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Tractor Talk Discussion Forum

valves through piston

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fixerupper

03-27-2006 06:54:33




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I Just tore down a 3406C Cat engine that broke both exhaust valves in #1 cylinder and the piston tops are cracked in the rest of the cylinders. This engine is in a truck that the owner bought used about a year ago but the previous owner said "It has a lot of power because I turned it up and put a bigger turbo on it". Of course this sends up some red flags but I have heard exhaust valves can be broken by over-reving when downshifting with the Jake on or a malfunctioning Jake not closing the exhaust valves in time. I know a lot of you guys are truckers or truck mechanics, so what's your opinion on this? Can the cause be all of the above or is there something else going on? When this goes back together the pump will be turned down. The turbo is probably trashed from bits of valve and piston going through it. Jim

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Mark - IN.

03-27-2006 20:43:19




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 Re: valves through piston in reply to fixerupper, 03-27-2006 06:54:33  
Weak springs have been known to cause that. Valves tend to float when a spring doesn't pull it back to seat, and the piston just don't know any better. Over reving can do it too, especially with weak springs. Yours sounds like a boost problem with cracked domes on all of the pistons though. Takes its toll on domes, rings, seals. It happens.

Make sure you pull the oil pan and get any chunks or cracked metal that may've come off of the undersides of those pistons or rings. Oil pumps are stupid. They don't know are just supposed to pump oil, and if chunks or debris end up in the pan and find their way to pickup sump and clog the screen, bad things like spun bearings happen too. Much cheaper to get them out before...

Mark

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CaptRon

03-27-2006 13:02:43




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 Re: valves through piston in reply to fixerupper, 03-27-2006 06:54:33  
third party image

I used to drive a cabover Pete with a 350hp Cat that developed a knock in the engine. While running with another driver I told him on the cb that I thought the my engine was knocking and he guessed that our light load wasn't working it enough. When I got back to the terminal I reported the knock to the trucks owner and the shop forman. They looked at it and the forman drove the truck bobtail a couple of miles and gave it a clean bill of health. So I went out right then and hooked a trailer full of minerals with a max gross and took off for Springfield, Mass. The next morning on I-81 the engine exploded at 70mph coming down a mountain in VA. I thought that I blew a tire and looked out the right mirror to see engine parts flying off the side of the truck along with a major smoke cloud.
One of the things that ran through my mind is that in the past when I lost power in other trucks and tried to down shift and the engine stopped turning then I lost power steering and the truck became seriously hard to control. I wasn't about to lose control of this truck at 70mph coming down a mountain. Besides, from what I could see this engine was a goner anyway.

To make a long story short, I got a call about a week later that the no. 1 piston had been destroyed when a valve dropped and the block was severed in half because I let the engine turn. They discussed this with the Cat dealer and decided that I had over-reved the engine and caused it to blow despite the fact that I had reported it knocking. They should have been pointing the finger at the shop forman and not me. I asked them if Cat said how much rpm would have cause this and they told me "at least 5000". I told them that this was BS as I couldn't have gotten that engine to 5000rpm if I had dropped it out of an airplane.

These people used to be friends of mine...used to be.

I was their best driver and posted the highest revenue in the company but they constantly harped on me about that Pete and how much I had cost them. I moved on and a couple of years later I see the old man comming at me in his two story Falcon on a road in Ga. When I called him on the cb the first words out of his mouth were about the Pete, I said "nice to talk to ya" and turned my radio off.

Btw, There was no other indication of valve or piston damage in this engine. The no. 1 cylinder was the location of all the damage, I saw the disassembled engine. The crank was good.

Actually I could write a book about the poor maintenance history of this company and how they always blamed the drivers for it.

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the tractor vet

03-27-2006 07:57:22




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 Re: valves through piston in reply to fixerupper, 03-27-2006 06:54:33  
Well i am one of those guilty people that can not keep his finger out of things and i have turned up diesels with bigger pump settings bigger turbos and may be just lucky that i never dropped a valve or put a rod out the side or broke a crank , but have seen stock engines that for no reason drop a valve or a head of a valve come off . Man made the engine and there for it will break . The last truck that i was driving started life as a 475 3406 cat but was uprateable to 550 well that was done then it was uped angain with a new after market chip that took it over 700 hp and it ran fine for me but it did not run as well as and old 350 small cam cummins that i worked over as it was faster on the same hills with the same weight as the cat . So why the cat ate a valve is anybodys guess it fall under the heading THAT SHUTT HAPPENS . IT's broke now so ya got to fix it. valves are not made from one pice of steel the heads are welded on so it may just have been a bad weld or a spring broke can't say with out seeing all the pices and parts.

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Bob/Ont

03-27-2006 07:23:24




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 Re: valves through piston in reply to fixerupper, 03-27-2006 06:54:33  
If it was from over reving all six cyls would be affected not just one. Was the lock nut on the Exh rocker tight? If the adjustment backs off the adjuster ball can get up on top of the cup on the push rod end and the valves shoved too far down, hitting the piston. The bridge adjuster coming loose can also cause valve trouble. I would throw six liner kits into it and a new set of crankshaft bearings while it's apart. Definatly set the fuel to spec and look at the turbo. If an old engine is down on power, people that know enough to be Dangerous think opening up the rack will correct things. My experiance tells me that a new governor spring, set of flyweights and thrust washer will serve you better. As the spring ages it gets weaker and the engine has to load down more to get full fuel. The flyweight leg ends also wear into the thrust washer and get harder to move too. All this makes for slugish governor responce when the engine is loaded down. Another thing to look at is the fuel timing advance in the pump drive, the center sometimes works loose and doesn't always drive as fast as the pump does. It spins the set of flyweights that control the timing advance.
Later Bob

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fixerupper

03-27-2006 08:34:12




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 Re: valves through piston in reply to Bob/Ont, 03-27-2006 07:23:24  
The valve lash was not loose and the bridges all look OK. One question I have in my mind is why both exhaust valves broke. My guess is one broke first and when it was banging around in there like a hammer it nailed the other valve, or both of them broke when they hit the piston for some reason, maybe a Jake malfunction on that one cylinder. I just don't want to do this job over again if this was caused by something other than just foolishness. Jim

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