what's the likely cause?

[DavePlum]

Yesterday I removed the cylinder head off my 80SG project because of a stuck open #2 exhaust valve. Not that I much experience with internals of combustion engines but just by looking thru the exhaust ports these have to be the worst looking valves I've ever seen as far as crud/corrosion on the valve stems. Giving the pistons a quick visual I decided (just out of curiosity) to measure the gap between the top of the piston and cylinder wall with my feeler gauge. When I was wiping down the top of #4 piston I could actually feel some movement of the piston. It actually will deflect .006" from the cylinder wall. Would this movement be most likely caused by badly worn rings? Pistons #1-#3 all measured around .004" while #4 was .008". I'm a newb when it comes to working with the internals of an engine and learning as I go, I only know enough to be dangerous.
Another question, I hear/read a lot about bent valves from being stuck open and making contact with the piston and would like to know if it's normally obvious if the valve is bent on just a visual inspection while it's still in the head or does it need to be removed and inspected/measured.
I don't see anything to indicate my stuck open valve made contact with the piston. Thanks

[mack]

Motor

Well I’ve been absent from channel 11 for several months now so I don’t know the history of bike. How any miles? The stuck valve was probably a result of being in the stuck open position for a long time. One cylinder is always open, to open air and if not exercised a bit during storage or proper winterization, things do stick. I assume you may have bent the valve trying to roll the engine over, maybe trying to check the compression. Now that the head is off it’s a great time to service it.It’s never a waste of time. The bent valve will be obvious on visual inspection. If you have a manual there is a servicing section for the head and valves. Not complicated at all. The geometry of the pistons’ movement up and down the sleeves tend to wear the jugs on the front and backs of the sleeves making them out of round. This could account for some of the movement on higher milage motors, but I suspect the rings are the larger issue. If your lifting the jugs off to replace the rings you can check the sleeves for symmetry at the same time. The manual has a good write up on the tolerances allowed. Several methods to do this without a bore gauge.

[Ranger_xs1100]

Another question, I hear/read a lot about bent valves from being stuck open and making contact with the piston and would like to know if it's normally obvious if the valve is bent on just a visual inspection while it's still in the head or does it need to be removed and inspected/measured.
I don't see anything to indicate my stuck open valve made contact with the piston. Thanks

Hi Dave, I don't think the valves can open far enough to actually hit the piston unless maybe it is the wrong head? I think they get bent from hitting each other. When the cam timing is too far off, both valves open at the same time and hit each other. Here is a couple of pictures of a badly bent valve. In this case it is very visibly obvious.

[DavePlum]

Mack- I really don’t know anything about the bike’s history. I bought it in late July in non-running condition and half way disassembled. The collected dust and cobwebs were free with the purchase. The odometer reads 13K but I believe the odometer has turned over. I do know it sat for a long time before I bought it. The valves hitting each other makes sense, I wasn’t sure how close to the pistons the valves got when they were fully open. In the pics you provided it’s definitely clear that the valve is bent, I don’t see anything like that on mine. I plan on removing the jugs to inspect, gain the wrenching experience, and to see what I’m up against as far as what’s needed to get it running. On the bright side I’ve only got about $250 into the bike at this point. And yes, I have a Yamaha service manual that came with the bike. thanks, Dave

[TopCatGr58]

Hey Dave,

When I did my rebuild the first time in 2000, I didn't realize that the intake and exhaust valves couldn't occupy the same space at the same time ....was turning the cams out of synch on the bench to try to do the valve clearance adjustment. I bent one and didn't know it, the valve retracted into the head just fine. Put the engine together, and did a static compression test, and found 1 with NO compression. Had to take the head/cam chain all back apart, then pulled the springs, and THEN I was able to spin the valve in the head, and THEN I could see the obvious wiggle/wobble it had...otherwise it looked okay!? Others will say you can pour fluid into the head with the valves closed and see if they leak, and the leaky one could be the bent one.

As to the piston/cylinder clearance, the manual should give you the specs. Good Luck.

T.C.

[Radioguylogs]

Hey Dave,

Others will say you can pour fluid into the head with the valves closed and see if they leak, and the leaky one could be the bent one.

T.C.

I had a bent valve in one of my triples. It had no compression in one cylinder.

I couldn't see which valve was bent while it was in the head.

It was easy to find by pouring a little mineral spirits in the head.

[CaptonZap]

If you take the cylinders off, you can measure the piston SKIRT clearance. You will find that the part of the piston that has the rings is smaller than the lower portion, (the skirt) by 20 or 30 thousandths, which is noticeable visually.
Piston clearance is measured at the skirt, 90 deg to the pin, since the piston skirt is not a true circle, but rather an oval, such that the portion of the skirt at 90 deg to the pin is the widest part of the oval.
Sliding a feeler gauge down the top does not give you a true measurement of the skirt clearance, since the feeler gauge blade hits the top ring before it can get down to the skirt.
If the mileage is indeed 13K, and that could well be, if it has been sitting for a long time, the rings are probably OK. Without a leak down test, it is hard to say.
BING or Google "differential pressure check" to see what is involved in that test. You might want to do that test before you remove the head for valve cleaning.
Engines that are in the condition that you described do not usually run long enough to go 113K miles. If you know an automotive machinist, you should take the head to him and get his opinion as to the condition of the cam lobes. Tell him "DO NOT TURN THE CAMS". If he does, he may bend a valve, as described above by TC et al. You should get the manual and follow the instructions about replacing the cam chain. That will put the cams in the proper position for removing the head.
Have fun
Have fun,

