Coolant leaking from crankcase

5 replies to this topic
  • chrisleika

Posted April 10, 2013 - 01:41 PM


Some time ago i discovered that coolan are leaking from the hole in the crankcase... bike was in the garage for a month, and i discevered coolant on the ground... why this small hole in crankcase is for??? Too big preasure in cooling system???
Posted Image

Posted Image

Bike Yamaha WR400F 1998.

  • Krannie McKranface

Posted April 10, 2013 - 02:23 PM


That is a 'weep' hole.
It is there to let you know when the impeller shaft or impeller seal are wearing out.
If the bike sits for a long time, the seal becomes less compliant, letting it leak.
It usually goes away, or becomes minimal with use.


  • chrisleika

Posted April 11, 2013 - 08:48 AM



Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • GuyGraham

Posted April 12, 2013 - 09:49 AM


Just done mine, as it was also leaking on bike start up, but after about 10 secs of the engine running it would stop.
Ironically, this has happen after the winter lay up, and it was fine when I put the bike away last year

I decided to replace the seal, as I didn't want it to fail mid race, dump all my coolant and the engine seize

You don't even need to remove the clutch cover to replace the leaky seal (well at lest on the 450 you don't, don't know about the 400)

Drain the coolant
Remove the water pump cover (no need for a new gasket, as its sealed with an o-ring and hence is re-usable
Undo impeller with 10mm socket - its a normal RH thread
Remove impeller from shaft
you will now see the seal that is the cause of the coolant leak
Lever the seal out with a screwdriver or similar

To fit new seal, lube with grease the ID and OD of the seal (OD of the seal so it will slide back into the housing with minimal effort, ID so it lubricates the seal in use)
Press back in with fingers/thumbs, with numbers on the seal face, facing inwards
Refit impeller and torque to 14Nm - use a bit of locitie on the thread
Refit water pump cover
Refill with coolant
Start engine and check for leaks
Go Ride :thumbsup:

No more than 30 mins start to finish

Edited by GuyGraham, April 13, 2013 - 02:09 AM.

  • JMathis

Posted April 20, 2013 - 04:12 PM


I have a question about removing the impeller. What holds the impeller shaft from turning when removing the impeller? I just replaced my stock pump and housing with the Boysen pump and housing but it was my understanding I had to remove the clutch cover as well as the side case to hold the impeller shaft. Now that I have done all of this, I have fluid leaking from the weep hole. I leave for Moab in 1 week so I need to get this corrected.

  • GuyGraham

Posted April 22, 2013 - 08:32 AM


End of impeller shaft fits into a recess in the end of the crankshaft, and hence is driven by crankshaft
This stops the impeller shaft turning when undoing the impeller


Join Our Community!

Even if you don't want to post, registered members get access to tools that make finding & following the good stuff easier.

If you enjoyed reading about "" here in the ThumperTalk archive, you'll LOVE our community. Come join ThumperTalk today!

The views and opinions expressed on this page are strictly those of the author, and have not been reviewed or approved by ThumperTalk.