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Bent Pushrod What to Do

Z

zwages

Member
I knew it would happen one day, I just didn't figure it would be today that I would declare an emergency. During steep turns near Morgantown PA I was greeted by a thump and loss of power. It was clear that I'd lost a cylinder so I turned and headed for KLNS. Its all fields between the two so the pucker factor wasn't that bad. Well, at least until the oil started to sprinkle onto the windscreen.

The tower was kind enough to get me to the ground right away and even give the runway a good scrub after I'd taxied over to Florintino's for a nice lunch to reflect on today's adventures.

Once the top cowl was pulled, I found a broken exhaust side stud that had let its parts fall into the intake side assembly. The intake pushrod was bent, it's tube was bent as well. Obviously I need to replace the obvously broken parts. What knowlege and operating experience can you guys bestow upon me reguarding cam health? Should I pull or replace the affected intake lifter? Have you seen cams fail after this sort of event? Any advice on the entire situation here is appreciated.

For reference, the engine is a o-320-H2AD that is 250ish hours since MAJOR by Tropic Air Power. The stud appears to have had a factory defect. Viewing the broken portion, there's a small amount of different grain structure.
 

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At 250ish hours since MAJOR, I would also be upset that the builder didn't clean and repaint the motor and install new valve cover gaskets....

I would talk to the person that said it was majored 250ish ago......:eek:
 
It is unlikely the cam has suffered any issues however the good thing about THOSE engines is you don't have to pull the engine out to inspect the camshaft or the face of the follower.

The first thing I would do would be to remove the pushrod and pushrod tube, then remove the follower and inspect it. From there you can inspect the cam face, as well as the hydraulic lifter and cup. If those were looking ok I'd pull the cylinder and get the necessary repairs made. Looking at the photos its hard to tell... is there any other damage under the rocker cover?

The lifter and cup will have suffered some pain. I would definitely replace those.

A good engine shop will be able to get you sorted out.
 
While anything is possible, it seems unlikely the cam or lifter experienced any damage. The pushrod is weak and takes the bulk of the damage. I would inspect the plunger assy if that has traditional parts. Lyc typically uses over size threads to hold studs in, so that cyl will likely need to go to a cyl shop or at least a machine shop to get it out.
 
I would be careful about assuming it is a defective stud. Typically you can not tell visually you have non-conforming grain structure. More likely you are looking at differences in the fatigue versus overload portion of the failure.

My concern would be that they misassembled something or torqued the stud too high, creating the failure on a good stud. You might have 3 more cylinders at risk if they repeated the mistake.
 
AaronG said:
I would be careful about assuming it is a defective stud. Typically you can not tell visually you have non-conforming grain structure. More likely you are looking at differences in the fatigue versus overload portion of the failure.

My concern would be that they misassembled something or torqued the stud too high, creating the failure on a good stud. You might have 3 more cylinders at risk if they repeated the mistake.
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Never seen one of these engines, but in the pics it looks like the stud has a square spacer that goes between two rails. You can see the oil staining covering about 30% or more of the contact area on the heads mating surface. It would appear to possibly be an issue with parallelism/flatness on the mating parts that allowed a rocking motion that would add a lot of fatigue stress to the stud. Never seen one of these, but guessing that the half ball is torqued down against the square spacer. This is an assumption, as most automotive rocker studs that I have seen tulip into a wide base flange in order to deal with side loading commonly seen with rockers.
 
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The lifter in an H2AD is like a car lifter. The follower and lifter are all in one. It is definitely on the replace me list! You will be able to check out the cam lobe way better than looking at it through the lifter hole, once the cylinder is off.
Good Luck,
Mahlon
 
Most H2AD engines, when majored, get what is referred to as the "T" mod; which replaces the removable followers with a non-removable mushroom style cam followers.
 
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The T mod is a different crankcase with larger diameter lifter holes to incorporate larger diameter "car style" lifters that have a larger outside diameter then the original lifters. In addition, the cam for the "Tmod engines" had wider lobes then the original H2AD cams. This gave the lifter and cam lobe a larger bearing surface on the T mod engines vs. the original design. They lifters are not mushroom style, still automotive style and can be removed without disassembly of the engine's crankcase.
In the certified world, non t crankcases can be modified to T style through STC.
Good Luck,
Mahlon
 
Mconner7 said:
I have bent two pushrods, both were stuck valves. I would ream the guides.
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I don't think this is necessarily the case here. If the exhaust valve is stuck closed cuz of the broken stud then the cylinder would be under compression as the intake valve opens. There's some overlap for scavenging so the intake would trying to open as pressure is highest...I'm guessing maybe enough resistive force to bend the intake pushrod?

BTW is that the baby blue Lycoming used to use? I'm glad they switched to grey
 
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I’m thinking the valve guide clearance was too tight and the valve seized, then when the cam tried to open the stuck valve, it bent the pushrod. Has the engine ever experienced “morning sickness” (rough running in the morning.. acts like a fouled plug but then starts to run smooth a few minutes later?
 
