400 lower rod bearing

My 400is running very tight and will rev up no problem but as soon as i get off throttle it stops instantly. The problem feels mechanical so i have have torn into the motor. I am down to the crank and nothing feels tight, but the left side of the crank has some serious blueing where the lower rod bearing pin is pressed into the crank. There is some slight blueing on the other side. Everything spins nicely and doesn't have any excissive play. Is it possible that the factory uses heat to press the pin in. Or does anyone have any other ideas. Thanks in advance.

[ April 16, 2002: Message edited by: Yamapimp ]

Check with the dealer or an engine builder. I haven't seen the crank out of one of these things yet but in general it is not normal for a crank to be blued near the pin. This typically would point to a lubrication problem. Make sure your oil pump is working and take the time to blow out the oil lines with some air to make sure they're not plugged. Since you noticed the blueing it may be worth while to have the crank inspected by a machine shop or motor builder to see if you need to replace any of the related parts (rod bearing, crank pin, etc..). If there was enough friction to get it hot enough to blue the metal then there's a good chance that you have some significant wear or damage. You say that it sounds like there is a lot of drag when you rev it and let off the gas, this sounds lubrication related too. The parts don't have to be "tight" to have a lot of drag, just missing lube will cause this type of friction too. Just my $.02, hope this helps.

Same on my '99 400 last year. Go ahead and have the crank rebuilt. It will save the top end plus other possible damage. Bluing is not normal.

I've taken my engine apart several times, and one of those times I put in a new crankshaft. The bluing sounds normal. They come like that from the factory. Before you blow money on a crankshaft, make sure that's what it is.

What's the real problem? Not having ridden it, I'm guessing it could be one of several things: any of the bearings pressed into the crankcases or the camshaft bearings could be bad, or the clutch could be dragging. Maybe the crankshaft bearing is bad. But it sounds OK from your description.

[ April 17, 2002: Message edited by: Rich in Orlando ]

Don't worry the blue on the crank is normal.The rod is weak if it is reved to much. You can put a 426 rod on the 400 crank and go with the 426 bore .And if you really want power stroke it to.

I just did the 426 rod and piston in my 98...so far so good. And yes the blue is normal (factory).

I bought a new cylinder. I talked to Eric Gorr and he would have swapped my 400 (420) cylinder, but the new one was less money (right at 200 bucks). I went with a Weisco piston with stock compression. It had a Weisco 420 kit with 13.5 to 1 compression before.

HEY YZ400COURT, you have to buy a new clynder or just bore the old one?? did you use a yamaha piston or wiseco??thanks jimbo

Interesting that these things are blued a bit from the factory. Is the pin welded in place? I'm wondering if the welding caused the blueing.

also another thing i am wondering, i know to convert a 400 to 426 you have to change the rod, my question, is the crankshaft re-buildable so you can take it apart to replace the rod or does it have to be un-welded and re-trued after splitting it????jimbo

The crank is pressed, and can be rebuilt. The lower bearing and pin are the same in the 400 and 426.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now