RC390 Update to leaking oil - Crack in engine case

samroy92

New Member
I replied in the other thread you posted about the leak pattern being exactly the same as mine. As soon as I replaced the shift shaft seal and sprocket shaft seal the leak went away. Just saying... could be that crack but I would check to see if it leaks without riding the bike in the same spot.

Thanks for the advice, I will check into this again. It seems from the oil location that it is seeping from a crack - not sure how it could travel from the shift seal. But it's worth a shot still.

Thanks,
 
Ever do anything about yours? I have the same leak too... [emoji17]

Sent from my SM-G960U1 using Tapatalk

My 390 is still in the garage...never got around to properly addressing this yet.

Taking into consideration that one of the rearset bolts goes into a through hole in the engine, I am going to look at that area first.

SUPER frustrating...
 

LunarModule

New Member
Welp, looks like mine is cracked in the same spot. It looks to me the stress of the bike on and off the kickstand. Maybe that has something to do with it. The mount next to it is basically that.
c74a202f33fe432617d0f896a4cbf4fc.jpg


Sent from my SM-G960U1 using Tapatalk
 

ReidMcT

Active Member
Premium Member
Elite Member
Site Supporter
Country flag
Just thought I'd follow up on this thread. I bought one of the broken bikes mentioned above and I did the crankcase swap. I'm attaching a few photos.

The crack was right where others above were. It was VERY hard to find. I used the magnifier on my phone and found it; almost invisible to the naked eye but showed up well on camera when I angled the light just right.
RC390 crankcase crack.jpg
Interestingly, I looked and looked, but couldn’t find a corresponding crack on the inside of the crankcase. Amazing how much oil this little crack could leak in such a short time.


Disassembly was not too difficult, and I found the service manual, which has many weaknesses, was excellent on engine disassembly/reassembly. I congratulated myself for buying three KTM special tools: the flywheel puller, the large castle nut wrench, and the wrist-pin circlip installer.
RC390 engine disassembled.jpg

RC390 major parts transplanted.jpg

It was difficult to align everything simultaneously when joining the case halves and trying not to smear the sealant or jam anything the wrong way. It would have been easier if I had the KTM special engine holding jig, but it is really expensive and I could do without it.

Anyway, though it was a painstaking job, it really wasn't all that hard. I took the opportunity to clean the piston, rings, head, and valves, and I replaced the cam chain that had a minor kink. Plus all new gaskets and seals, including the improved water pump seals.

I started it and took a short test ride today. Nice.
RC390 finished.JPG
 
Last edited:

Fasteddy

Member
Country flag
Just thought I'd follow up on this thread. I bought one of the broken bikes mentioned above and I did the crankcase swap. I'm attaching a few photos.

The crack was right where others above were. It was VERY hard to find. I used the magnifier on my phone and found it; almost invisible to the naked eye but showed up well on camera when I angled the light just right.
View attachment 6469
Interestingly, I looked and looked, but couldn’t find a corresponding crack on the inside of the crankcase. Amazing how much oil this little crack could leak in such a short time.


Disassembly was not too difficult, and I found the service manual, which has many weaknesses, was excellent on engine disassembly/reassembly. I congratulated myself for buying three KTM special tools: the flywheel puller, the large castle nut wrench, and the wrist-pin circlip installer.
View attachment 6468

View attachment 6470

It was difficult to align everything simultaneously when joining the case halves and trying not to smear the sealant or jam anything the wrong way. It would have been easier if I had the KTM special engine holding jig, but it is really expensive and I could do without it.

Anyway, though it was a painstaking job, it really wasn't all that hard. I took the opportunity to clean the piston, rings, head, and valves, and I replaced the cam chain that had a minor kink. Plus all new gaskets and seals, including the improved water pump seals.

I started it and took a short test ride today. Nice.
View attachment 6471
Just curious, which new water pump seals did you use?
 

ReidMcT

Active Member
Premium Member
Elite Member
Site Supporter
Country flag
450 EXC seals, same ones I used in post awhile back, do you have your old cases? would be nice to figure out the weep hole but I need to see the inside of the casting for a measurement.
Yes, tell me what pics/measurements you need and I'll do what I can.
 
Top