Bent Push Rob Saga

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dusterbuster5621

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Denver, Colorado
TWO BROKEN PUSH RODS Saga! Hi all!! I have a 318 in a Plymouth Duster I rebuilt with my dad about 15 years ago. The car/engine hasn't been driven very much, probably less than 1,000 miles.

So the complete back story:

A few months ago I noticed that cylinder two (front most passenger side) exhaust wasn’t getting hot. But the engine was starting and running alright without that cylinder. I pulled the valve cover and found cylinder twos intake push rod bent very bad. Ordered some new ones. Put the new push rod in and unknowingly reassembled the rocker shaft incorrectly with some of the spacers in the wrong places. Not knowing this at the time, I tried starting the reassembled engine and heard some terrible knocking, so I opened up the valve cover and found the new push rod on cylinder twos intake bent and cylinder 6’s intake push rod bent.

I disassembled the rockers. I checked the valves and lifters and from what I could tell they were moving fine. Nothing seized. I used a scope to look at the top of the pistons and did not see any clear contact/damage. This is also when I realized I had assembled the rocker assembly with the spacers placed incorrectly. So I reassembled with new push rods on those cylinders and the rocker spacers in the right spot. I turned over the engine by hand. Everything moved fine but there was a strange hissing sound that I couldn’t determine where it was coming from. I turned it over with the starter and coil disconnected so it wouldn’t start and everything sounded fine. Then I tried to start it. The engine started up immediately but there was this terrible knocking sound so after a few seconds of it running I shut it off. I cringed as I opened up the passenger side valve cover expecting more bent push rods but found them all straight. I reassembled and tried starting again and now the engine is struggling to start and is making a ton of popping sounds. Some air even shot out of the intake air filter. I wish I could post a video on here to show you.

I’m stumped. It seems like my timing is off now? Maybe? Curious to hear everyone’s thoughts and what I should check next.
 
I’d go for the simple stuff first.

Pull the vc off the other side and look things over.

On the side you’ve been working on, pull that cover back off.
Turn the engine over to the point where each rocker is all the way closed.
When each is in that position, make sure the rocker isn’t able to flop back and forth between the valve and the pushrod…….. but it should slide fairly easily side to side.
 
Timing chain can also be the culprit if not installed properly or if bolt has loosened from cam.
 
yeah, if it wasn't rebuilt only 1000 miles ago i would have said timing chain stretched and jumped teeth. easiest check for stretch is to turn the crank back and forth at the pulley bolt while watching how far it moves each way before then rotor arm moves. it shouldn't move very far at all.
neil.
 
@GTX JOHN I did a compression test and here is what I found:
  • Cylinder 1: 102.5 PSI
  • Cylinder 2: 120 PSI
  • Cylinder 3: 105 PSI
  • Cylinder 4: 15 PSI
  • Cylinder 5: 110 PSI
  • Cylinder 6: 115 PSI
  • Cylinder 7: 112.5 PSI
  • Cylinder 8: 115 PSI
The real kicker here is that I checked Cylinder 2 and 4's compression about 2 weeks ago and both were around 80 PSI. So something is wonky.

@PRH I checked under both valve covers and things look fine. No obvious bent pushrods or anything looking out of place.

@tonysrt the timing chain was replaced during the rebuild.

@mygasser can you describe the test to check for stretch in the timing chain more? What is the rotor arm you are referring to? The distributor's rotor?

@cookietruck
no I haven't pulled the distributor.


*big sigh* it is seeming more and more likely I am going to have to tear the engine apart to figure out what is going on. With Cylinder 4's compression way down, the popping and air coming out the carb, it seems like the timing chain may have skipped? Any advice on the best way to check for this before pulling the front of the engine apart?
 
it seems like the timing chain may have skipped?
Click to expand...

Not at all likely(and how would that affect only cyl #4?)
Probably a bent valve in number 4 cyl.
And not likely bent “just a little bit” either.

A relatively easy visual inspection for that is to look across the tops of the retainers for that side after removing the rocker shaft.
You’re looking to see if one of the valves/retainers on #4 is noticeably lower than the others.

If that doesn’t answer the question, I’m sure a leak down test of #4 would.
 
Last edited:
I googled Denver, and found the elevation there to be 5350.
I checked the Wallace calculator and found that a low-compression (8/1) 73 up 318 should be making ~112psi, so it looks like the valve-timing is good.
But, it looks like 5 of your cylinders appear to be on track, therefore
3 are bad, therefore;
the next move is to do a LeakDown test on those bad cylinders.
and because the one is extremely low; baring having inhaled a foreign object, my guess is that those three have bent valves, which means,
it's valve-job time.
Because of the bent pushrods, I'm blaming it on tight guides, so I'd do both sides; or you could be taking all apart again soon.
--------------------------------------------------------------------------------------------

