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How a simple valve adjust can ruin your whole day.


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Hi everybody! It's me, the "I can't believe I did that" guy..... :stupid:

Well, believe it or not, I have another "learning experience" to share. So grab a coffee and get comfy.

Working on a new friend's 78z over the last few days, trying to get it ready for an "out of province inspection". Your Canucks will know what I mean. When moving a car to new province, everything has to work, and no rust holes or no insurance. Quite a challenge for most Z's, this one is no different.

While doing a million other things, we decided to investigate a small engine "rattle" that happens under any amount of power "on" application. Not good. Bad bearing, slap happy piston, loose valve train? Let’s find out.

Started with a valve lash check and setting. Off comes the valve cover. Well, it came off but the gasket didn't so much, had to be scrapped off. Then there was some RTV stuff left over that was doing a great job of gasket sealing to peel off, and we all know how you don't want those little tendrils of that crap getting in the oil, so while I was peeling and digging it off, I stuffed a standard blue shop rag sheet in front of the timing gear to prevent RTV bits from going down there, cuz most of the RTV was on the front part.

Finished the clean up and proceeded to start checking valve clearances. Right away I notice that #1 rocker is super loose. Have to jack up the adjuster about 6 feet (slight exaggeration) to get .010 clearance. Yeah, because the valve is stuck partly open, the spring pack is about 0.10 lower than its mate next to it. I can push it down and it pops back easy enough, but right to a nice stuck position. Alright, there’s the rattle source. Stuck/bent exhaust valve. Proceed to start spinning the engine with my remote start button and checking a few more looking for loosy gooseys .

Just about then, the car's owner who's working with me tells me to hold it, that blue rag is caught up in the timing chain/gear. Sure enough, I'de left that thing down in the timing cover and it got dragged up and is now nicely distributed over the cam gear

between it and the chain. We're talking a thin synthetic blue towellete here, we just grab its bits and spin the engine and get it clear. Its shredding into bits and pieces its so thin and weak. No biggy right? Read on....

Finish setting the valves, which are all somewhere from spec to spec x 3 too wide, slap the cover back on with a new gasket and try to start the car. It cracks but we're getting no fire. Nothing. Ok, what did we do?

First I had just yanked the plug wires off, didn't check what order they were in, they were just zip tied together, not in nice ordered clips, so who knows how the PO had left the timing or drive spindle. Could be anywhere. So set the engine to TDC

and the dizzy is pointing to one, so just put them back on again in stock order and position. Still no go. Check spark, good, all cylinders. Check each injector with noid light. Flashing. Check fuel pressure. Good. oh oh...... What the hell have I done.

Swap ECM's. Nope, same. Threw timing light on #1 and check while cranking. Yup, right at 15' ish, good enough. Hmmmm, spark, fuel, timing, what else do you need? Compression???? Pull the plugs and stick the comp gauge on #1. I know its going to be 0 with that bent valve. Sure enough. Then I check #2. Its 0 as well..... ?????? And 3..... and 4 ...... oh no...... and 5, and 6, all = 0.000000000 How is this possible?

To make a long story short, hour and half later, heads on the bench, all 6 exhaust valves are bent. Yup, all six. Nice nick in each piston, nice shiny spot on each valve face. The timing chain slipped on the cam gear by about 5-6 teeth when that thin, feather weight, POS blue paper towel got in between there, causing a not too surprising detrimental effect on piston/valve clearance. When you think about the force required to stretch the chain to make that happen, I initially dismissed it as impossible. No way that piece of fluff did anything bad. Yeah, right.

What I hadn't considered was the incredibly loose stretched timing chain on this old well worn L28..... I can *almost* slip a tooth by hand it's so loose with no tensioner pressure. Just took a little old piece of near-nothing to bunch up "just so" and make it happen. I'll bet you couldn't do that again if you tried on purpose all night long.

I had actually noticed that the cam seemed out of time earlier but didn't catch on. When I was setting TDC to check timing and dizzy alignment after it didn't start, I was peering in the oil filler hole for #1 bunny ears, and noticed that the centerline of them wasn't at 12:00 at crank TDC, more like 10:00....

So all you engine tinkerer's out there, beware! Even the slightest mis-step can lead to disaster without you even knowing you done it... Runs, set the valve, doesn't run. How do you get that wrong?

Of course now I’m thinking about that initial stuck valve that I found before the engine turned with the rag in it. Just how did that happen? Maybe the chain had already slipped a couple of teeth before I got there due to that crazy loose chain, just enough to take out that one valve, and my rag trick just finished off the other 5 by slipping it a couple more.

Let’s just say that’s true and not ALL my fault.. Yeah, that feels better. A little….

Well It’s a good thing I have all them old heads in the shed..... Eenie meenie minie moh...

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That is the first verified account I've read of a chain actually jumping a tooth (or two). It gets proposed all the time but I've never seen it verified that it actually happened. Thanks for the story.

You probably saved the owner the cost of new pistons from the slippage happening at high RPM. He owes you.

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My last valve adjustment (actually just an inspection) I left fuel injector wires where they got pinched between the valve cover and the head. Took me weeks to figure out what the heck had gone wrong with the FI, in the meantime ordering lots of replacement parts.

Like you say, a moment of carelessness can result in carless-ness.

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I agree. Chain tooth jumping was just a dirty rumor for me as well until now. Though it seems to take quite a combination of a loose chain and some "help" apparently.

Thanks for the additional support to the "it ain't ALL my fault" threory as well....

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Don't feel all that bad. I was helping my brother out before he went for a long trip in Northern Arizona (he drives a 620). I figured that it be nice to get him a new fan clutch as the old one was stuck on - as in it was on all the time. Well in 620's the waterpump and fan is one piece (yay) so new waterpump sweet. All was going great right until the end, when I tightened the one LONG bolt - you know the one that goes into the block. Took it about 1/16 of a turn to fast and CRACK! Snapped right off. Long story short, three front cover replacements his truck is in running shape.

As for chains, I did mine a year after I got my car, as it had an annoying rattle - sounded like the chain (which is still there mind you). The old chain I took out was roughly 2" longer then they new one - and based on records I had my car sat for 5 years and didn't move. So its possible that your friend's chain stretched even more.


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Wow that's hard to believe! Its scored all the way around!

Its not clear just how close the chain must be to the cover until you see something like this.

Does this mean it was loose enough to be "floating" off the teeth enough to be rubbing the cover over that much of its circumference? Hard to imagine how this looks running, but there's the proof.

I haven't really looked under the cover of mine, other than to note how black and grundgy is looks.... I'll report back.

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I will add to this since that is my valve cover picture. Blue and I did a valve adjustment on my car several weeks ago due to a slight rattle we heard. We kept the blue shop towels away and got everything in spec, now its as quiet as a sewing machine. My chain has no shortage of tension on it and I cannot hear it scraping or touching the valve cover when its running. Even used a mechanics stethoscope and all seemed fine.

It is a very unfortunate series of events but it happens to all of us. Add to the fact all of our cars are 40+ years old, many with questionable past ownership and maintenance. All of this seems to magnify any failures we experience, weather they are created by us or by sheer wear and tear.

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