400ex rebuild, a couple questions

[levonskinator]

I recently got a 400ex for $200! I was told that it smoked bad and had low compression, but I never saw it run. I was friends with the previous owner and took his word.

I took apart the cylinder, assuming to see a gouged out sleeve, but the sleeve is as smooth as can be. The piston, which I think is stock, had a little bit of a carbon buildup but nothing too bad.

I was hoping that I could just replace the rings and hone the cylinder and I would be good to go, but I have a few worries. The piston seems to have some wear marks on the skirt and burn marks around the rings. I attached some pictures of it.

I understand that the stock cast pistons are weaker than forged weisco ones, but money is very tight right now. I was hoping to go to a 440 later when the money comes in, but I don't want to spend money on a piston if I plan on getting a new one in the future.

My main question is do you think that the piston is compromised due to these wear marks, or should I be fine for a couple of months? Thanks in advance for any replies

[levonskinator]

the other side

[jdog75]

if i were you i would check to see if the cylinder is a perfect circle. it may look like it is but if its even a litlle off you wont get the right amount of compression. i just did a rebuild and mine looked fine besides it needded a hone but with a closer look it turned out to be slightly ovalish. so if i were you i would probly do atleast an 86mm bore job just to make sure you get everything out and you are starting fresh

[levonskinator]

the burning I was talking about

[levonskinator]

How would I go about checking to see if the bore is ovaled or a perfect circle? A micrometer?

[jdog75]

i did it by just lookin at the piston an looking at the wear on it if its worn and scuffed on the skirts it was probly ovaled a little and to seal the deal i put the piston in and tryed to wiggle it if it wigled at all in the cylinder its either ovaled or completely worn all the way around

[levonskinator]

What did you do, bore out the cylinder to what size?

[jdog75]

well i went to a 416 wiseco 87mm 2 over bore with 11:1 compression with gttstuds and magnum stage 2 cam but if i were you id go atleast to a 407 86mm 1 over bore and the rest depends on what your looking for

[levonskinator]

Alright, thanks for the response, I'm going to wait for some other responses to get a better idea but I am thinking that you may be right

[togup]

before you make any purchases take it to a machine shop or a machinist and get it checked out, that piston aint burnt ,you may only need honing rings and gaskets

[lester985]

i agre with jdog go on ebay get a 87mm piston kit and be done with because if u hone it and put new ring soon u will be doing it all over again but thats a good deal for 200 bux i paid 100$ for mine

[bigbad400]

i dont think theres anything wrong with that piston. you should have it checked by the shop but you should be able to hone get rings and gaskets and put it back together, they also have .5 overbore rings for stock pistons too you can get a set for like 35 bucks. you still aint buying a piston and you still fixed the problem.

[levonskinator]

Alright, thank you for the responses. I am going to go to a shop and have the piston and cylinder looked at to make sure the bore is not ovaled. Hopefully it is not and i can get away with a hone and rings.

[levonskinator]

Also, considering I am staying with a stock cam do you feel that I will need to upgrade to a crf450 cam chain or will the stock one be fine. The chain seems fine, it was nice and tight when i removed it.

[jdog75]

well the chain tensioner might be all the way out and it might be a good idea to replace it when your motor is apart. and ebay is the way to go i got a wiseco 11:1 416 namura gaskets new cam chain and magnum stage 2 gasket all for 300 shipped to my door and i went and bought the hd studs and the only problem i had was a defective headgasket but namura is garbage anyway so i upgraded to the sparks one and its been good so far