Personal musings; (I gotta say that, else certain individuals will castigate me)
BTW-1
Seized valves occasionally happen to engines that sit unused for long periods of time out in the environment. Especially in environments with large temperature swings and high humidity.
Over the last 55 years, I have had a couple of engines bend valves on me, even now, I have one here.
BTW-2
Because your elevation is so high, you may have noticed that your 318 is pretty sluggish at your current pressure. This is because even tho your current compression ratio may be 8/1, that, because of your elevation, the EFFECTIVE ratio may be as low as 6/1, a full point lower than at sealevel.
Even if you increased the Scr to 9.2, the Effective-CR only comes to 7.1..
At 10.25, the Effective-CR finally gets to 8/1
BTW-3
To get the sealevel pressure back, the Compression Ratio needs to come up to 9.2/1.
and for performance, to at least 10/1,
but not to exceed 10.25 unless you don't mind running 91 gas.
If you decide that you want to bring the compression up closer to 10/1, then I highly recommend to just start with a 360. It costs about the same money to bring the 360 up to pressure, but it will be vastly better at low-rpm. The Wallace points to 51% better, with the 360 at 10/1, versus your current 318
BTW-4
If your Duster has to operate at other elevations, then, you have a problem. You have to limit your cylinder pressure to whatever lowest elevation you will be operating at, which keeps the engine out of detonation; and then, at 5350, it is what it is.
The best solution is supercharging.
But, alloy heads are a reasonable compromise.
With those, you can set the pressure to near 200psi at sealevel, for instance, and the pressure still be near 170 in Denver. Obviously, the 360 thus set up, will be light-years stronger at low-rpm, than the 318, even similarly set up.

Whatever you decide;
Happy HotRodding.
 
@AJ/FormS, thank you for all the by the ways. The car has sat without running a lot over the years. My dad and I rebuilt it together then my dad passed away in 2016 and then it sat in my mom's garage until 2022 only getting started about every 6 months. I swear we had built it to be closer to a 10:1 static compression ratio but I was 13-15 years old when we rebuilt the engine so I may be misremembering things. I actually have a 360 block that I bought a year or two ago that I have been thinking about rebuilding but gotta save for a down payment on a house first! haha

@AJ/FormS and @PRH I don't think I'll try a leak down test. I think you're right that the valves are likely bent. I am going to start prepping to get the heads off. Remove coolant and start disassembling the top of the engine. Should I buy all new valves, guides, retainers, and seals to rebuild the heads?
 
Last edited:
Should I buy all new valves, guides, retainers, and seals to rebuild the heads?
Click to expand...

I’d want to assess what you had before ordering anything.

More than likely, some machine shop services will be required.
 
The Saga continues!!! I got the passenger head off yesterday and it looks like I was wrong about the valves hitting the pistons. Cylinders 2, 4, and 8 show signs of the intake valve hitting the pistons. (See pictures) I haven't taken apart the head yet. A couple questions:

  • Is this enough damage to the top of the pistons to warrant replacement? I was hoping I could deburr them in the block, clean everything really well, and then reassemble.
  • I am going to take apart the head later today but this brings me back to timing issues and if I should check out the front of the engine as well. Does this indicate to anyone that the timing was off? Or is still the most likely issue valves got stuck (in the open position) and started hitting the top of the pistons and bending the push rods?

  • IMG_8981_2.jpg
    IMG_8982_2.jpg
    IMG_8987_2.jpg
 
Hi All! Just took off cylinder 4s valves and have a picture attached. That was the cylinder that had terrible compression. They seem to be in good shape and straight when I put a straight edge next to them. What are thoughts on this? It is looking more like a timing chain skip from my perspective. I am going to tear into the front of the engine next.

IMG_8990.jpg
 
if the cylinder only had 15psi compression the. One of the valves is likely bent a bit.

What size cam is in the motor?
 
it's highly unlikely that the timing chaing skipped. shearing a crank or cam key would be more likely-- unless it's a nylon toothed sprocket, then all bets are off.

that cam isn't ridiculous in the slightest, but i always check P/V clearance above .450 and if your pistons are positive deck to achieve that 10:1 static it is very possible they hit.

as an aside, what kind of rockers are you running?

IMG_8990.jpg


see that mark? that's galling. it's indicative of the valve stem rubbing the guide, which means the valve is bent.

so you have multiple touch ups on the piston crowns, which means you've obviously got piston to valve clearance issue. this would lead me to believe that one (or more) of the following is going down: the timing set was installed improperly, the cam lift is not what is on the card, the pistons are positive deck with a very thin head gasket, rockers are 1.6:1

another, albeit distant possibility, is that the guides are too tight. this can hang up the valve and allow for contact and then congrats: bent valves!

last order of business: yes, you can clean up the pistons in the block. knock down any high spots, dress any sharp edges, clean it up real good and ship it.
 
I see the heads have the two groove exhaust valves.
Those typically are used in truck/industrial applications with rotators and short springs.
They usually don’t play well with .450 lift cams, so just make sure someone has verified all the parts being used are compatible.

The intake valves contacting the pistons, barring some other yet to be discovered issue(like stuck valves or pushrods not properly seated in the lifters), would usually only happen if the cam were way advanced(as in installed wrong).
 
dusterbuster5621 said:
@mygasser can you describe the test to check for stretch in the timing chain more? What is the rotor arm you are referring to? The distributor's rotor?
Click to expand...
yes the distributor's rotor. with a socket on the crank pulley/damper bolt rotate anti clockwise until the rotor moves, then rotate clockwise until it moves again. if the crank' can rotate more than a few degrees between rotor movement at either extreme then the chain's stretched bad. hope that makes sense.
neil.
 
I think that when you cranked it over with the rocker arms spacers incorrectly, and heard that initial noise,- that an out of place rocker/s trapped a valve/s open causing the interferences.
jmo
 
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