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Hello All! Well I got the 3406B Cat out of our Autocar apart today. It did have Cat Pistons in it, part# 9Y4004. 5 out of the 6 have cracks in the skirts. One has the skirt borke off. All the cracks are on the Right Hand side. It had 1 & 3 Liners leaking coolant into the Oil. Thats what started all of this. The liners look good, not eat up on the outside. The O-Rings thats a diffrent story! Mostly GONE! What may have caused the pistons to crack? Thanks for the Help!!!
Life Is Short, Enjoy It! J.R.Smith  Tatum,Texas
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Ok i will guess here on something. Looks like this is the two piece piston.If so then the jake brake has hung open the valves or the RPM have exceded a safe range and valves have made contact with the pistons. If so there will be valve marks on top. If so then this means the piston has cocked in the cylinder due to a valve piston contact and this will put excess load on the skirt.Best guess i can come up with. I have seen it many times with the C12.Crack the skirt and the pistons rocks over center of the wrist pin and locks in the liner. The it ends up with a side window in the block.If the crown is cracked it is from hi load from the wrist pin up to the top. This does not look to be cracked to the top.
glenn akers
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 Glenn, No Jake's on this one. I have been cleaning the pistons up, just to look at them. 3 of them have cracks that go all the way up to the 2 ring groove. Or that is as far as I can see see it. # 1 & 2 rod bearing have a place in them that the lining is coming off. About 3/8" wide and 3/4" long, about where the rod bolt is. The other half of the bearing looks good. The Crankshaft is smooth and is round and within specks with the mic. So here we go!
Life Is Short, Enjoy It! J.R.Smith Tatum,Texas
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ok then if the crack goes up but it realy goes down from the ring groove to the bottom of the skirt then that is a one piece piston. And if not valve markings on top of piston then look at the wrict pin bore and see if crack started there. The crack goes down instead up. If not valve marks on top of piston then only thing i who think is fast timing and too much fuel.I dont think coolant temp has hurt it because no scoring on the skirts.
glenn akers
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| man what a sick feeling looking at them pistons sorry that you arwe having that kind of luck . let me know if I can help you in anyway.Tony
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Be very carefull of the piston cooling nozzles. If you didnt remove them prior to removing the psiton/rod assy. then I would suggest replacing them. If they get bumped/bent in ANY way during pushing the piston/rod out they WILL fail upon rebuild, usually first hill loaded=scored piston.
Scott Waggoner
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| What kind of coolant was used in this engine?
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chrisL (2/3/2010)
What kind of coolant was used in this engine?Leaking lower liner seals is a common failure on a high mileage Cat or poor cooling system maintence. Depends on when it was rebuilt last time but Cat had alot of O-ring failures in mid-nineties.
Scott Waggoner
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| I was just wondering if they were using the CAT ELC coolant thats based on Dexcool and if that ate the liner seals or had anything to do with the seals.
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glenn akers (2/2/2010)
ok then if the crack goes up but it realy goes down from the ring groove to the bottom of the skirt then that is a one piece piston. And if not valve markings on top of piston then look at the wrict pin bore and see if crack started there. The crack goes down instead up. If not valve marks on top of piston then only thing i who think is fast timing and too much fuel.I dont think coolant temp has hurt it because no scoring on the skirts. Glenn I think has hit it on the head, too much fuel and advanced timing. I personely am a big fan of fuel if it's done right and you watch your pyrometer, and don't be afraid of down shiffting. Great Lakes and Pittsburg preach retarded timing and a lot of old Cummins guys like advanced timing, but I believe in top dead center, thats where it was designed to be at. All of this is just my opinion and not meant to be critical. Scott sure told you right about the oilers, and I would make sure the pyrometer probe is good and that all the connections are tight going to the gauge, and as I told you in another post be sure to have the head checked before going back and as an after thought check the timing advance close too. good luck, Lynn
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Hello All! On Coolant, I have no idea. The liners look good on the outside as well as the inside. The block and head look good and clean on the coolant side. I went to Holt-Cat in Longview,TX. yesterday. 2 of the engine builders looked at the pistons. Thay said that they have never see cracks like that in a 3406B???? More to come later! Thanks for the replys!!
Life Is Short, Enjoy It! J.R.Smith Tatum,Texas
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| Extended higher rpm use with big fuel and fast timing could do that too.Seems like most of the cracked skirts Ive seen were on engines that ran over 2,100 on a pretty regular basis. Lynn, We used to run our Cats retarded, ran 1,800 to 1,900 constant with 13 overs and 3:70s, running from Maine to the coast.Running stock the engine felt like it was working to hold those rpms, advanced would feel like you were on the govenor even when you had some left, retarded felt like it was loafing along, till you tipped youre foot in, then it was right there.We got a little better mileage running them retarded too.John
John Costley
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| Engine timing from only my opinion: We have all heard engine "pinging-detonation" in a gas engine. That is caused from combustion too far before TDC-top dead center trying to push the piston back down the cylinder, example pedals on bicycle on a steep hill trying to go backwards. This causes extremely high cyclinder pressures and potential engine damage. In a gas engine with spark ignition the spark could be too soon, BTDC or the piston could be too hot from previous combustion causing compression ignition like a diesel does. Bad gasoline-low octane will burn faster causing compression ignition before the spark. That is why higher octane gas burns slower. In a diesel engine if we inject the fuel into the compressed air too soon it will do the same thing, try to push the piston back down. Here is the kicker so to speak, you will most likely never hear detonation from pre-ignition in a diesel engine. What you will find is engine damage. That damage in my experience will be cracks in the piston tops, sometimes confused with "heat checks" It is actually pressure cracks from high combustion pressure-has to go somewere, think of piston as catchers mitt. This also places very high loads to the rod bearings and crankshaft. The highest cylinder pressure should occur at 15-20 deg ATDC on the power stroke. We can advance the timing with higher engine rpm because the higher rpm is easier to overcome the mechanical leverage to move something. With higher boost pressures from turbocharging we also need to allow for later injection timing because we have higher intake air pressures and heat. On a Cat mechanical engine I do not advance the injection timing, esp. with this wonderful fuel we have. I might adjust the rate of mechanical advance but I do not adjust the total or the start-stop. If I put a better turbo on someones truck I always check the static and dynamic timing because of the higher cylinder pressures it is then going to have. I find injection timing to be very criticle to engine longevity and performance. Only time I have seen piston skirts broken like that is from the oil level being too high in the crankcase. That put outward pressure on the inside of the piston then cracking the skirts. Seen pressure cracks from the piston crown down through the ring grooves to the wrist pin before but not up from the bottom. Like I said this is only my opinion from my past experience.
Scott Waggoner
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Did JC say "retarded" ?? lol
Mark Brubaker { "known associate"}
Rochester, Indiana
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Mark Brubaker (2/5/2010)
Did JC say "retarded" ?? lolHuh, thats funny, I typed disadvantaged.Just kidding, guess Eddy got the politically correct glitch fixed, lol.John
John Costley
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