Possible Top End 2007 Big Bear 400

970 Views 4 Replies 3 Participants Last post by stho0785, Dec 6, 2017

stho0785

Discussion Starter · Sep 18, 2017

Comrades,

I have a 2007 yamaha big bear 400 that is starting to have a small amount of oil seepage from the bottom of the cylinder. I am preparing myself to take the top end apart. It runs great and has no issues. I am curious what approach other folks have used. I have rebuilt my raptor 350 and it was relatively cheap.
I am just going to take the top end apart and have the machine shop mic the cylinder. If it needs to be bored or honed, I'm game for either. I was considering going with a bit higher compression piston I have read about often, but not sure yet. I am not wanting to do a crazy race build or anything like that. I have put an hmf exhaust on it and rejetted the card, so it runs great. Any recommendations on piston brands? Should i get the head rebuilt with the valves, etc. . .?

Thanks for yalls time.

Seth T

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stho0785

Discussion Starter · #2 · Nov 27, 2017

update`

So, I took my top end apart and everything really looked great. Can still see cross hatching on cylinder. Big bear has 1400 miles. Gonna bore cylinder to the next size up. Looking at pistons, the oem is a pretty good value compared to whats available. Purchased a webcam, 388 and the valves. Figured I'd just stay with the stock compression as the only place I could find a higher compression piston was FST and it was around $200. OEM was half.

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dezz

· #3 · Nov 27, 2017

Why bore a perfectly good cylinder and go up size on the piston? You don't gain anything by doing it. The only time you should bore a cylinder is if it's worn out of limits. Once you over-bore, you can't go back. So if something happens later on and the cylinder needs a bore, you'll no longer have that option. You'll need to replace the cylinder.
A higher compression piston will gain you some extra performance, but a slightly bigger piston will not gain you enough to notice.

If it were mine, I would clean and lap the valves, install new stock piston rings and get back to riding....You'll fix the leak and have ultimate reliability.

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jvictory1974

· #4 · Nov 27, 2017

X2 on what Dezz said or at the most just hone the cylinder and put in new rings

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stho0785

Discussion Starter · #5 · Dec 6, 2017

update

I had to get the cylinder bored up to the next size as it was out about 3 thousandths. redid the valves and got it all back together. It's broken in, but I'm kind of kicking my self for going with the regular compression piston..........it runs well, however. I purchased the service manual also based on recommendations I have seen on this forum. Thanks for the all the info.

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