Broken Piston Bits in Sump

[Charles H Jacobus]

My guess is that, during the rebuild, a piston with a longer skirt was used. On start-up, the crank took a bite or two out of the piston skirt and resulted in a drastic reduction in engine price for you......

Now you have the chance to rebuild it right.

Chuck

[C J Murray]

Well, it's safe to say that you have a big problem. The engine has to be taken 100% apart and an extensive cleaning done including removing the oil passage plugs in the crank and the case. You may or may not find some good parts in there but I would not be hopeful.

I often disagree with Al Zim but he is correct that you have a top quality machine shop clean, inspect, and measure all the parts that you would like to reuse. They will tell you what to recondition and what to replace.

If you are very mechanical and very careful and very clean you can rebuild one of these engines but if you have doubts about your ability you may just want to give the job to somebody that you are certain has experience with these engines.

Remember that doing your own rebuild only saves you the labor portion, not the parts or the machine work which is often way more than half the cost of a rebuild.

[Wes Bender]

+1 on the quality machine shop. Competition Engineering would be my shop of choice, but I'm not that far from him. I still feel that an owner who has a decent amount of mechanical ability will be able to take the time to cc the heads very accurately, along with other time consuming tasks. I think you can save quite a bit on labor, but my main reasons for doing my own rebuilds are the satisfaction of doing it and the ability to control all facets of the job.

Bear in mind that I'm not trying to discourage folks from having a good reliable rebuilder do the work. There are many on this forum who definitely SHOULD have a shop to do it.

[David Jones]

I agree in principle with Wes that one can rebuild a 356 engine if you have the aptitude but only if you also have some engine building experience. I built many F Vee engines and quite a few 356 engines and found that it is very easy to make a small error that can cost a large amount. I have had my share of post rebuild issues though most were minor except for one that was a machine shop error when they machined a flywheel off center which I only found after rebuilding it twice because I could not track down the source of the vibration. Comes back to using a trustworthy machine shop and then double checking everything twice. It is very time consuming building a 356 engine so you need to be sure everything is in spec when you do the final assembly which is why I usually do at least one if not two initial assemblies. Read about and think about it and use a good torque wrench then use the correct torques. Measure and weigh and then do it again. Test fit and rotate then check for errors again. Even now after about 60 years of rebuilding engines I still set off on that first trip with a new engine with a feeling of anxiety. When racing even after maybe 1 to 2 hours on the dyno I could not relax in the race car till I had at least one practice session complete. If you do decide to build your own engine find someone who is building an engine and ask to hep even if it is a water-cooled lump, you will benefit from the experience.

[Ron LaDow]

...You CAN rebuild your own engine. [...]
However, with the right tools and books, you can do just as good a job and get the satisfaction of doing it yourself.

Mixed thoughts on this.
Pro: Yes, with the right resources, you can and you can actually do a better job than most shops, because you're likely to spend the time that you wouldn't pay for to get a really good result.
Con: Those resources are not easy to come by. The factory did not help by introducing changes as often as they did, and some issues (like, oh, 01 rods not clearing certain cams) are not well known and maddening to diagnose when you get cylinders fitted to the pistons and find the thing won't go round.
Suffice to say you can, but to get really good results, you need to plan on several mock-builds to certain levels (and the resultant disassemblies) before the final build.
(oops; looks like David covered some of the issues)
(also oops; Cliff, too. The "refresh" function seems no longer automatic)

[Dan Mazur]

I really appreciate all the input on this engine, and the wisdom regarding tackling a rebuild.
Just to assuage some concerns, let me share that Im a mechanical engineer and comfortable with careful, high tolerance work. Ive have been studing up on the many details of these engines and I'm finding David's point about the many variations in parts is right on and sobering.

This engine is not matching numbers to any of my cars so I figure its a good one to start with. Im up for the challange and besides, at the end of the day, I figure I've got all you great guys, and gals to help me out!

