So I am doing a complete build/rebuild on my 23 yz250x and may or may not have made what I would call a noob mistake. When I went to install the new crank bearings, I did not heat up the cases enough and ended up having to tap the bearings in. No big deal I thought, it happens, continued with assembly. Got cases together and there was a slight side load on the crank bearings. Lightly tapping the inner races did not remedy. Tore it back apart and ordered another set of crank bearings. When tapping the bearings in, they had scored the bearing seats in the case halves and failed to seat properly. Now this is where I may have made a mistake. I used very fine grit sandpaper to smooth out the scoring and make the bearings seats smooth again. Got the new bearings, made sure I heated the cases up enough this time, and the bearings fell right in like they should. What I'm worried about is whether or not I ruined the bearing/case fit by using sandpaper to smooth the surfaces out. For about a minute or two after I dropped the bearings in, I could spin the outer race in the case by tapping it with a punch. No longer can after they cooled down though. How much of a risk is there on these of spinning a crank bearing? Has anyone here done what I did and had it fail or not fail? I don't want to waste my time on assembly if it's just going to fail on me
For sale: just rebuilt…going in different directions…
Lol. Definitely won't be for sale. Im in way too deep in this thing at this point to not make it perfect. I really can't find anything anywhere on the net about whether or not a little light sanding is enough to make a crank bearing spin. If it is I'll just buy new cases and do it again. If it isn't an issue though I don't wanna spend more money for nothing
Did you sand the entire bearing bore, or just the area it galled? I’d probably run it if it were mine, but you can replace the bearings again and use Loctite bearing retainer if it makes you feel better.
The whole bore with 1500 grit. There was scoring most of the way around
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If you're that worried, pop it back out and reassemble with a bearing retainer epoxy.
Try warming up the case halves around the main bearings with a heat gun and an IR thermometer to get it up to 200~220 F to simulate running conditions (i.e., specifically get to those temps and don't overshoot it by going hotter) and see if you can spin the outer races within the cases and/or easily lift them out.
If they stay in place, and if you were relatively even all the way around with the light sanding you did to the aluminum like you mentioned in your last post, I'd be confident that the main bores are still plenty concentric with the right amount of interference-fit to the bearings, making it a-ok to run.
Thank you for the very well thought out reply. I did just as you said and found that they are still pretty tight in there. However, I went ahead and ordered a new take off set of case halves anyway. I am super OCD about this stuff and I know if I put it back together I'd be thinking about it the whole time I'm riding it whether it fails or not lol. Mistake made, money lost, lesson learned. I don't feel too bad however since I have no plans of ever leaving the yz platform so having an extra set of case halves to use in an emergency ain't too bad an idea.
That's a safe call. I've never been mad at myself for being too thorough with my bike work.
You will most likely be okay but if you are worried, take them back out and use green bearing retainer loctite and put them back in place. If it is that bad, you can stake the bore where the bearings go. I've used the green loctite on lots of older cases where the bores become oversize
Short of us guessing or giving anecdotal experience, any machinist or builder is going to tell you there is only 1 sure way to know. Disassemble and measure the bearings and bores to determine if you still have the correct interference fit. Pay extra attention to the left side case. The YZ is already known for being prone to cracking around the bearing pocket on that side.
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