[mack]

Motor

Ok so you think you’re up to your arse in alligators but really this is fairly standard stuff. Don’t panic none of what you need to do next is over your head. You will need a decent set of tools but you can beg, borrow those. Before you try to pulling the jugs, why not service the head, reinstall and do the compression test before you approach pulling the jugs and replacing the rings. These bikes are very easy to work on. Pulling the jugs can be a little difficult if the head stanchions are plugged with crud. A special homemade tool might be required. For the cost of time and money for an additional head gasket, service the head, do a compression test and ride the crap out of it and see if it doesn’t come into it’s own after five or eight hundred miles. That will tell you if you need to go deeper. Delving into these motors and trannies is fun but you have a year round riding season; where, I for instance have about five months a year to do more in depth stuff. The compression test will tell you if it has clocked 13 K or 130 K. Just be cool man, it’s all good. No matter what you need to do to get this dude running, this site has the expertise. Remember; always do the simple things first.

[skids]

Remember; always do the simple things first.

! yes !

[DiverRay]

Remove and clean the valves, Check to be sure they are strait as best you can, re-lap, install with new seals and put it back together. As the others have said, these are engines that will go a LOT longer than most people realize. You can probably put another 20K on the engine IF it really has 113K!

[DavePlum]

Thanks for all the replies, all suggestions and info is helpful. At this time, this is what I know: (just so everyone knows...the cylinder head is OFF the bike)
The cylinder head has a stuck open exhaust valve on #2 and is possibly bent as well. I suspect (just based on a visual inspection) via exhaust ports all four exhaust valves need to be replaced due to excessive corrosion on the stems. I read a post on this site not long ago when someone stated their cams showed delamination (not sure it that's correct term) on some of the lobes, mine also have that. It would probably be cheaper to just replace this head but that's not my biggest concern. I'm more concerned why I can actually move the #4 piston with just my finger. It moves a lot more front to back vs left to right and when I move it front to back, you can hear the pinging sound as it comes into contact with the cylinder wall. None of the other cylinders do that so I'm thinking I might be looking at a reworked top end AND work on the bottom end. (Now that's not a pleasant surprise)
I've also got a 79SF project bike that is only missing the cylinder head and the engine rotates as it should. The odometer on that bike reads 29K but who knows if that's correct! I already pulled the engine off the 79 and it sits on a furniture dolly in my garage. I've also stripped the bike down and cleaned everything and boxed up all the parts I didn't need for the 80 so it's just a matter of putting everything back together. I'm thinking I might just switch gears and run with the 79...what do you guys think? Good idea, stupid idea, what?

[mack]

The

trouble with heads is that you might be buying one that has less life on the seats than the one you currently have. Go buy five gallons of diesel and a large paint pail. Put the existing head in the pail and immerse it in diesel. That will free up anything stuck, valves included. It's not uncommon for the rings to stick as well. That's why I said to run the bike for 500 miles and let it go through several hot, cold sessions before you even do a compression test. Most if not all of us have had this same situation. 9 times out of ten the bike runs like crap for 200 miles then bang, everything free's up and your crapping your pants and your face is sore from smiling for a 100 miles! Do the head first, reinstall and make sure the carbs are spotless inside. Dial them in then ride that dude. If it doesn't come into it's own within a thousand miles then you can pull the jugs and do the rings and wrist pins. Trust me on this.

[TopCatGr58]

Hey Dave,

Well, regrettably, for the optimal performance, you will not want to put the 80 head on the 79 engine, because the valves and cam profiles are different, different sizes, and so the piston recess are not going to be adequate for the larger 80 valves.

But you may not have been thinking of doing this swap, as much as just getting a head/valves/cams for ONE of the engines, and the 79 sounds like it might be in better condition...ie. no wiggle room with the piston vs. the 80 engine.

The 79 has a higher piston dome, and 0.2 more compression ratio than the 80 engine stock. And since the 80 piston is more suspect, the 79 sounds like the better option. HOWEVER, have you moved/rotated the crank to observe the complete cylinder wall on each piston/cylinder checking for obvious signs of scoring or other marks of excessive wear??? And while the piston is at the bottom of it's travel, you can do a fair measurement of the cylinder diameters to check for out of round condition. Without a head, you can't necessarily to a leak down/compression test, so you don't know what condition the rings are in. But if the cylinder measurements are very close to spec, then you may be able to assume that the 29K odometer reading is reliable, and you may be able to just do a light HONE and put a replacement head on and have an engine for 70+k miles!?

T.C.

[mack]

Wait

are you saying you can't roll the motor at all with head off?

[TopCatGr58]

are you saying you can't roll the motor at all with head off?

Hey Mack,

I think he couldn't rotate the 80 engine only because of the stuck valve, and that's why he stopped and took the head off. I think he should be able to rotate the crank/pistons now tha the head is off!?

T.C.