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I don't think a sticking valve caused the bent pushrod...it's the intake.

I like to see what Mahlon thinks but I suspect the rod was bent from the intake valve being resisted by compression, jug was under compression because the exhaust rocker arm broke off and exhaust valve couldn't open. The exhaust stroke turned into another compression. Since there's valve overlap as the piston approached TDC on exhaust stroke the intake valve was trying to open at near peak pressures...peak considering this was just proceeding the power stroke and there was nowhere for the hot expanding gases to go and they were just re-compressed.

If this is indeed what happened, I would take that entire cylinder off and have it IRAN'd. I would also get some guidance on whether the connecting rod should come off and be inspected. Not sure what could happen when a piston attempts to re-compress an expanding combustion process. Those hot expanding gas are still somewhat burning on the exhaust stroke.
 

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bent push rod

Those rocker studs get specific torque.
I wonder if it was over tightened - no washers under the
fulcrum nut

mg
 
avrojockey said:
I don't think a sticking valve caused the bent pushrod...it's the intake.

I like to see what Mahlon thinks but I suspect the rod was bent from the intake valve being resisted by compression, jug was under compression because the exhaust rocker arm broke off and exhaust valve couldn't open. The exhaust stroke turned into another compression. Since there's valve overlap as the piston approached TDC on exhaust stroke the intake valve was trying to open at near peak pressures...peak considering this was just proceeding the power stroke and there was nowhere for the hot expanding gases to go and they were just re-compressed.

If this is indeed what happened, I would take that entire cylinder off and have it IRAN'd. I would also get some guidance on whether the connecting rod should come off and be inspected. Not sure what could happen when a piston attempts to re-compress an expanding combustion process. Those hot expanding gas are still somewhat burning on the exhaust stroke.
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The pushrod will easilly push the valve open, even if the cylinder is under compression. Ever removed a valve spring? I put 100 PSI of air into the cylinder and can open the valve by pushing on it with my palm. Typical pressures at the top of a compression stroke are only around 160 - 180 PSI. Valve surface area is small and pushrod can EASILLY deal with this without bending. It is true that the gas will expand on the power stroke, but the volume of air ingested was typical therefore the exhaust stroke will not apply significantly more force, with both valves closed, than a compression stroke. I suppose that the air has expanded a bit as it is still hot, but we taking something like 10% more volume. Still within the strength of the pushrod. The serious forces are the rapidly expanding highly compressed air at TDC. It's all pretty much over by BDC. Far more likely that when the stud broke, the parts landed in such a way that they restricted the pushrod from moving the intake rocker. Also possible that you experienced a stuck exh valve and it broke the stud. The latter being something the rest of us would never see as we have very beefy rods to support the rockers. I personally don't know if the weak point is the stud or the pushrod on that engine.
 
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Looked inside yet?

I'm watching this progression closely, as I have an H2AD on my RV-4. Im also an AP/IA and I rebuilt my engine from an old run-out core I got from a C-172. I'm fairly well versed on these engines, which don't have anything common to the "other " O-320's, yet they are still very reliable with plenty still running hard. Have you pulled the plugs and looked inside yet? I haven't seen it in the thread that any internal cylinder inspection has been done yet. I've seen plenty of bent pushrods from an object ingested. As others have mentioned, the Ford type liters can pull right out to examine the cam, T or no T mod. May also look at the valve spring (inner) for failure which allows the valvetrain to get sloppy. The broken stud does question if the fulcrum was improperly seated, and the stud was flexing for the last 250 hours. Hopefully a cylinder R&R/IRAN will get you in the air again, but I am curious about what can be seen inside that cylinder.
 
lr172 said:
The pushrod will easilly push the valve open, even if the cylinder is under compression. Ever removed a valve spring? I put 100 PSI of air into the cylinder and can open the valve by pushing on it with my palm. Typical pressures at the top of a compression stroke are only around 160 - 180 PSI. Valve surface area is small and pushrod can EASILLY deal with this without bending. It is true that the gas will expand on the power stroke, but the volume of air ingested was typical therefore the exhaust stroke will not apply significantly more force, with both valves closed, than a compression stroke. I suppose that the air has expanded a bit as it is still hot, but we taking something like 10% more volume. Still within the strength of the pushrod. The serious forces are the rapidly expanding highly compressed air at TDC. It's all pretty much over by BDC. Far more likely that when the stud broke, the parts landed in such a way that they restricted the pushrod from moving the intake rocker. Also possible that you experienced a stuck exh valve and it broke the stud. The latter being something the rest of us would never see as we have very beefy rods to support the rockers. I personally don't know if the weak point is the stud or the pushrod on that engine.
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Ahhh okay...the intake valve FODing out makes more sense.

I would be interested in knowing from the OP if it showed signs of valve sticking, ie. morning sickness
 
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