2007 Yamaha big bear 400 top end rebuild

[Charlesbrown]

I was given a 07 big bear 400 with all the options. The prior owner ran it through water and it started smoking  so he took the head off and gave up. I have  a bunch of parts in boxes and did a quick inventory.  I need a jug, piston, rings etc and thinking it’s better to buy a complete top end. Does anyone have a recommended site to buy parts? I plan to go oem, no china junk but if there are quality aftermarket manufacturers I’ll do an upgrade. 

[Mech]

Running bikes through water doesn't generally cause them to start smoking. Even drowning them in fresh water doesn't make them smoke. It sometimes bends the con-rod though.. I'd be a bit cautious about spending money on it till I'd checked the con-rod and bottom end felt ok.

Are the piston and barrel missing ?  I'd try fitting new rings and two gaskets and get it running if the parts are there.. then test the whole quad before overhauling the engine...  if in fact it needed overhauling.

[Charlesbrown]

I have a jug but the cylinder has surface rust and I need to look at it closer but I’m betting it’s scorned.  Piston is missing. Oddly I have two heads.   I can freely turn it over by hand. Prior owner thinks they got sand in with the water. I’m perfectly comfortable breaking the lower end down this one is worth keeping. 

[Mech]

Oh ok. I've never seen sand get into a cylinder after a dip. Never seen one start smoking either.

I was thinking that perhaps they were bluffing and it really had a bent rod..  haha..  Suspicious mechanic..  I always cross check what my customers tell me..

[Charlesbrown]

Visually the rod looks fine. 

[Gwbarm]

Souds like a nice project, looking forward to it, keep us posted. I would flush the bottom end with some type thin oil maybe even diesel and put it back together with new parts. OEM parts are expensive but for this I would go with them, also check the cam and chain for proper specs.

[Mech]

You know how to check the condition of the big end bearing..  By wobbling the rod sideways and feeling for play, and pushing it up and down feeling for play at the point where the wear on the crank pin happens, and rotating it on the crank pin feeling for roughness ?

[Charlesbrown]

The rod has a lot of play moving it left to right. No play or roughness when rotating the crank.  

[Gwbarm]

There should be some side to side play, but shouldn't be excessive, their shouldn't be any up and down play, once you get oil back in the bearing side play should be minimal. 

[Mech]

If the rod moves sideways more than about three mm it's a sign of wear, if it has six mm play it's bad.

The wear on the crank pin happens when the crank is about twenty or thirty degrees after top dead center. That's the place to check for wear or roughness.

Feel for end play of the rod till you find that loosest place, the difference will only be minute though so you have to feel for it very carefully. The best way is to rest a thumb or finger on the crank web as you use the same hand to pull and push the rod. Your finger, bridging between the rod and the crank, will detect very small movements.  Once you have found what seems like a looser place, compare it to other places around the crank pin, and feel for roughness when you press the rod down hard and roll it over that most worn bit.

If you can feel end float, or roughness, it's worn.

[Charlesbrown]

Thanks everything seems tight.  I am going to get a new jug, piston and rings and go from there. 

[Mech]

Ok.

[Gwbarm]

I think you are good to go, I just brought back one of those engines they are very tough, I was very impressed with how tight and smooth everything was after so many years of neglect.

[Charlesbrown]

[Gwbarm]

• It will be nice when you get it all together, looks good, just needs some work, and a pressure washer.

[thelostjs]

ahh let the dirt pile on. if you keep the airfilter clean oil filled and the tires pumped it'll last a long long time. I'd love to see a build log for this as i'm wanting a big bear irs super badly. 

if you'll sell it I'd think very seriously about a cheap cylinder and piston from ebay. but since they are under 100 bucks it does make you wonder if they are ok to use!

I'd say it comes down to your usage case. if you are going to use it weekly and run it like a sport quad  then might as well spend up and get some nice oem or name brand parts because it sounds like you got a great deal on it. im actually jealous!

[Charlesbrown]

I probably won’t use it much. I loose interest once I fix something but I hate Chinese parts. 

[kp3004x4]

Just out to say that's a nice bike. Looks like it was never rode hard and put up wet as they use to say. Good luck on your project.