The bits in the sump came from the skirt on the #3 piston per the attached picture. Ill tear the rest of the engine down this weekend and seek out a good shop to do the machine work.

The key question for me is why did this happen? The bores are 86 mm, consistent with a big bore kit, and as I understand, the standard big bore kits should work with the 912 counter balanced crank.

Of note, I grabbed the piston and pulled it up and down and the assembly had more play than I expected. Could worn out bearings (there are three in line in the piston assembly) lead to interference between the crank and piston skirt?

-Dan

[Ron LaDow]

Dan,
Sounds like you are willing to assemble and find the resources; it is great fun when one of your 'creations' turns out to do as expected or better!
Regarding 'why', it's odds on we'll never know, but you will find, on fitting the cylinders over the (ringed) pistons that getting past the oil control ring can be difficult. If the last builder resorted to a hammer to resolve that difficulty, s/he would have caused exactly what you've found.
Still have no idea regarding the black semi-circle; what is the material?

[David Jones]

With reference to getting the oil control ring into the cylinder I have found that a hose clamp can work very well if difficulty is encountered with getting the oil ring compressed enough to slip into the cylinder.

[Martin Benade]

It does appear to have been run for a good while based on the amount of carbon on the piston tops. Maybe cracked upon assembly and failed sometime later. Too loose bearings usually make the piston hit the head, not the crank, in my experience.

[Wes Bender]

It's possible that the previous owner who did the rebuild will be the only one who knows exactly what happened, and he's not likely to chime in.....

Like Ron, I'd really like to know what that black object is made of. I can't think of anything in the engine that resembles it. Maybe FOD in a piston engine?

[David Jones]

The black object kinda looked like the rubber insert out of a plug socket.

[Bill Lawless]

Fun Read!!!!
If you picked up the engine cheap enough your still a head of the game..
I paid some good money for a 912 engine and it needed a full Top End rebuild, Piston were dinged, Broken Value springs.. etc.... Did it all by my lonesome
Alll-in-all I got a nice running engine now and I'm good..

Nothing worse then seeing sealant inside the engine... That's an OMG moment for sure

[Al Zim]

For the past week I have thought about the comments that Mr. Wes Bender and Mr. Ron Ledow stated about do it yourself engine building. Today Zim's finished modifying engine sheet metal so that alternators can be fitted to engines with European heaters. A half day job that no one else has attempted. Over the 50+ years that I have been in the Porsche business I have managed to acquire most of the factory tools for 356 cars. I have also spent hundreds of thousands of dollars on shop equipment and acquired a good selection of good used 356 parts to draw from. You are a mechanical engineer and starting to rebuild an engine and you do not know what it takes to clean the oil passages in the engine or the size of the plugs and taps that you will need to purchase on this first step of your rebuild. I am sure you do not have a dial bore gauge or a trimichrometer to measure the clearance between the piston pin and the piston pin bushing. How do you size your valve guides to fit the head? Do you have a diamond hone to properly size the guides after they are installed? Finally do you have a inspection micrometer to properly measure the valve stem wear? What would you have to do to successfully use a standard mike on the valves. Porsche was successful because it was an engineered automobile with extremely capable designers. You car was not dessiggned like an English car and you cannot put it together as if it was an MGA. al zim

[Dan Mazur]

Al, Wow! Really? What a total buzz kill.

I dont quite know where to begin, so Im just not going to say anything beyond this;
I do have a dial bore gage and a trimic, and precision calipers and micrometers, but why on earth would I use them when I have a CMM machine?

I posted here to try to understand why this piston failed. Those of you who were kind enough to reply with constructive comments, I thank you. I'll end with that.

[Martin Benade]

Al, that's not a polite answer. All your questions of what he doesn't know or have, the answers are easy to obtain, and likewise the tools, you cannot predict what he has or can get the use of. Another thing easy to obtain is the correct spelling of LaDow.