CAT 3208 new knock

The piston I replaced looked similar. It had similar discoloration from the high temperature. The third pic from the top it looks like the skirt of the piston has wear? I was told when a aluminum piston gets too hot it will change shape. How did the crank journal look? If it is in good shape be careful not to scratch it during reassembley. How dose the cylinder wall and Rod bearing look? Hopefully good. Remember I've done this but still consider myself a rookie.

There are devices with a camera at the end of a flexible tube(I`ve seen one but no idea what they are called), you can view on a screen in the head unit areas you cannot see into. Do they take pics, I don`t know, at least you`l get to see the crankshaft. Try ebay.

This type of camera should get you a look at the crankshaft. 720P HD Inspection Camera 3 5 Inch Wireless Monitor DVR Recording SD Card | eBay

"I was told when a aluminum piston gets too hot it will change shape."

Piston mfg understand this , so pistons are machined oval, and expand closer to round under normal operation.

The piston skirt that is in contact with the cylinder wall in line with the rod provides most of the anti rocking , so has the most contact.

That piston is toast!!! But that connecting rod looks fine....and that is my very ignorant opinion!!

Connecting rod looks ok, but it could still be bent or twisted which will wear the bearing shells, piston, cylinder wall.

One way is use plastigauge. If bent, the plastigauge wont be evenly compressed all the way cross the journal face.
If a rod is bent even a tiny amount, the developed thrust will cock the piston and cause accelerated wear, combustion creates huge pressures which will be transferred at odd angles not straight down to the crank.

But, yes, it is most likely ok.

Is that a 3 ring piston? If so I've never seen a 3208 with a 3 ring piston. All I've ever seen had one compression and a oil ring. Aftermarket? Has someone been in the engine before?

bad injector spraying too much fuel????

Three ring piston. Most 3208's have two ring pistons, not sure of the 210. Might be aftermarket?? Rod could very well be bent. Either have it checked at a machine shop, or replace it. Dimensionally checking rods is not super easy unless set up for it.

How is cylinder bore?

Ski in NC said:

Three ring piston. Most 3208's have two ring pistons, not sure of the 210. Might be aftermarket?? Rod could very well be bent. Either have it checked at a machine shop, or replace it. Dimensionally checking rods is not super easy unless set up for it.

How is cylinder bore?

Click to expand...

Just looked up the part numbers for NA pistons in my old parts book. Looks like all the NA and turbos use the same piston. They are all 2 ring pistons. Rebuilt my port 375TA the end of last year. Used all CAT parts with a rebuilt rod exchange. Cost for a rod exchanged was around $275 each. If you are planing on doing a replacement in frame then I would just do the rod exchange. If not then when you pull it down a good rebuilder should be able to do an exchange or re-work with warranty for less.

Bottom line IMO someone has been in that engine before. I would be curious if all the other cylinders have 3 ring pistons or just that one? Maybe that cylinder has had issues before?

Cat does make three ring pistons and I think those are fitted in 435hp, but those use lower comp ratio than lower hp machines. Some folk when rebuilding a lower hp 3208 will chose to put in three rings from Cat. Less blowby.

This is where it is very good to be nice to a knowledgeable marine Cat parts guy. The good ones can sort out what to get.

yes, it's a 3 ring piston. Which apparently makes it harder to replace. I contacted a diesel shop in Tacoma and they're supposed to be looking into it and calling me back. The engine was rebuilt in '03-04 and the only I thing I could find about it says it was rebuilt back to stock.


I found the only "machine shop" we have here, and he's not sure what to do with a connecting rod to make sure it isn't bent or twisted, but said I could go ahead and drop it off next week and he'll check it out.


I was wondering about the injector too, but the only guy in town that can pop test them says he's too busy. It will probably be months before he could do it, so I might try to find someone in Seattle that I can send all of the injectors to for testing just to make sure.

I think the cylinder bore will be ok, but I'm going to try to have the mechanic that's been helping me some take a look for a 2nd, and educated, opinion.

What happens if I put a 2 ring piston on and the rest are 3 rings??

You could put a two ring piston in and you won't see any effects. But try to find the Cat three ring. Try to avoid aftermarket parts.

I'm not a CAT owner and this may be the wrong piston, BUT. . . . If you go to Amazon.com and search 3208 CAT Piston, this is one that comes up and it says it's OEM CAT. There are other sellers on there too which you can search as well.

