some frame rails soon.....

[sandblaster]

11-5-11
Old carriage goes to the recyclers...
Not really, keep all the pieces, you never can tell when one will come in handy


Next

Next

Next

Now.... it is too late 
I am also keeping track of cost and the offset of selling the left overs.
I will post all info when it is done. (Gulp)

[stevea100m]

I would save the yoke and do it like im doing my build so you don't haveve to make your own

[sandblaster]

I considered that.
However, I like the looks of the clean unadulterated frame tubes.
It was not a matter of function, only looks 

[sandblaster]

Ordered the frame rails today from JFAB.
THANKS!

[sandblaster]

1-12-11
Ok, I've been really busy and have not been able to do much to the bike.
I did manage to do a bit more today.
Just for grins I went to mount the Swing arm to the engine and frame.
Guess what? It does not fit.
The engine cases measured 94.3MM or 3.713".


The inside to the inside of the swing arm bushings when pushed toward the center as they would be when mounted measured 93.95MM or 3.699".


That leaves a interference fit of .35MM or .014".
My solution was to remove material from the Collars that are on the outside of either side of the swing arm.
The book shows that they are 6MM thick.
However, I measured them at 5.95MM or .234" thick.
I precision ground (Bench grinder and a 17MM rod) each to 5.59MM or .220" thick.


Everything fits together nicely.


These Collars are hard anodized so, I may have to have them anodized again depending on how fast the ground soft aluminum surface wears.
My next project is to check the swing arm to right engine case clearance through it's full range of travel as I believe it may be a bit too tight and I may have to machine the swing arm a bit to make it fit correctly.
I wouldn't want to bottom out my suspension and crack my right engine case 

[livenlife699]

Looking good! I just drilled out my cases on sunday with a 43/64" and it was perfect, No fitment issues at all I was able to mock it in the frame with swingarm in about 15 minutes!!