The Stovebolt Site Forums The Shop Area Engine Shop Worn valve guides?

1956 half-ton with a rebuilt 235 in it. Engine was built by a company in Kansas City whose name I forget (Engine Specialists?). I have about 20,000 miles on the engine. Yesterday, it started running rough and smoking. I put a vacuum gauge on it and the needle fluctuates at idle between 18 and 24 inches (fast movement). Needle steadies when you accelerate. Smoke out the tail pipe is really hard to read. Sometimes it looks white. Sometimes it looks blue. My oil consumption is up, too, though the truck does leak quite a bit. This all started really suddenly.
Anyway, just for kicks, I reset the timing. Doesn't make a difference. Plugs, points, and condenser are only a few months old. Been about six years since I adjusted the valves.
Could this be caused by anything besides worn valve guides? And if it is the guides, what does this entail? Is this something you can do yourself? Anyone ever been through this?
The only other thing I would mention is that I JUST filled the tank with gas. Twenty minutes later, this problem started. Kind of grasping at straws there, but it is odd all the same.

It is more likely something other then the valve guides. With it idling pull one plug wire at a time & note how much it effects the idle. The cyl. that does not change is the one with the problem. Pill the plug & read it for carbon or oil fouled. Check compression on this cyl. Make sure you have some valve clearance on the valves for that cyl.
That should give you a good start to figuring the problem.
George

Thank you, George. I will be doing that.

What George says makes sense, of course. Fluctuating vacuum gauge like that can be a burnt valve or stuck, also. After doing George's suggested checks, a leak down test would further isolate the issue, if you still haven't definitively found it. Time since last valve adjust isn't as important as miles, assuming there isn't an issue with a stuck valve due to varnish from lousy or old gas. Normal interval by the old Chevy documentation was 5,000 miles between engine tune-ups, which included valve adjustment.

You may already know all this... but here goes. Compression should be about 130 # with all readings within +/-5% of one another. One or more cylinders that are "out of family" will help isolate the problem area. If you have one or more low compression cylinders, a squirt of motor oil through the spark plug hole and retest will tell you a lot. If the compression comes up after adding the oil, then it suggests stuck or worn rings. If the cylinder under test is still low and doesn't respond to the oil, that suggests a valve seating issue (burnt or stuck).

For context, I should add, I'm thnking that you may have a stuck valve, since the recent fill-up was mentioned. If that loosened crud in the tank, it is possible that the crap released contributed to a stuck valve...

Hope this helps.

Ditto to all the above, plus try a power balance test. Run the engine at a constant speed a little above idle (1000-1500 RPM) with a tachometer and a vacuum gauge attached. Pull one spark plug wire at a time and record the RPM and vacuum drop. A cylinder that's not pulling its share of the load will have little to no RPM or vacuum change. Good cylinders will make a noticeable change when the spark is disabled.

A stuck or burned valve is probably causing the rough running. Valve guides usually cause a puff of smoke from the exhaust after coasting and getting back on the gas, or on startup after sitting overnight.
Jerry

Thank you, Doug. That's useful information. I did the test suggested by George and it seems to be isolated to the #5 cylinder. There was no change in the idle when I pulled that wire and the spark plug is black and kind of wet looking. All the other plugs are dry and tan. I had to stop for the day but I was going to check and see if that plug is making a spark at all. Or maybe the better idea is pull the valve cover and see if either one of the #5 valves are stuck? This happened so suddenly that I have to believe the cause was also sudden, like a sticking valve and not the gradual wear of the valve guides. I'd appreciate hearing how you guys with a lot of time and experience with these trucks would go from here.

Do a compression test first, after confirming that the valve adjustment on that cylinder hasn't tightened up for some reason. If the plug is black and oily, you've either got a really bad valve guide, or the oil is getting past the piston rings on that cylinder. Dry, black soot indicates the plug isn't firing for some reason- - - -shorted plug, bad wire, even one flat lobe on a distributor cam can kill a single cylinder.
Jerry

So, first make sure there is no stuck valve and then do a compression test?

