Missing Cylinder Head Side Plug - found the hard way (bent valves)

My bike is a 1981 GS550T with the 650 top end modification. It had been leaking oil from the front left side at the cylinder gasket for a while and I had attempted to fix it in the past but without any luck. Well I finally have a garage to work on it so I did a deep dive into the cause of the oil leak. It appears the dowel pin didn't fit too well keeping the cylinders from sitting perfectly on the bottom end. So the leak was fixed and the engine was back together, and I put a couple hundred miles on it. Then there was an oil leak from the valve cover.

The gasket kit I purchased for this rebuild didn't have the correct valve cover gasket, I had tried using a liquid gasket maker, and that didn't work. So off came the the valve cover again with a correct gasket in hand. This was going to be a speed round of checking valve clearance one more time and getting the cover back on so I could make it to a group ride that afternoon. Turns out when I first put the engine together, I didn't do the valve shims correctly because all of my valves had too much clearance. I think I put the shims in originally and checked for clearance without rotating the engine to fully seat the shims.

I get all new shims in with correct clearance and its time to button it all back up when I realize I only have 3 of the 4 semicircle side plugs. Why do these even exist to begin with? I must have dropped one and it rolled under something, but whatever. I have spares. So I forget about the missing part and move forward with installing the valve cover.

It wasn't long and I was out on a little test ride. The bike is running really good. I feel like I can tell a difference with the proper valve shims. Then the worst thing happened. The engine shut off and I heard some terrible noises. I immediately pulled in the clutch and came to a stop. The starter would not turn the engine and if it was in gear the rear wheel was locked. I pushed it 2 miles back home pretty worried.

Back off came the valve cover. All 4 intake valves were stuck open. I can rotate the engine but only like a half rotation. Finally it came to me what happened. The semicircle shaped side plug must have fallen down into the timing chain cavity when I pulled off the valve cover. Some of them will stick to the valve cover and some stay on the head. Once I pulled off the head I was able to fish it out. It was stuck between the sprocket on the crank shaft and the timing chain. It must have bounced around for a bit before it became lodged onto the crank sprocket and knocked it out of time.

I have 4 ruined intake valves. Luckily, the damage is only with those valves. And I happened to have valves on the shelf. I didn't make it to my group ride that afternoon. 10 hours later I have new intake valves and fresh oil.

What are my lessons? Slow down. Be Diligent. Anytime the engine is open I must have a microscope on everything around the opening.

Good lesson there, and if your pistons and guides are still OK then you got very very lucky. Also I wonder how many spares you do have on the shelf, not many people carry "spare valves" you see.

When the cilinder head is manufactured the camshaft bearing bores are precisely manufactured by line-boring through and through. The clearance in the side of the head is needed for the long boring-bar creating the half moon shaped openings.