Looks like it may be a 3 ring or may not even be the right piston, but worth a look anyway. $55 + $14 shipping

CAT PISTON 7e4729, 7e-4729 CTP Model#3208

https://www.amazon.com/gp/offer-lis...0?ie=UTF8&condition=all&qid=1470449333&sr=8-5

There maybe some weight difference between a 2 ring and 3 ring piston. May not really make much difference. However you will need to use a bore gauge or measure the existing piston to determine what size over it is.

Ski might correct me but my memory tells me that there are different rods for different pistons and wrist pins. I could however be way off base. As for the rod, like I said earlier it might be best to just do an exchange with CAT. It's not worth the cost difference to make sure it's ok.

Look on top of the front cover ( under the coolant tank) for the engine build tag. You at some point will need the serial number and the series number to facilitate getting most parts. I would sure try and find a good CAT parts guy who can help you locate the proper piston. May take him/her a few days but it would sure be worth the wait to find a match. Keep us posted on you outcome.

but the only guy in town that can pop test them says he's too busy

Most ANY injector repair shop can test an injector.

A truck, farm place or industrial engine repair will be fine.

3208 is ancient , they have all done them.

Just take it there , it will be faster for the shop owner to test it than make out paperwork and have you wait.

FF said:

but the only guy in town that can pop test them says he's too busy

Most ANY injector repair shop can test an injector.

A truck, farm place or industrial engine repair will be fine.

3208 is ancient , they have all done them.

Just take it there , it will be faster for the shop owner to test it than make out paperwork and have you wait.

Click to expand...

Thanks, but we don't have those places here. We are a small, remote fishing town with very limited resources and somehow only one diesel shop in spite of the number of boats here. Kind of blows my mind, but it is what it is

I found the guy in Washington that did the rebuilds and called him this morning. As soon as I told him what it was, he knew what I was talking about. Sounds like it's an aftermarket part, which I'm not thrilled with but it will match the rest of them and should be available from what he said. He's going to get some info for me and call me back. He also said he and can test the injectors if I mail them down to him. Hopefully this will all still have a happy ending and I can get back on the water in the next couple of weeks

Thanks again everyone for the help!

I would pull at least one other piston, from the other end, and make sure it is also 3-ring. If not, I would pull all and replace all with factory spec.

Rob can you get some clear, in focus pics of some of the cylinders including pistons? Include the problem hole.

Some good pics of the top of the failed piston and the bottom of the head on that hole.

Pics will help figure out what happened.

I prefer Cat over aftermarket, but unless you want to replace all of them, probably best to go back to the same brand.

Kudos on finding the guy that built the motors. Be nice to him as he will be a wealth of info. Including whether bore is std or oversize. Offer to pay him for his time, it will be worth it.

3 ring piston

I am do not know much about 3208 but found this

The pistons shown are all three-ring pistons. Caterpillar produced a three-ring piston only for some turbo application but never for a naturally aspirated engine. Instead, two-ring pistons were used in these versions. The two-ring piston was only available as a conventional type ring groove while the three-ring pistons use a top Keystone ring (see illustration).

Use the three-ring piston as your choice of piston: it’s a superior replacement for the two ring and you’ll never use a two ring in place of a three ring. Naturally aspirated pistons come with three different compression ratios: 16.5:1, 17.5:1 and 18.2:1. Turbo pistons primarily have a compression ratio of 16.5:1, though there are some lower horsepower turbo engines for California applications, which have a 17.5:1 compression ratio.

Easy to forget the relative remoteness, problems, and cost of getting things in and out, of Sitka. I did when raising getting a flexible camera, sorry for that. For that reason alone, focusing on the precise issue, rather than a total rebuild, makes sense.

Ski in NC said:

Rob can you get some clear, in focus pics of some of the cylinders including pistons? Include the problem hole.

Some good pics of the top of the failed piston and the bottom of the head on that hole.

Pics will help figure out what happened.

I prefer Cat over aftermarket, but unless you want to replace all of them, probably best to go back to the same brand.

Kudos on finding the guy that built the motors. Be nice to him as he will be a wealth of info. Including whether bore is std or oversize. Offer to pay him for his time, it will be worth it.

Click to expand...

