[supersport#69]

My 68 400 ws motor had lower compression in #3 cylinder and mechanic believed it to be burnt exhaust valve.
Her retested compression on all cylinders before pulling heads for valve job. The low cylinder was 115. The other 7 cylinders were 160-170.
So the backstory is it was smoking on pass side, pulled valve cover and a rocker nut somehow backed off and rocker was loose. Smoking stopped after tightening rocker nut.
The valve that looks burnt is the intake, same valve nut backed off.
Push rods and rockers all look good. There doesn’t seem to be a theory for why that nut backed off off causing intake valve to do that.
He said cylinder walls and pistons look normal. There’s 67k original miles on the engine.

[dataway]

How long before you start putting it back together?

[supersport#69]

Dropping heads off Monday at the machine shop. Probably a a few days there so guessing week after next.
What are your thought on the looks of the valves?

[Sirrotica]

Before you ship the heads off to the machine shop, you should take a hammer and bounce it off those valves in that cylinder a couple times on the end of each valve to make sure there is no carbon buildup holding them off their seats.

After you bounce the valves off the seats a couple times fill the intake port till the fluid is up past the valve head, and seat with any of these liquids, kerosene, diesel fuel, solvent, etc. With the head sitting on edge observe the fluid to see if it's leaking into the combustion chamber area past the seats. Set the head on the opposite side and fill the exhaust port and observe for leakage again. You shouldn't get any leakage past the seats if they're in good shape.

Looking at the valves while assembled can't tell you much unless it's really burned, or bent enough to be able to see with your eye.

This is how I've tested valves and seats this way for 45 years, it's pretty much an infallible test. It keeps you from chasing a problem that isn't there. Grinding valves and seats when the problem is ring seal, can be a costly mistake.

[supersport#69]

I’ve decided on valve job either way so I’ll see in a few days on valve conditions.
Is it absolutely necessary to surface heads? I believe this is the first time off the engine.
I’m thinking all new valves and hardened exhaust seats but no idea on rest yet.
Head gaskets I bought are fel pro 8518. I don’t want to increase compression if I can help it.

[Sirrotica]

Originally Posted by supersport#69

I’ve decided on valve job either way so I’ll see in a few days on valve conditions.
Is it absolutely necessary to surface heads? I believe this is the first time off the engine.
I’m thinking all new valves and hardened exhaust seats but no idea on rest yet.
Head gaskets I bought are fel pro 8518. I don’t want to increase compression if I can help it.

You're still not going to be sure if it's valves, or rings if you don't test it with fluid before they disassemble the heads. Would really suck to assemble the engine, and have the same problem, then have to pull it down again for a broken ring. If the valve leaks you're sure what problem you're chasing. The valve may not show if it's a minor leak. 115 lbs may just not show up on the valve, or seat upon disassembly.

A leakdown test before pulling the heads would have isolated the problem, but to my knowledge it wasn't done. Putting oil in the cylinder, and repeating the compression test would have isolated the problem, but from your posts, you haven't said it was done either.

Since the head is now off you can't do either test, and you're guessing. It gets expensive when you guess, and are wrong.

You can do whatever you want as far as valve job for preventative maintenance, after you find the cause of the low compression. Not knowing if it's a valve or rings, you could waste a ton of time, and money for nothing, then have to pull the head again to replace the rings on that cylinder.

I'm basing this on nearly 50 years making my living wrenching, I'm trying to save you money, and time, but it's your car, your decision, and your money.

Cutting .010 off of the heads, just to true them up isn't going to make any difference that you'll notice. Any machine shop worth dealing with, will have a straight edge, and feeler gauges to check for flatness, determining whether they need cut, or not. In my experience Pontiac heads and decks are usually flat if they're low mileage, and haven't been overheated severely, but this can be checked as already described.

If you are going to keep the non adjustable valve train with stock factory nuts, it's imperative that the valve stems are all level, and at the correct height after the valve job is finished. If they're not, you're not going to be happy with the results. It's possible to have either ticking noise, or a valve staying open off the seat, depending whether they are too long or too short.

Hopefully you have someone doing the work familiar with Pontiacs, and the necessity of keeping all the valve stems equal in length within height specs. Good luck with whatever you choose to do...........

[supersport#69]

A leak down was done but he said he just did it old fashioned. He said adding oil to cylinder and retesting didn’t change the compression.

I am inexperienced at this stuff but will absolutely have the machinist do that test you suggested before he tears the heads down.

I agree not wanting it reassembled just to still have a low cylinder.

Thanks for the advice. I should drop the heads off tomorrow morning so maybe he can do that test tomorrow.

[Sirrotica]

If compression test done with oil, without any change from a dry test, it likely is a valve leaking. Good to know it was done both ways. Fluid leaking by valve will make it certain you're not chasing a ghost.

[supersport#69]

I got a curve ball on machinist first look before doing anything. He said the head shows the gasket may have been blown between the two middle cylinders and was surprised the one read 160 plus. There is some pitting in that low cylinder too.
Also the drivers side gasket was starting to blow.

I don’t have a real temp gauge but I don’t recall any overheating issues and it was running ok.

Mechanic thinks it may have just been ready to blow but wasn’t sure. If blown already. The cylinder compression was 160-170 in question.

[salem1912]

You did say # 3 cylinder right? Not sure I follow you with the smoke coming from passenger side.
you wrote "The valve that looks burnt is the intake, same valve nut backed off"

Also check for a wiped camshaft while you have it apart.

[supersport#69]

I’m sure I said wrong cylinder number then. Passenger side second from front. Smoke was always on that side.

Yes it appears the nut that backed off is the same cylinder that was low.

I’ll know more when heads are torn down.

[supersport#69]

Update: there was lots of rust in the heads and he found what might be pin holes so doing a pressure test. A few of the valves were brittle it sounds. He said he tossed one a foot and the head of it broke off. At least four valves are bad. And he’s thinking I had them removed just in time.

I’m just hoping I don’t need a replacement head/s

[supersport#69]

Update: heads passed pressure test. This makes sense since no steaming on pistons, water leakage, etc.

Low compression was caused by valves not seating on that cylinder. He was surprised it even ran. The heads and gaskets show evidence head wasn’t sealing and he thinks passenger side was blowing.

The mechanic checked the cam when doing compression test again before removing heads. Push rods all looked good.

Knock on wood- I might be ok once the heads get put back on.

I’m not sure on cost yet but the initial plan is hard seats on exhaust, new springs and maybe all valves. The guides I’m not sure of yet.

[supersport#69]

Heads are being finished up tomorrow. All new parts except for guides. I couldn’t believe the condition of some of the valves! The metal stem had gotten thinner toward the base cause by rust apparently. There was lots of rust on some intake seats he said too. He was surprised it even ran. He said the rust was so thick I couldn’t be getting much flow at all.

I’m excited to see how it runs after hearing how bad they were.
I’ll post some pics after I get heads Wednesday.

[Greg Reid]

Just goes to show you...just because something is original doesn't mean it should be considered sacred. This stuff doesn't last forever!

[supersport#69]

He suggested changing the studs and rockers (I think) to big block Chevy to make valves adjustable. I just had him leave them as they were.

He paints them grey but I am trying to find the blue I used last time. Dupli color 1616 but it seems to be nowhere I can get fast.

[PONTIAC LARRY]

With stock studs and rockers just get some 3/8 poly locks and set your valves precisely with a light adjustment (1/4 turn off 0 lash) ..more rpms !

[supersport#69]

Here is what I had.

[supersport#69]

After some blue on them they’re ready.

[694.1]

Dude. You are SOOOOO lucky!