Two Stroke Motocross
Two Stroke Motocross Forum => Technical => Topic started by: beckvall on November 20, 2010, 12:14:27 PM
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Hello! I have a honda 125 SRS 2003, and I caught sight of CR500-f, and every bit eager to convert my 125cc to a 500af. which is the case? how much must be done over again to get the engine to fit?
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Service Honda has built conversions using a CR125 rolling chassis. Of course you will need to modify the frame and many other areas of the bike to get everything to work just right. Stiffer suspension springs would be a must.
I believe that the bike in this article used a 125 frame.
http://twostrokemotocross.com/wp-content/uploads/2008/08/service_honda_500af_vs_crf450.pdf (http://twostrokemotocross.com/wp-content/uploads/2008/08/service_honda_500af_vs_crf450.pdf)
This guy in Europe built his own, this article should help get you started on the right path.
http://twostrokemotocross.com/2009/12/want-to-build-your-own-aluminum-framed-kx500-benny-lindberg-shows-you-how/ (http://twostrokemotocross.com/2009/12/want-to-build-your-own-aluminum-framed-kx500-benny-lindberg-shows-you-how/)
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alright that sounds good. how much more dose the 500 engine weights? the 125 is about 15-20 kilos
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I'm not familiar with the letter designations you used with the 125 and 500. I'm assuming you
have a 2003 CR125 and you want to put a late model (89-01) CR500 engine in it.
I am building something similar right now using a 2001 CR500 engine and a 2008 CRF250 frame.
You will be using a gen 3 frame for your project and it is a fairly straight forward swap.
For frame mods, you will have to cut/grind/clearance, whatever it takes, the "Y" to gain
clearance for the exhaust flange. Both upper and lower engine mounts need to be relocated and
a new headstay has to be fabricated to accomodate the 500 head. Other than the frame mods
you will also have to figure out a way to mount the pipe and silencer.
Here is a link to a website showing many pictures of this same conversion:
http://www.bannedcr500riders.com/board/viewtopic.php?t=1840 (http://www.bannedcr500riders.com/board/viewtopic.php?t=1840)
Good luck with it.
dogger
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this topic made me think seriosusly about an project :D
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I'm surprised that people are able to wedge that 500 engine into the 125 chassis.The CR125 typically was about 1" (24mm) shorter then the larger CR's in the wheelbase and since they used the same basic swingarm and suspension, the loss was in the frame length.
I've done conversions myself and one thing I've learned (the hard way) is that it is critical that the crankshaft centreline be positioned correctly in the chassis both in the vertical and horizontal planes. The crankshaft is a gyro that affects every aspect of a motorcycle's handling characteristics and when you have a crank the size of a CR500 you better make sure it is in the right place.
Engine output, weight and vibration qualities all affect the design of a motorcycle chassis as well. Most modern 4's have engine balancers not for rider comfort but rather, engines that vibrate less have lighter frames. All things considered, 125 frames can be made far lighter then open class frames.Offsetting this is the fact that Honda, as a cost cutting measure, apparently uses the same basic chassis in a number of different applications.
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it is critical that the crankshaft centreline be positioned correctly in the chassis both in the vertical and horizontal planes.
A very true statement. With the 500 conversion, the effort is to get the engine level ("Honda" logo
on the clutch cover) parallel with the bottom frame rail and using different spacers at the swingarm
to lign up the countershaft with the rear sprocket.
The gen 4 conversion I am doing is a bit more comprehensive. I am replacing everything from the
"Y" to the pegs - the whole cradle - in order to gain the clearance for the exhaust flange, right side
clutch cover bolts and to gain a little more room between the head and fuel tank. This was actually
done on the gen 5 frames as Honda lowered the center of gravity 1" to improve handling. I also think
it will be a cleaner, more OEM looking conversion. I also have to deal with airbox/airboot problems.
The stock CRF airbox/airboot does not line up or reach the 500 carb. Most folks fab up some kind
of adaptor plate to mount to the airbox that allows the use of a 98/99 CR250 or other longer airboot.
Other problem areas are the lack of clearance between the fuel tank and spark plug, coil location,
radiator conversion and pipe/silencer. The gen 4 CRF250 frame is what AJ at Service Honda is currently
using for his conversions.
Compared to everything required in a gen 4 conversion, the gen 3 is a much better choice from an amount
of work viewpoint.
dogger
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Hey BeckVaLL you need to take a close up picture of that srs finned head, that,s too cool. Where do you get a head gasket for it. I order a bunch of stuff from the USA srs importer then he went out of buisness in 05.
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allright i can take i pic when i get it back from the engine specialist. i by the Orings at the lokal bearing shop. they hawe all the stuffs from bearings to Orings and more.