Bent pushrod mystery

Little history, 1976 CJ-5 with a 304. The vehicle had sat for about 10 years before it was given to me. I found it had a bent push rod when I fired it up in #3 cyl. Replaced, now ran good. Jeep sat idle for another 2 years while I upgraded the running gear and other stuff. I took it out on it's maiden voyage last Saturday to run in the new gears. Ran it up and down a long gravel hill for about 30 mins. just to warm up the diffs. Took it home to park it. It ran beautifully all during this time.
Tonight (two days later) I fired it up again and it ran awful. Narrowed down to a cylinder, found bent push rod.(#4 intake). Checked the rest on that bank, also found #8 intake bent. I checked to see that the valves were free by bumping with the butt of a 3 lb sludge. I could feel good spring rebound so I'm assuming they are free. I don't know if the P/O changed a cam or springs. I didn't see anything obvious that performance work had been done. Because it was running perfectly when I shut it off on Saturday I'm assuming this happen when I again restarted it tonight. If anyone has any clues as to what might be going on I'd be anxious to hear.

Could have a valve sticking open, it would only take one cycle of the piston with a valve open to bend a pushrod. Could be a stuck lifter. Hard to say. The whole set of pushrods can be changed for $50, including gaskets. Since it sat for 10 years, probably without being started, there is probably varnish and traces of rust from moisture and the acids left in the crank case by combustion. You might try running it for a while with some sort of crankcase cleaner additive (maybe BG44K MOA).

Bob
tufcj

I had the same problem but I ended up breaking one and bending 3 others 2 at almost a 90. No bent valves, no witness marks on the pistons, no bad rockers or stuck lifters. Im pretty sure the valves stuck in their guides. maybe a little condensation or moisture got in there and the valve stem stuck to the guide enough to bend the rods.

My first thought was that after I shut it off the valve stem bores contracted as it cooled and maybe had enough grip on the stems to initially hold them due to an accumulation of varnished oil on them. I'm thinking about just replacing with oem rods and trying it again. Nothing else makes sense.

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Originally Posted by ironman_gq

I had the same problem but I ended up breaking one and bending 3 others 2 at almost a 90. No bent valves, no witness marks on the pistons, no bad rockers or stuck lifters. Im pretty sure the valves stuck in their guides. maybe a little condensation or moisture got in there and the valve stem stuck to the guide enough to bend the rods.

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By the way, Did you determine what caused your issue and what did you do to rectify it?

No I ran it a few cylinders short for about a week then pulled the motor and did a full rebuild. everything looked fine when I took it apart but I replaced the valves anyway just to be sure, guides looked good and nothing else was bent/broken so it got new bearings, rings, cam. intake. lifters. timing set and got a full cleaning and ground the valve seats

OK, Mystery solved. I did indeed have 3 stuck valves. Luckily they stuck in the closed position. Tens years of sitting under a tree and a steady diet of non-detergent motor oil prior glued the stems quite firmly to the guides. When I first checked and bumped them with a hammer handle I must have glanced off the top of the stem and down onto the spring cap. Felt like it was moving at the time. Anyway, I pulled the springs off and brushed acetone on the stems at the top of the guide and heated the guides with a heat gun. Things slowly started to move. I just kept applying acetone and working the valve up and down until enough of the varnish was dissolved and the valve moved freely.
I think the heads may have been reworked just prior to it being put out to pasture. One of the intake valves obviously had been replaced. The keepers and retainer cap was very different from the rest so I'm assuming it may have been a sbc valve. All the interface between the stems and guides were pretty close tolerance as if it hadn't been run much before it was parked. This only compounded the sticky valve issue I was left with. It's running pretty damn good now. I ran a can of Sea Foam through it and now the plugs look oil free on initial inspection. There was quite a bit of blow by before.

I think the chevy valves have a different stem diameter than the amc valves and are a common performance swap as they come in more sizes and are much cheaper than stock amc ones.