Collapsed Piston

[Jeff_Sigrist]

I have about 6,000 miles on the rebuild of my 1275. This is a .040 overbore with a Kent 266 cam and dished 3 ring pistons.
The initial 3,000 miles I ran Valvoline 20w-50 racing oil. I then switched to mobil 1 synthetic. We recently drove to Florida, putting about 2,000 miles on pulling a 500 lb trailer. This was all freeway, with a lot of miles at 4,000+ rpm. On the way back I started having a knock. The oil pressure never changed so I drove it the rest of the way. Since the rebuild the engine has never overheated, or been past 5,000 rpm.

I finally had time to tear it down today and found that #4 piston has collapsed. The skirt is about .015 undersized compared to the others. I could actually see it rock as I turned it over once I had the head off. The wrist pin is also binding in the piston when you rotate it. Fortunately it looks like the scuffs in the cylinder will hone out. They'll have to as I am not going to re-bore it at this point. Everything else I have looked at so far looks good.

Now to locate a piston and get this back together for Missouri.

So, what causes a piston to collapse? Is there anything else I should look at which might be related to this before I button it back up?

Thanks,
Jeff

 

[PAUL161]

Jeff, I've seen pistons with holes burnt in the top of them and the domes collapsed due to detonation caused by carbon build up and timing too far advanced. You shouldn't have any carbon at all in your engine. But it sounds like the cylinder temperature was excessively high if the collapse was due to excessive heat. Timing, fuel mixture, Even a remote possibility of a poorly made piston might be the cause. I think you would have to have the piston analyzed to see if it was heat or material failure and go from there. I'd be leery of the other three, if they came in a set.

 

[Hap Waldrop]

Well from your discription, it sounds like the pin gaulded on the piston, even thought the pin press fits in the rod, you still need to size the piston pin bores in the piston sometimes, never assume they got it right at the factory. The other thing you have to worry about in a press fit piston is start up lubrication on the pin and pin bore in the piston, I use Goodson Tool press fit lube, it is designed for this very purpose. You could have had proper clearence but at initial crank up this area was dry, aluminum from the piston can start to gauld to the pin, eventually if you get enough tranfer it can lock the motor up, your didn't go that far obviously, but it does sound like the piston was unable to rotate back and forth and this can cause piston skirt and clyinder wall damage. You should be able to get a single piston for the engine, try Gordon at The B Hive in Clemson, SC, he should be able to help you. I would check that bore out carefully and make sure the piston to cylinder clearence is right with the new piston (about .0025" measure opposite the pin bores on the piston) If you see that is is not, say bigger than .003" or deep scratches that won't hone out, then consider sleeving that cylinder.

 

[regularman]

Yeah, something going on with the wrist pin there definitely. The pins only get what gets splashed up on them, could have been that a pin got dropped and a small scratch got on it and the tolerance was tight. I like to use engine assembly lube or grease on them. I know must of it get squirted out when you push it in, but some stays on the pin for a good run in. Also, you might try you local Car Quest if you have one. I have had good luck getting single pistons from them the same day, but be prepared to pay twice what you would for one out of a set and its usually a higher end one like "perfect circle".

 

[Jeff_Sigrist]

Thanks all,
The other 3 pistons all look good. There are no indications of the wrist pins binding on them and the pistons show no wear.

There are no signs of excessive temperatures that I see. There is a small amount of carbon, but I attribute that to running a bit rich part of the time.

I have not had any issues with the timing. I think it is set at 10 degrees BTDC with the vacumn advance unhooked. I normally run 89 octane with no detonation in evidence.

I have had some issues trying to get the mixture correct, and am going to order new needles as I think mine are worn a bit. I don't think I ever ran it lean long enough to cause this kind of issue, but maybe. I have never had any indications on the spark plugs that I was running that lean and certainly there is no damage to the piston tops or cylinder heads that I would expect if I was running excessively lean.

The machine shop who did the machine work on my motor fitted the original pistons and rods and I have no idea of the condition or what lube they may have used, but the shop has a good reputation and has been doing automotive machine work for many years.

Eric Jones of Riverside motors here in Ohio is going to see about getting me the piston. I will have the machine shop assemble the new piston and rod and be sure they are aware of the issue.

I will hone the cylinder and check the clearance on the new piston carefully. Hopefully it is not too far out and I won't have to sleeve that cyclinder. I prefer to drive in the summer and do this kind of work over the winter.

Jeff

 

[JerryB]

"""""""""""" I then switched to mobil 1 synthetic. We recently drove to Florida, putting about 2,000 miles on pulling a 500 lb trailer. This was all freeway, with a lot of miles at 4,000+ rpm."""""""""""

The operatve words here are "lot of miles at"...."4000 rpm" and "pulling a 500# trailer".

You will probably find the motor has detonated itself to death. look for telltale aluminum flakes on the spark plug porcelain.

 

[Jeff_Sigrist]

No aluminum on the spark plugs. No pitting or other evidence of detonation on any of the pistons. The only thing out of order appears to be #4 piston skirt and the sticking wrist pin. Well, there are also the markings on the cylinder wall. I can't catch a fingernail on them but we will see.

Thanks for the input,
Jeff

 

[RonMacPherson]

When you talk about markings on the cylinder wall, can you do a little more description.

Do you have any horizontal dark rings around the cylinder wall? In more than one height on the cylinder wall? If so these are overheat indicators.

From your description so far, either it got hot around one cylinder or you suffered a wristpin/rod failure.

 

[Glen_B]

Post a picture of the piston?

Glen

 

[Jeff_Sigrist]

Ron,

The marks run vertical with the bore. It appears to me that as the piston collapsed perpendicular to the wrist pin, it expanded parallel to it. This appeared to have pushed out the area where the curve of the piston skirt intersects with the flat causing it to expand into the cylinder. You can see this area in the attached picture. It is the same on all four "corners". This left visible marking in the cylinder wall.

The bronzing of the area around the rings is the only thing I have seen which showed any sign of heat. Wouldn't this discoloring be normal?

Glen,
I tried to get a picture of the cylinder wall also, but could not get anything that was clear.

I should have the new piston, rings and wrist pin tomorrow. If I can find time to have the rod pressed on by Friday, I may be driving again this weekend.

Thanks,
Jeff

 

[RonMacPherson]

Sounds like you're on the right track. A quick hone of the cylinder, accompanied by good bore measurements, then proper piston, wristpin installation and good to go.

Forgot to mention that if it overheated with horizontal marks, then the rings would be fastened more to the piston, usually frozen into the ring lands.