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Bottom-End Rebuilds!

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I was just wondering what percentage of riders really rebuild bottom ends in-house. If you do, please comment below any tips or suggestions for first timers. I'm actually looking into a CR250 motor that I could experiment with, as I am too scared to take apart my normal bike. Thanks!

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I was just wondering what percentage of riders really rebuild bottom ends in-house. If you do, please comment below any tips or suggestions for first timers. I'm actually looking into a CR250 motor that I could experiment with, as I am too scared to take apart my normal bike. Thanks!

I'm in the middle of one right now.

Also my first, 05 crf250. I'd suggest a tech manual be first thing. Gives you all the tolerances you will need to know. I'm going to use a micrometer and measure all the gears bearings etc. And feeler gauges for rod slap. Clean everything very well. And reassemble. I may do this differently than other people as well. I work at a john deere dealership and everything has to me checked per manufacturer specification. But I also shade tree and made stuff happen with I had for the other 20 years.

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I tried one and dropped my transmission when taking it out.  had to have someone else put it back together because i kept putting in a fork the wrong way.  but that being said i will do my next one.  the only thing i did was had the crank rebuilt and balanced by our Honda shop.  all the seals and bearings are easy if you know how to remove them.  use a video to see how they are installed.  and get a manual

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take pics as you go can help at reassembly a month later, LOL     Draw pics onto cardboard and punch holes with #2 philips where all the cover bolts go, fill the holes as you take it apart.  Try to take stuff apart as an assembly keeping stuff in order and stowing it separate from other parts gallon zip locks work good.

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I'm going to have to try one day but there are a few steps that I am unsure about and hesitant to embark on. There's too much on the line. I don't want to misplace one bolt/nut/washer/etc... and then start it up, only to have it seize after 2 minutes. 

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The first one I did was a crf250, have done a few since. Buy the right tools, get a manual, take pictures, and keep the trans together when you take it out. Easy as pie, just take your time.

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Already said, but I'll add another - a manual , proper tools and patience! 2 strokes are super easy, as long as you take your time. Label everything, take nothing for granted, and it'll go smooth. At work, I have access to an endless supply of small metal cans with plastic lids (look like coffee cans) On a full build, I'll have as many as 10-15 cans lined up against the basement wall, some of them only holding maybe a few bolts. But everything is separated, organized and labeled, so reassembly goes quite smoothly.

For me, it was well worth the money for the case splitter and crank puller (Tusk brand, $120 eBay) I tried to rig up my own tool, but the right tools may very well prevent a more costly replacement of cases, etc. So I went ahead and bought one. Torque wrenches, feeler gauges, telescopic bore gauge (top end) and micrometer. It can add up, but pay a shop to do one job, and you've already spent the money you saved by not buying the tools.

Like the previous post said, just keep the trans together and pull it out as a unit. Easy peasy. Even if it does come apart, there's not much to them. With a manual, you can easily put it right back together. Only a couple gears will come off the end of the shaft. The rest are held in place by circlips.

Plus, after you do a full build, you get to go on TT and brag about it ;)

Edited by Bron-Yr-Aur

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The right tools, the service manual, zip lock bags and a sharpie, big clean work area, lots of patience and perseverance, mechanical knowledge and some wd-40. There is a lot to it, but it gets easier after several rebuilds under your belt. I don't think it is for beginner mechanics, unless you are really determined. First rebuild might cost as much as paying somebody (or more) after you add all your time and tools.

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Like everybody else said above-get a manual, and label EVERYTHING as you take it apart! If you remove a side cover-keep the bolts that hold that side cover in a plastic bag with the cover. Remove your shift linkage? Keep it all together in a zip lock plastic bag. Just keep all parts together, and it will all go back together easily. I also like to put a paint dab on pieces that can go on either way (like a countershaft sprocket as an example). Either way, I always know the paint dab faces out/away from the engine. when I remove things like the stator, I put the bolts that hold the stator back into the holes after the stator is out of the way. That way when I go to put it back-there are the bolts for each hole already there waiting for me. You can't do this for everything, but anywhere I can, I do. In case you get sidetracked and it takes weeks, or months, or even years to put back together having the bolts in their corresponding holes takes out a lot of guess work.

 

You are working on a 2 stroke so it is MUCH easier than a 4 stroke. Before you start tearing the engine down, clean your workspace. If you drop a clip, or tool, or a small spring, you want to be able to find it easily. Even if you have to buy a bed sheet and use it to cover your work surface, you want to be able to see where small parts fall. The last time I did my 2 stroke I had it complete apart, case halves split, in under an hour. Even for you it will be that simple. Use the RIGHT tools. You don't have to buy a case splitter set up you can actually use a steering wheel puller kit from Harbor Freight and it will work just as well as the case splitter. You WILL need a flywheel puller though so go ahead and buy that now before you get into tearing it down. E-Bay is your friend for those.

 

It's not hard to do really. I think for most it's just the fear of the unknown. If you get stuck on anything, just post pics here and we will guide you through the process.

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I was just wondering what percentage of riders really rebuild bottom ends in-house. If you do, please comment below any tips or suggestions for first timers. I'm actually looking into a CR250 motor that I could experiment with, as I am too scared to take apart my normal bike. Thanks!

I do my own, for sure have a manual & a case seperator (motion pro). Might want to have an experienced freind help you the first time.

Other than that, some common sense & patients. And the very good advice given above.

Edited by Throttle5

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Had mine apart a few times, replaced a couple bearings. 

Service manual has what you need; though there's some shortcuts when you're comfortable with it.

 

I don't have, nor have I used, a case splitter on mine.  Dowel pins line everything up, never really saw the need for one. 

 

It's intimidating the first time you do it.  There's a LOT of little parts/pieces that all have to be in the right place for everything to work (Gear alignment, shims/spacers).  Really not that much to it though.

IMG_20140802_155115755-L.jpg

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If it's something your only gonna do a few times in your life you might considering farming it out. I paid $150 labor to have a lower end done because I was too busy with other stuff.  Guy did about 4 bottom ends a week and knew the motor well enough to check it for cracks and wear in places he's fixed them before. Welded 2 places on my cases I would have missed.  No regrets paying the money.

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It seems intimidating, but it really isn't that hard. As stated here already, have the right tools and take your time, and having a service manual is a must. Putting parts in baggies and labeling everything is highly recommended, and taking pictures as you disassemble will help if you can't remember how something goes back.

 

One tip about installing your new crank bearings is to heat the case halves in the oven, and freeze the bearings. I forget what temperature (not that hot, some searching should find you the right temp), but it heats the cases evenly, and with the cold bearings they should drop right into place. Do not reach for your hammer when installing these bearings. Oh, and you probably only want to do the oven thing when the wife isn't going to be around for a while, for some reason they aren't that fond of the idea of baking engines in the oven.

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It seems intimidating, but it really isn't that hard. As stated here already, have the right tools and take your time, and having a service manual is a must. Putting parts in baggies and labeling everything is highly recommended, and taking pictures as you disassemble will help if you can't remember how something goes back.

One tip about installing your new crank bearings is to heat the case halves in the oven, and freeze the bearings. I forget what temperature (not that hot, some searching should find you the right temp), but it heats the cases evenly, and with the cold bearings they should drop right into place. Do not reach for your hammer when installing these bearings. Oh, and you probably only want to do the oven thing when the wife isn't going to be around for a while, for some reason they aren't that fond of the idea of baking engines in the oven.

You can also use oil on a hot plate and set the case halves in oil. And I believe it's no more than 300 degrees could be wrong tho.

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Watch youtube a couple times first should be fine that's how I learned

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