YZ250 Bottom End Assembly- How To / Walkthrough

Here is a detailed walkthrough of how to do an excellent job rebuilding the bottom end on a '99 through current YZ250.

[@camilogomez6538]

this guy really knows what he's doing

[@andrewdi8561]

Great video , Never new about releasing the side load , thanks for sharing your knowledge

[@ntinkham20]

Wow! Awesome tutorials, Chuck! I learned what I got wrong on my first bottom end rebuild years ago. The tap of the inner race of left side crank bearing to free up the crank. I'll never make that mistake again now.

[@Dobemandan]

Nice work 👍🏻

[@RamRaidAt4am]

When you installed the clutch side crankcase seal initially it appeared that you installed it with the spring facing outwards ( as I believe it should be? ) but only moments later in the edited video it appears you reversed the seal and now the spring was facing inwards towards the crank/centre of the engine. Is the general rule not that the spring faces towards the medium that it is sealing?

[@ChuckfromTrueTech]

The purpose of the crank seal is first to seal in crankcase pressure. Of course it is also responsible for preventing oil from entering the crankcase but that is secondary.

[@Thepianobird]

very informative video. At first I did think you realized the the seal can rub on the bearing. Do you have a source for the bearings? Subbed...

[@ChuckfromTrueTech]

The bearings are easy to cone by if you search by size. YZ250 uses 63/28 C3 or C4 bearings. Just buy Japanese, IE: Koyo, NSK, NTN etc. I rarely buy OEM.

[@Dsm152]

Interesting. Never thought of pulling the crank into the case via the inner race of the main bearing, if you went too far you would start to press the bearing out of the case though? I see why you choose your own method for ease of setup however it doesn’t always go that smoothly when using heat. If using heat to put cases together you have to be very well timed and ensure your puller is close by ready to go if the bearing suddenly stops. First few engines I ever built I was scraping and cleaning the yamabond off the cases and starting again

[@ChuckfromTrueTech]

You won’t press the bearing out of the case, but you can put side load on the bearing the opposite way or pull the crank out of true. You’re right that it can be tricky to get the timing right with heat.

[@Dsm152]

@@ChuckfromTrueTech I need to try tapping the inner race of the main bearing on this RM125 I just built, crank is very slightly resistant to turn. Used a puller on both sides to seat the crank however still not perfect. Will remove the crank seal and try hitting the bearing lightly

[@motown0723]

Where do you purchase your bearings? Are they koyo?

[@ChuckfromTrueTech]

I have several suppliers, but most of my bearings come from Winners Circle in Toronto. The bearings will be Japanese. Either Koyo, NSK, Nachi or NTN.

[@Chris-uh3cm]

Thanks. Can I send you my engine / transmission to be worked on?

[@ChuckfromTrueTech]

Definitely.

[@Chris-uh3cm]

@@ChuckfromTrueTech great how can we exchange info? Do you have a website?

[@ChuckfromTrueTech]

@@Chris-uh3cm Yep. www.truetechmc.com. My email is [e-mail].

[@IBotBurchell42097]

Crank dropped in nicely, but i am worried i overheated the bearing with a heatgun 😢

[@ChuckfromTrueTech]

If the bearing turned blue it’s overheated.

[@IBotBurchell42097]

It didnt go blue like the bearing ive just purposely overheated, my concern is because as i rotate the crankshaft, holding the case i can can hear almost a train wheel sounding affect along with feeling holding the case, first time doing this so i am abit clueless, im probably gonna just run it and if all fails learn from my mistakes, costly tho 😅 thanks for the quick reply , and also the video helped alot thanks

[@IBotBurchell42097]

Well i pushed the crank back out, I did overheat the bearing and warp it slightly not that i got it blue buttt a golden colour on one side of the inner and now has a notchy feeling on that bearing causing that feeling i was one about, as the bearing in the other case is smooth like butter, just purchased a laser heat gun temp guage lol, once again thankyou for the video

[@axkoula7851]

This is attention to detail. I would have never thought of bearing side load but on the other hand I have never assembled a bottom end myself.

[@ChuckfromTrueTech]

After probably over a thousand bottom ends, this is my preferred method.

[@Rusty-Brown_]

******NO SEALS ON TRANS BEARING ???? HOW ABOUT CLUCH OILING ?????????

[@ChuckfromTrueTech]

Why would you want a seal on the tranny bearings?

[@Rusty-Brown_]

for oilinh on clutch shaft i saw a vid on it@@ChuckfromTrueTech