Here are the pics. The holes in the top of the bad piston are there because I couldn't get it out all the way from the bottom so I added a few screws to attach a small cable to and pulled it from the top. Any thoughts on the stain marks on one of the cylinders? It's the one next to the bad one, if that matters. My son cleaned it today hoping to find a part number on it. In the pic of the full head, the far left spot is where the bad piston was.

I definitely don't want to replace all of the pistons! Hopefully the mechanic that did the rebuild does call me back and I can get the parts soon. Thanks!

I'm no expert, though can you catch a finger nail on the ridges in the lining in the cylinder you pulled the piston from? Just curious.

I think the stain mark is from where water set in that cylinder? Any indications of a blown head gasket?

Looks like 3L had water sitting in it, also a little bit steam cleaned on piston crown. Is that a crack in cylinder above the water stain? That irregular mark rising vertically? Can you catch it with a fingernail?

4L has had some metal transfer from piston, maybe some scoring. Looks like piston got hot and then friction caused more heat, typical snowball type failure progression.

Were you running it hard when you first had the problem? If engines were babied for many many hours pistons can get carboned up, then when run hard the carbon can be packed in the clearance between crown and bore and when piston tries to expand, friction ensues.

Ski in NC said:

Looks like 3L had water sitting in it, also a little bit steam cleaned on piston crown. Is that a crack in cylinder above the water stain? That irregular mark rising vertically? Can you catch it with a fingernail?

4L has had some metal transfer from piston, maybe some scoring. Looks like piston got hot and then friction caused more heat, typical snowball type failure progression.

Were you running it hard when you first had the problem? If engines were babied for many many hours pistons can get carboned up, then when run hard the carbon can be packed in the clearance between crown and bore and when piston tries to expand, friction ensues.

Click to expand...

The only thing I can catch a fingernail on is the top of the cylinder I pulled the piston from where the metal transfer is. Can't feel the line in 3L, and can just barely feel the line in 4L with my fingertip but not enough to catch a nail. I'm still holding out on my plan of honing it and getting 4L cleaned up enough that it works, but none of that matters unless I find a piston

The only water I know of that got in there was from pulling the head and apparently not having drained the coolant correctly. It sat in that cylinder for less than 5 minutes, probably more like 1-2. I don't know what to look for in a blown head gasket, but I wasn't losing oil or water, always had good oil pressure and water temps.

From early May to mid-June, I put over 100 hours on it. Any free time we had, we were on the boat. In the 3 years the PO had it, he put 200 total on it, and last year I want to say it was only about 30, so it definitely wasn't getting used much. I was running about 10-11 knots and slowly accelerating when I first heard the knock. Cruising is about 8-9, WOT is 16-17. I wish I could tell you the RPMs but tachs only work when they want to (for now...I'll fix that once I have 2 engines again). In the times out before that, I had pushed it just to get to know the boat, especially after I fixed the trim tabs, but I tried not to be stupid about it.

Like Ski said the top of the piston in that cylinder has been steam cleaned. You can get staining from coolant standing but it would take longer than just a few minutes.

Back to what was said earlier proper diagnosis of the failure is important. Let your local guy who's stopped by look at it give his opinion before spending $$$$.

what do you mean by steam cleaned? Is it because it's so clean compared to the others? I mentioned my son had cleaned it to to try to find the part number, but maybe I wasn't clear enough that he cleaned that piston in the cylinder with the staining. Prior to that, it looked just like the other two pistons. Or are you seeing something else? Thanks

If he cleaned it, that would explain it being clean!! So it was black and dusty when you pulled the head off?

That stain down at the bottom of 3L would not occur in five minutes. Would take a matter of weeks. But you can see the stain on both the bore and the piston and the levels line up with what water would do.

Can you feel any roughness on that 3L stain on the bottom of the cylinder bore?

Ski in NC said:

If he cleaned it, that would explain it being clean!! So it was black and dusty when you pulled the head off?

That stain down at the bottom of 3L would not occur in five minutes. Would take a matter of weeks. But you can see the stain on both the bore and the piston and the levels line up with what water would do.

Can you feel any roughness on that 3L stain on the bottom of the cylinder bore?

Click to expand...

yes, it was black and dusty just like the others. The mechanic that did the rebuild thought there should be another part number besides the one we found on the bad piston. We thought maybe it was illegible due to the beating it took, so he cleaned that one to look. Unfortunately there wasn't anything different and no part number anywhere else that we can see.

I can feel a definite texture change where that stain is. Nothing extreme, but it is a little rough