Hi all, I'm planning to split the engine on my DT400 twin shock, to re-seal the crankcase halves. The last time I did this job (on a DT125) was in the late eighties. I have a few questions:
1. When splitting the cases, which case should I retain all the innards in? the RHS?
2. Will I need any special tools? I have the tools to remove the flyheel/magneto. I don't plan on replacing the bearings, so presume I won't need a bearing puller?
3. Are there any good videos to watch (everything I can find is for the DT400 MX), or any good blogs / how to articles etc?
4. Anything else I should do 'while i'm in there'
Many thanks............
 

By no means an answer to all your questions…but just my opinion…when you open the patient you should open your wallet. At least you have some experience. My DT400C was my first and I only wanted to do it once. So I sourced all Yamaha bearings and replaced. The old crank bearings were noticeably looser than the new ones. I bought it not running so no history. I would at least plan on crank bearings and of course ALL engine seals…Yamaha seals not after market. Look up the heat/freeze method for bearing removals and installs.

Also check for wear on the big end bearing. While the crank is out is the time to repair if needed. There are wear limits to check in the repair manual. And of course give all internals a good look for worn or chipped gears, galling, etc…

A crankcase splitter will make the job a lot easier but you mentioned you’ve already had them apart so in theory it shouldn’t be too difficult.
The only real advice I can give from my recent experience is buy the crankcase sealant from Yambits, I need to check the brand but it’s from Japan and excellent quality.

Phil, I have a case splitter and crank insertion tool you can borrow.

Me being as tight as a fishes a---s i only replace what needs doing if the rest of it looks & feels serviceable, within reason. But looking at that kickstart spring i'd be tempted to do it as is internal. Not sure if the 'do' fail--but!!. Something else too but forgotten what it was.
www.partzilla.com/catalog/yamaha/motorcy...tarter-250b-c-400b-c

Oh yeah--crank, only way to be sure is to split it but if it looks & feels ok then should be good, feel how much rock it has which can be a bit deceiving as the cage can wear the rod near the edge or about 5mm or less each end 'guessing'. But if it feels ok it'll be good. I could easily find old 214 DT RT rods worn like that.

Not wanting to start you worrying but here's a link to a post i just started on conrod wear.
www.yamaha-enduros.com/index.php/forums/...wear-rod-rock#134459

Hi - thanks for all for your responses.
Pete-RT1 - Pete, many thanks for letting me borrow even more of your tools! I sent you a text on Wednesday.
DT Tim - thanks, I'll order the crankcase sealant from Yambits.
My only outstanding query is which crankcase half should I retain all the innards in? does it matter? will it be obvious when I start splitting it???
Thanks again...

If ya not intending to do mains, & the left one being the tight fit on the shaft plus in the case so i'd pull the right case off as the right side main is a slide fit on the shaft or very light tap with a nylon faced hammer on the end of the shaft to help separate the cases as might resist slightly if it's never been off. Better still use a case tool to push it away if ya have one.Also a light tap of the gearbox main shaft & lay shaft. Just be careful incase something hangs up like the selector drum. Take out the plunger & spring for the drum on the other side because the drum might lift away from it then the plunger will jump in behind which won't matter at that point but sure will for assembly.
Talking right side bearing--If say you had a clean crank on the bench & a new bearing then it should hand push on the shaft with near enough to zero clearance on the right side, just a firm push.--held tight by the crank nut & gear etc which is why i'd go for that side. None of that applies--or reverse applies--when talking late IT175 & IT200's with back to front motors.

Looks like the service manual shows leaving everything in the left side case (Mag side). I just did a 71 CT175 engine last week and did just that, removed the right side case and left everything in the left side. Worked very well. Then i ran duct tape around the left case and main shaft and everything stayed put even when i man handled the engine and removed the crankshaft from the left case.
When you do this, its easy to make sure your in neutral when you go back together and you can go thru the gears by turning the axle drive shaft and turning the shift drum by hand. Engine fell right back together too. No issues at all.