Valve clearance has a tendency to tighten up as the valves sink into the head a little from normal wear. I'm not as concerned with a "stuck" valve as I am with one that's lost enough clearance that it's staying slightly open instead of getting seated properly. The only way a valve cools itself is by transferring heat to the cylinder head through the valve seat every time it closes. If you haven't been adjusting the valves every time you do a tune-up, you might have a burned valve, not a stuck one. Hydraulic valve lifters have made us lazy over the past few decades. These old mechanical lifter engines need more TLC than one with hydraulics..
Jerry

OK, thanks. Should I get the engine up to operating temperature before checking valve clearance?

Yes, if you're going to do an adjustment while you're in there. Right now, I'd just like to see whether or not the valves on the cylinder with the miss have any clearance at all. Adjusting a valve that's already burned is like giving CPR to somebody at his funeral.
Jerry

Since the engine has an issue, I would suggest checking clearance cold. It should be above the specified clearance by at least 0.003" when cold. At-temp clearances will tighten up, so if you have around the spec clearance, you probably don't have a valve staying open, so can do the compression test at that point. Final adjust should be done hot, of course, but let's get the engine running on all cylinders first.

OK, I'll check for clearance and report back. If there's no clearance, should I assume there's a burnt valve or should I adjust and see what happens?

No clearance would just suggest the valve has sunk in the seat, and the rocker is holding it open a bit. Since it came on suddenly, I wouldn't expect it to be burnt, but it is possible. If you find no clearance, then adjust to spec plus some. Do the compression test. That will tell you if the valve seals or potentially burnt. If the compression reads low, I would still do the squirt of oil in the spark plug hole, and recheck, which will tell you valve or rings. If at that point, the compression still doesn't raise in that cylinder, that would make a burnt valve a likely culprit. Again, you probably know this, but don't forget to hold open the choke and throttle plate when doing he compression test, so it gets a good slug of inlet air.

Here's the Cliff's Notes version of doing a compression test:

1. Make sure all the valves are closing- - - -precise adjustment isn't a concern right now- - - -just be sure there's at least SOME clearance when each valve is closed.

2. Remove all the spark plugs. Block the gas pedal wide open and make sure the choke is fully released.

3. Disable the ignition. On a pedal start truck just leave the key in the "off" position.. Disconnecting the primary wire at the side of the distributor is a foolproof method regardless of how the coil is wired.

4. Spin the engine with the starter after connecting the compression gauge to the #1 cylinder. Let the compression "hit" 3-4 times. Record the reading and move on to the next cylinder.

5. Repeat step #4 until you run out of cylinders.

6. If the compression readings aren't within about 5% of each other, squirt a couple of tablespoons of motor oil into the cylinders one at a time and repeat the sequence for a "wet" compression test. This temporarily seals worn piston rings. Little, or no pressure increase will be seen in a cylinder with a bent, stuck, or burned valve on a wet test.

Two adjacent cylinders with low or no compression pressure probably indicates there's a head gasket blown out between those cylinders.

Good luck!
Jerry

First off I would check the plug wire & dist cap on #5 cyl. The wire should have no more then 10,000 ohms per foot of length. If you don't have an ohm meter change the wire with another wire & see if that changes the miss to the other cyl. Examine the inside & outside of the dist. cap for carbon tracks or cracks around #5 term. Check the spark plug for cracked porceline.
George

I have a broken pushrod at the intake valve of cylinder #5. It's the mushroom part at the top. It split lengthwise and half of it was lying in the bottom of the rocker arm space. The rod isn't obviously bent, though I understand you'd have to take it out to know for sure. I was thinking if it was bent it might indicate a stuck valve. Otherwise could just be metal fatigue but I'm just guessing. I checked all clearances and every valve has some clearance when it's closed though a couple are very tight, and that's when they're cold. So I'm thinking take the head off and get it to a machine shop? Replace the pushrod myself and check for free movement of all the valves? Would appreciate any suggestions. Thanks.

