what's the likely cause?

[DavePlum]

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."

Valid point, but I've been in contact with andreas and he's got a complete 81SH cylinder head that has 50K miles on it for a really good price. It's my understanding a 81 head will work on an 80 engine. Someone correct me if I'm mistaken.
I'd hate to purchase it then install it only to find out I have big problems with pistons/rings and have to pull it all apart again.

TopCat "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."

See above response to Mack

"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."

This is what I was thinking, although I've spent so much time already on the 80SG I'd hate to abandon it now not to mention I like to finish what I started (within limits)

Mack "Wait, are you saying you can't roll the motor at all with head off"?

No, the 80 rotated with the head on, I haven't rotated it since pulling the head off because I was waiting for an extra pair of hands to keep tension on the cam chain while I rotated it to look at cylinder walls. The 79 I got without a head and it rotates fine but I haven't inspected for scoring/wear or out roundness on either engine yet. Andreas also has a complete 79SF cylinder head he's offered to sell me.
I think I'll have to take a step back, take a deep breath and make a final decision on which route I think I will go. Cost (of course) is always one of the major driving factors in this decision. Thanks guys, Dave

[motoman]

80,81 and XJ have same larger diameter valves.

[trbig]

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.

If you look at this picture, you can see pretty clearly that this valve was hit by the piston. (See the carbon smeared off and bent towards the outside edge.) You can see marks to the outside of the exhaust valve as well on the 2nd pic, showing it had some contact as well. If your cam's were lined up, no matter how bad your timing is off with the crank, your valves won't interact. At top dead center, there's not much room. If the valve stems are severely caked with carbon, they can get to a point to where the valve can't close. Many times, the hot gasses will cut a groove in the valve seat or valve face if it isn't sealing properly, but if that carbon is holding the valve open much at all, a piston strike can occur.

While a severely carboned up valve can mean bad valve seals, it's usually a sign of a motor that had worn or stuck rings and burning a lot of oil. As these motors run, at the bottom of the stroke, and the crank running "Backwards" like it does, it shoves the piston forward and up which causes wear at the bottom front of the piston skirt and will also egg shape the cylinder walls front to back. It can also smash the top front of the piston down onto the first ring if there's a lot of wear, keeping it from expanding/contracting, causing blow-by and oil consumption. You'll usually notice some piston slap going on before that happens though, if you know what to listen for.

Just my suggestion to check things over well before throwing back together. It'd be a shame to do all that work and still have the original problem.

[trbig]

No, the 80 rotated with the head on, I haven't rotated it since pulling the head off because I was waiting for an extra pair of hands to keep tension on the cam chain while I rotated it to look at cylinder walls. The 79 I got without a head and it rotates fine but I haven't inspected for scoring/wear or out roundness on either engine yet. Andreas also has a complete 79SF cylinder head he's offered to sell me.
I think I'll have to take a step back, take a deep breath and make a final decision on which route I think I will go. Cost (of course) is always one of the major driving factors in this decision. Thanks guys, Dave

If the head is off, there's no need in worrying about tension to the cam chain. You can drop the chain down and to the side if the crank sprocket and it won't even move it then. You just don't want it getting all wadded up in a bunch at the bottom.

[TopCatGr58]

Hey Dave,

Yes, an 81 head will fit/work on an 80 engine. I just checked the Official Microfiche, and the 80 and 81 cams have the same part #'s, so hopefully they also have the same lobe profile/specs...I think Scott/3Phase had done some investigation on the differences between the years...at least on the timing adv units, and he may also know about the cams. The 81's TCI and timing parts are different than the 80's, but I would hope that the cams are the same as the 80's. BUT, with that suspicious loose piston, I would be hesitant to put that much $$ and work into that engine just to have that cylinder go bad.

Now, perhaps Andreas might also have some 80/81 jugs/sleeves and stock pistons that do not show much wear, are not loose, and would be able to send you a replacement cylinder sleeve and matching piston/rings that you could then swap for your excessively loose one!? The sleeves aren't hard to get out of the cylinders, just ask Tod/TrBig!

Yep, without a head/cams, just support the chain with a clothes hanger, just to keep it out of the way and not too close to the crank, and you will then be able to rotate the crank just fine.

Hope this helps you with the decision making process.

T.C.

[DavePlum]

TC- because of that loose #4 piston I've decided to focus on the 79 for the time being. The 80 will go on the backburner and get parked in my backyard with a tarp over it where the 79 now resides. Already sent Andreas a check for a 79 cylinder head (missing from bike when I bought it) and will likely be ordering another fuse box upgrade from you soon. thanks for all the help, Dave