Crank oil seal YZ400F

7 replies to this topic

Posted March 06, 2007 - 07:18 PM


I just took the right side cover off to replace a leaking gasket and noticed the crank seal was chowdered up a bit and the spring was exposed about 1/3 of the way around the seal. I know the previous owner and he said he replaced the water pump seals but not the crank oil seal. The seal lip and spring was facing the crank when I took it apart after removing the washer and snap ring. Is this the correct direction? It seems backwards to me but with it facing the other way there would be a chance the spring could come off and enter the oil system. My manual doesn't detail the seal direction and my searches turned up nothing.

  • grayracer513

Posted March 06, 2007 - 08:43 PM


I'll bet it does if you look a little closer. It says to install all oil seals with the manufacturer's markings facing toward you unless otherwise specified. You're right; it was backward. The lip of the seal goes down into the bore. The lip always faces the fluid it's sealing. In this case, it's sealing oil pressure in that little cavity in the case, where it's being fed into the nose of the crank.


Posted March 06, 2007 - 09:23 PM


The problem with this seal is that the mfg numbers are on the rim of the lip side and the flat side has no marks.
I went into the service dept of my local bike shop after ordering parts and asked them and after about 10 min with the shop supervisor looking through manuals, he went and got one of the techs who used to be a Factory race mechanic. He said on the race bikes they used a totally different seal than the stock bikes. He agreed with me that it looked wrong and the lip or apex should face the fluid pressure from the pump but he couldn't be sure and mentioned posting on TT.

  • grayracer513

Posted March 06, 2007 - 10:04 PM


Well, that's the downfall of using the "marks face up" method. That really never occurred to me in 30 some years of professional experience until I read it in a recent Yamaha manual. I've always gone by facing the lip toward the load. Either way, your suspicions were correct.

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Posted March 07, 2007 - 07:53 PM


I take it you have been into the right side case of the YZ before.
I spoke with the previous owner today. He is actually the second owner and he told me the first owner was a neighbor of a friend and suposedly, the engine was never apart. So it leaves me wondering...... was the seal installed backwards from Japan? I looked through my manual and found the paragraph you were talking about in the general info section near the front of the book. The manual goes into detail for the water pump and shaft seals but mentions nothing of the crank seal. Not even replacement or inspection.

  • grayracer513

Posted March 07, 2007 - 09:49 PM


Anything's possible, but no damage apparently came of it. The crank assembly is all rolling bearings, and requires very little oil pressure. The system is designed to maintain pressure at the cam bearings even if the crank seal leaks quite a bit.


Posted March 19, 2007 - 09:19 AM


I found some information with photos posted by Ziggy in the yzf250 forum that solved the mystery. The seal in his yz250 was found the same as mine. Manufacturing marks up with the spring facing the crankshaft. This position was confirmed to be correct by a Yamaha tech per Ziggy's post. I installed and ran mine yesterday. I checked for pressure at the cylinder oil feed and everything is working good. The Yamaha manuals do a poor job covering this seal inspection and install. I believe the seal is installed in this direction so if the spring were to come off the seal, it won't enter the oil system and possibly be pushed into the crankshaft oiling hole. With the seal installed with the spring toward the crankshaft, the spring will stay on the crank hopefully under the washer and snap ring which is how I found mine. The pressure must be low enough so it doesn't really matter if the seal is opposite the load.


Posted April 02, 2007 - 09:21 AM


I just bought a used WR400 bottom end for a trans swap and the seal was installed with the seal away from the pressure but mfg marks up with the spring side toward the crank. This again confirms to be the correct install. My crank had a slight burr on the end from machining that probably cought the seal lip and caused it to fold over when the previous owner installed it. I took a small sharpening stone and removed the burr and greased the seal and crankshaft end before assembly assuring a nice slip on fit. If you take a flashlight and look into the oil filter housing, there is a small oil galley hole where you can see the back of the seal with the crank sticking through to check and see if it folded over.

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