Honda CR125 crank swap conundrum

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3/4/2018 8:00 PM

Working away on my 1997 CR 125 and after some looking on here I saw that several people have used the Honda crank from a 2005-07 in older 1990-04 motors. I liked the idea of having a solid crank versus the old sheet metal stuffer style so I can easily change rods myself in the future.
I especially liked the new style crank was a little cheaper also (146.00 bucks, crazy for a OEM part)! So I ordered the newer (13300-KSR-730) crank. I knew it was about 80 grams lighter (77 according to my little scale) than the old style but reckoned that might only result in a faster revving engine, bonus on a puny buck-n-quarter right?!?
Well my parts arrived from Partzilla and with childish giddy I opened the box and sifted through the parts with Christmas like joy.
But after the initial buzz wore off I started sorting and assembling, I did some checking on the crank and to my surprise the O.D. of the crank halves was 1mm smaller diameter and the distance between the halves was 1mm wider. This would create a lot less crankcase pressure. Now that's about technical as I get on engine theory and I always recon that the slide rule nerds at Honda know more than 99.9% of us will ever about such things but just wondering if anyone has done the swap noticed this and if so was performance was affected? Also if anyone has a 05-07 engine split and could measure the dia of the crank areas in the cases i'd be curious to see if Honda made them smaller to keep the volume up and thus crankcase pressure up. I know transfer port shape and even pipe shape comes into play at that point, heaven help us at that point...

My thought right now is to return the crank and get the original style. As far as I'm aware Honda CR's never had issues related to this (the trusted engine builder I got to ask about it today said a bike with too much crankcase compression would have jetting issues) and if they did it sure took them long enough to address it, (7 years and we all know in MX thats dog years!)

3/5/2018 9:44 AM

A lot of unknowns there....if you can still get the Oem crank from Honda at a reasonable price I would return the newer crank...them older Honda 125"s were pretty reliable, I"m sure the 97 Oem crank will be fine..

3/5/2018 10:01 AM
Edited Date/Time: 3/5/2018 10:02 AM

Yep that's what I'm doing. New OEM 90-04 crank is only 170.00 (cheapest cranks I've ever seen, can't justify a rod rebuild at those prices)! The 05 crank is going back in the post today.
Hopefully cylinder will be done at recoater's soon and I can get her buttoned up!

3/8/2018 3:54 PM

It’s a beautiful thing! Proper crank showed up on my doorstep today, slipped right in to its new home already. Lesson learned the hard way. And I’ll never know how the other one would have ran but for any poor souls who stumble across this hopefully it will help their decision on what to do.
Oddly Hot Rods only offers one crank for 1990-2007. But I always believe those parts are to fill a void and make stuff run with performance being a low concern to them.

3/8/2018 6:38 PM

I may be horribly naïve, but I would have lost almost no sleep installing that new crank. And I would bet any performance difference would been hard for my butt to notice.

3/8/2018 7:56 PM
Edited Date/Time: 3/8/2018 7:58 PM

Micahdogg wrote:

I may be horribly naïve, but I would have lost almost no sleep installing that new crank. And I would bet any performance difference would been hard for my butt to notice.

Yea. If i were building to sell I would have, hell i'd have left the old one in it, a kid could ride it around the yard forever as good as it was. But I'm going to race it this year at Unadilla (the home of horsepower, LOL) and there is about 35 lbs more of me than there was in 1997 so every pony counts. Not to mention I'm used to modern 450's. Recon I'll be wringing the guts out of this poor little thing! Can't wait!!!

3/8/2018 9:30 PM

Gotcha, yeah better to not experiment too much then.