2006 Yamaha big bear 400 4x4 cylinder rebuild

[JNM]

Hello,

Acquired a used big bear 400 4x4 that would not start and debugged to compression issues. Tore down the head and determined the lower jug and piston were no good. Cleaned everything up well, got a lower new jug, piston, and all o ring / gaskets, and reassembled. Head bolts tightened to 30ft lbs which is slightly beyond spec of 29, but not grossly over.

ATV started right up after fix, but after idling about 5 mins, I noticed a small amount of oil weeping from the front bottom of the jug; you can see occasional oil bubbling out to surface through gasket. Note: I did ensure to include the rubber o ring on the bottom of jug. I also get white smoke, which appears to be coming out the spark plug side of the head gasket. The head gasket was a 3 ply metal gasket.

When I originally tore down the cylinder, there was black residue on all gasket mating surfaces. Should I have used a little gasket sealant on all gaskets? Everything I read seemed to indicate this should not be required. Looking for advice to help prevent these leaks assuming I tear back down and redo a second time. Thanks

[Mech]

If you used genuine gaskets then you shouldn't need sealer on the gaskets, but I always do. I use loctite master gasket and I strongly reccommend it if you are getting a sealer. I'd use sealer on the cylinder base gasket if I was doing it. The paper gaskets can get slightly damp without sealer sometimes, but they shouldn't let oil weep out like you are describing from the cylinder base. I'd strip it and check the assembly job and the O ring seems right. While it's apart I'd check the barrel oilways and the head oilways for a restrictor. Some makes have a little bung with a small hole somewhere in the oilways going to the head to preven excessive oil up there. Trouble is the tiny hole blocks up real easy. If the restrictor is above the base gasket perhaps there's too much oil pressure there, though I'd doubt it is the problem.

You deffinetly should not use any sealer on the head gasket. Head gaskets have to get their heat away to the cylinder and head, and sealer prevents that heat transfer. I've had slight puffs of smoke out of those metal laminate gaskets before, but only a very little and only when you give it a sharp rev. I've always just retightened the head after it's cooled down, and again after a week or so, and they are normally good.

The fact it's white smoke from the head gasket is a bit strange. Normally white smoke means a lot of oil. The combustion gases if they leak, shouldn't have much oil in them... not after it's had a first run anyway. Perhaps there is an oil leak at the head too ?

Edited June 6, 2022 by Mech

[Mech]

After thought... Most cylinders with an O ring at the bottom I've seen(hundreds), whether it's a big O ring around the cylinder sleeve, or a small O ring on one stud where oil gets fed to the head, there is one flat face and one countersunk surface. The gasket goes against the flat surface, and the O ring goes into the countersink.

[JNM]

Thanks for the detailed replies…. 
I definitely followed what you outlined and placed the gasket on flat surface surrounding the small Oring and the large oring around the jug.  I am worried I did not counter sink it enough into that groove before installing though. 
One follow up question ….  is there a good way to test if oil is getting up to the head?  I was looking for a bleeder screw, but not sure mine has one, I’ll double check.

 

[Mech]

I'm not familiar with that model, but the oil supply to the head mostly comes up a stud, then to the cam up one of the small bolts holding the cam cover on if it has one.. Some makes have a copper washer under the bolt where the oil supply is. Otherwise it will probably be coming all the way via the head stud. That should have a copper washer too. Some manuals suggest loosening the bolt/nut to check for oil when you first start them. Otherwise just take off a valve adjusting cover and it should be oily in there..

The o rings should fit themselves to the countersink. They are normally a snug fit on the cylinder and/or dowel, and as long as they are the right size, that is the right cross section diameter,  they will get pressed into place. When you fit the o ring it should be just slightly proud of the surface so it will be compressed during assembly. Just how much proud is a bit of a tricky thing to say.. but I'm sure you will figure it out if you have another go. O rings come in a range of cross section diameters, but in the range you are dealing with there will probably only be two sizes that would be in contention for the job. One will be a bit too small and another will be a bit proud(the right one).. or there might be one that will clearly not compress because it's too fat.

And, I'd be pretty sure the oil leak will be the oil feed, not the cylinder base o ring. The stud is near the edge of the gasket, and has some oil supply to it, while the base ring is a long way from the edge of the gasket, and doesn't really have much exposure to oil at all.

The oil supply o ring... there should I think be a thin walled locating dowel it goes over... You do have that ? Without it the o ring will crumple in around the stud instead of being compressed..

[JNM]

On this model, the jug bottom has 2 dowel pins on the corners, one on the exhaust side, one on the intake side.  The exhaust side, which is close to my leak had no oring, just a slip fit of the pin.  I will check the parts diagram again, but pretty sure this is correct.   so seems to be relying on the gasket to seal, assuming oil is even traveling in that cavity.  It may be head bolt only.  

The weeping seems less after running the engine a few times, but still there even if slowing down.  Not sure I want to torque the head bolts tighter but starting to wonder if they needed a little more tightening from the beginning.  some have suggested not to reuse the old bolts as they may be throwing off the torque reading off.  

What’s more baffling,  After 10 mins of driving in only first and second gear it stalled out and would not start.  Checked compression and it went from 200 to 90.  I let it sit over night and rechecked compression and it went back up to 130 and I was able to start it.  I have never seen anything like this.
 

Going to remove valve covers and see if oil is making its way to the head. Something is clearly amiss,  so going to tear back down and give it a second look.  In addition to proper seal, I think I now also have to consider oil blockage and/or pump pressure issues, recheck valve seals.

[Mech]

The loss of compression when warmed up might be tight valve clearance...