7th cylinder connecting rod in PIECES....

AMC friends,

I built a 360 last year, and threw a rod through the oil pan a few months ago. I had just changed the oil after a few hundred miles and added zinc in as prescribed.

10 minutes later I was working on carb tuning and blipped the throttle on the gas pedal with my hand, and it overrevved and the damage happened. I didn't feel like I had revved it that hard, felt more like a tap.

Good news: block from what i can see is ok. Crank has bad scarring.

Questions: could the ZDDP addition have something to do with it? What could I have screwed up on the build? Are the cast iron rods weak? (all parts were checked at machine shop pre-assembly)

Reason I'm asking is that I'm going to rebuild in the fall. I'll have machine shop go through it again, will likely need a new crank. Would love advice if I missed something so I don't repeat my mistake. Thanks!

Shawn

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Did you use new rod bolts? What I'm seeing by the bent rod cap is that a bolt may have failed or come loose. I always replace the rod bolts with ARP, then have the big ends resized. It's always hard to say. Were the rods magnafluxed? A fine crack could have escaped a visual without magnaflux.

If you need a rod, I have a few spare 360 rods that you could have for the cost of shipping.

Bob

Quote:

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Originally Posted by tufcj

Did you use new rod bolts? What I'm seeing by the bent rod cap is that a bolt may have failed or come loose. I always replace the rod bolts with ARP, then have the big ends resized. It's always hard to say. Were the rods magnafluxed? A fine crack could have escaped a visual without magnaflux.

If you need a rod, I have a few spare 360 rods that you could have for the cost of shipping.

Bob

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Thanks so much Bob! I may take you up on the offer. I did not replace the rod bolts, and in hindsight I will. Rods were magnafluxed, so my initial assessment was that a rod bolt failed.

No ZDDP cannot do that ya knucklehead. Your over-revving did that but maybe there is more to it. You may or may not have had good valve spring pressure and you most likely hit a valve. You dont break a rod like that spinning fast unless the piston hits something or a bolt fell out.

Quote:

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Originally Posted by jeepsr4ever

No ZDDP cannot do that ya knucklehead. Your over-revving did that but maybe there is more to it. You may or may not have had good valve spring pressure and you most likely hit a valve. You dont break a rod like that spinning fast unless the piston hits something or a bolt fell out.

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I'm confused, thought zinc was necessary for break in, and flat tappet cams. Used with Rotella 15-40

Springs were new and matched to summit 8600 camshaft. 3angle valve job with new seals.

I always add ZDDP for the first couple of oil changes then just quality oil and filter. ZDDP additive wouldn't have caused any failure like you had. I like NAPA Gold or WIX filters, I wouldn't put a Fram filter on any of my engines. I hadn't thought about a dropped valve, but it could be, so you'll want to pull a valve cover. I still think, based on the bent rod cap, that a rod bolt failed or came loose. If you're lucky an additional turn of the rod journals, new bearings, and a new rod should put you back on the road.

The 7-8 rod is the one that fails most in an AMC engine, since it receives oil last and gets the least. Did you add the lifter valley oil mod? If you did, check that you didn't screw the fitting in too deep and block the oil passage. If you completely disassemble the engine, which I would recommend, look into the cam bearings that Bulltear sells that reduce oil to the cam journals. They still get enough oil, but it increases oil to the last main/rod.

Bob
tufcj

Quote:

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Originally Posted by tufcj

I always add ZDDP for the first couple of oil changes then just quality oil and filter. ZDDP additive wouldn't have caused any failure like you had. I like NAPA Gold or WIX filters, I wouldn't put a Fram filter on any of my engines. I hadn't thought about a dropped valve, but it could be, so you'll want to pull a valve cover. I still think, based on the bent rod cap, that a rod bolt failed or came loose. If you're lucky an additional turn of the rod journals, new bearings, and a new rod should put you back on the road.

The 7-8 rod is the one that fails most in an AMC engine, since it receives oil last and gets the least. Did you add the lifter valley oil mod? If you did, check that you didn't screw the fitting in too deep and block the oil passage. If you completely disassemble the engine, which I would recommend, look into the cam bearings that Bulltear sells that reduce oil to the cam journals. They still get enough oil, but it increases oil to the last main/rod.

Bob
tufcj

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Bob, I'm not clear that turning the journals is possible without pulling the crank (and pulling the engine). Do you have a suggested easier option for getting this fixed without pulling engine or disassembling down to short block?

I did not do the oil mods, as it wasn't intended to be a high powered option. I'll order the special cam bearings for sure if I end up tearing it all down.

Shawn

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Originally Posted by morehops

Bob, I'm not clear that turning the journals is possible without pulling the crank (and pulling the engine). Do you have a suggested easier option for getting this fixed without pulling engine or disassembling down to short block?

I did not do the oil mods, as it wasn't intended to be a high powered option. I'll order the special cam bearings for sure if I end up tearing it all down.

Shawn

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It's not. You're going to have to pull the engine and remove the crank. With the breakage, you're going to want to completely disassemble and clean all the oil passages anyway. You might be able to salvage the crank if it's not too deeply scored, hopefully the piston is also salvageable. Look carefully at the bottom of the piston walls on the 7/8 cylinders, and at the cam. It's possible that the rod could have hit the cam and damaged lobes or cracked the cam. No cheap or easy way to repair a thrown rod. Just salvage what you can.

Bob
tufcj

Quote:

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Originally Posted by tufcj

It's not. You're going to have to pull the engine and remove the crank. With the breakage, you're going to want to completely disassemble and clean all the oil passages anyway. You might be able to salvage the crank if it's not too deeply scored, hopefully the piston is also salvageable. Look carefully at the bottom of the piston walls on the 7/8 cylinders, and at the cam. It's possible that the rod could have hit the cam and damaged lobes or cracked the cam. No cheap or easy way to repair a thrown rod. Just salvage what you can.

Bob
tufcj

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Right, figured! Ok will get to work and get back to you on progress! Thanks Bob.

Hey Bob, I'll send you a DM with my contact info, would love to take you up on the offer for the connecting rod!

Shawn

Hey everyone, just following up here with the update! We found a single rod bolt nut at the bottom of the pan along with a twisted rod cap and bearing. We think the nut wasn't torqued correctly and the rod cap twisted and resulted in the rod breakage.

Took the block out and got it into pieces to examine each. The cylinder walls were not scarred, so were able to be honed again. Turned the rod journals to 30 under for the crank scarring. Got a new piston and rod (Thanks Bob!) for #7. Using all new rings, all new ARP rod bolts, and since the metal got into 1 cylinder, had to get a new camshaft and lifters because one of the lobes started to go.

Picking up the short block tomorrow, and hope to have time next weekend to get it all back together! Will send pics as it comes along.

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Bob
tufcj