Do a hot valve adjustment and drive on- - - - -chances are the broken pushrod is the only problem you have. Just be a little more conscientious about adjusting the valves- - - - -maybe every 5-10K miles. Give it a few miles of running on all six before you go looking for gremlins that might not be there!
Jerry

Yeah, I don't know what I was thinking with the valve adjustment. Probably, "I'll get to that as soon as I can." Which turned out to be too late. So to get the broken pushrod out I assume the head has to come off, correct? I'm also assuming you have to buy an entire set of pushrods. Is that also correct?

No, unless there's a piece of the pushrod that has fallen down into the side cover, just pull it out the top. You can loosen up the adjusters and push the rocker arm(s) sideways against the spring on the shaft to remove/install the broken one. If you can't reach the broken piece, pull the side cover. No need to remove the head. New pushrods for stovebolts aren't made any more, to the best of my recollection. Get a length measurement of a good-condition pushrod from your engine and PM that dimension to me. I'll send you one.
Jerry

Sorry, I posted before I researched. I saw an old thread where you explained how to remove a pushrod. Since half the head of the rod is missing, it slips right past the rocker arm. Meets some resistance when the lower end hits the hole above the lifter. I'll take a closer look. Thanks for the offer. I'll let you know about dimensions.

I got the pushrod out. It's not bent at all. I rolled it along a piece of flat 3/8" aluminum stock and the heads at either end don't wobble at all. Sighting down it shows it's straight, too. Checked the aluminum with a straight edge to make sure that was flat. Is there any way to make sure that valve is moving in its guide? Jerry, I'll e-mail you with some info.

Use a pry bar under the rocker shaft to push down on the rocker arm tip. You should be able to move the valve through its full travel without any binding or sticking. It would be a good idea to run a little "Sea Foam" additive in the oil for a change or two and also put some in the gasoline if you suspect there might be gum on the valve guides. I add the stuff to the fuel on both my motorcycles, and it really does improve the performance, IMHO.
Jerry

You can still get pushrods. Truth be told, the aftermarket replacements are stronger tube construction as opposed to the solid rod construction of the originals. The length of the pushrod depends on the style of lifter. Early to middle 55 and back solid lifters motors used a taller lifter, looks like a milk bottle. And in fact many people refer to the those style lifters as milk bottle lifters. Those motors use the shorter push rod. If a 56 and up truck motor used a milk bottle lifter from the factory, I have never seen one. Later 55, and all 56 and up truck motors used a shorter lifter, resembling the hydraulic lifter used in the Power glide car motors up to 55, and 56-62. Those motors used a longer push rod.

Here is the catch. Sometimes there is a mismatch between the ball on the rocker adjuster and the cup on the pushrod. The ball being too big for the cup. What happens is that the ball is contacting the just the rim of the cup and pounds on the cup until it splits.

If you are replacing pushrods, inspect the condition of the ball adjusters, they should be relatively round. use a little red or blue dykem on the ball adjuster and the cup of the pushrod, and run the ball adjuster in on the pushrod by hand. Take a look at the register marks where the dykem wore off. You want the ball in the cup, not just on the rim.

Just to update this thread, I checked to make sure the valve in question isn't stuck. It is not. Moves freely up and down with pressure from a pry bar. So it looks like the pushrod just gave it up all of a sudden.

It's possible to order custom length and diameter pushrods from Comp Cams with any tip shape and size you want. I used to do it pretty regularly when we were building roller-cammed round track engines with special ratio rocker arms and non-standard valve stem lengths. They do get a little spendy when getting them made for a 1930's designed glorified tractor engine, though!
Jerry

I got a spare pushrod from Hotrod Lincoln. Installed it, adjusted the valves hot, and the truck is running perfectly. Don't know why the old one broke. Maybe just metal fatigue. Anyway, back on the road.