400ex head gasket teardown

[dealer]

my headgasket is blowing spits of oil all over my rear shock. I am going to replace the head gasket . I was wondering if anyone knows if I have to take the motor off the frame or I could sneak the head out of the top. I had a trx 250 frame with a 300ex motor in the past and I had to take the motor out to do this job. I was also wondering since I am not sure what previous owners have done to this motor how to look for modifications such as piston size and valves. For all I know this motor could have blown the head gasket because it has a bore kit. Is there any way of measuring to see what size piston it has. The last time I tore down a 300ex motor I had trouble getting the timing right.

1999 400ex

[Chino886]

you do not need to take the motor out of the frame, just piece by piece it out of the frame...you will not need to take the bottom end out....

[underpowered]

no need to remove the motor, plenty of room to take it apart in the frame. to check for mods measure the bore with some calipers. to check for a aftermarket parts they are usually stamped on it somewhere. on teh piston there will be a number on the top of it, scrape the carbon buildup off (If present) and look for a number on top of the piston. also a piston like Je will have a JE symbol cast into the underside of it. in the number on the top of the piston there will be the size if it is not stock. Ex. a wiseco 426 piston is 4628M08800 this is an 11:1 88mm piston(Last 4 digits is size in wiseco) a higher comp piston will have a dome on it while a stocker will be relatively flat. for the cam, it will have a number either engraved of cast into it if it is aftermarket. a Hot Cams for example has Hotcams cast into the cam itself.

Timing is easy. put the engine on TDC according to the marks on the flywheel and then align the two marks on the cam sprocket with the head and the third mark will be point upwards.