Splitting the case pays off
 

I removed the rods from the crankshaft today.
The first five bearings were OK, but the last one was completely worn out :eek:

http://forums.pelicanparts.com/uploa...1082242180.jpg

The reason I started working on the engine was two broken head studs. I decided not to cut any corners but split the case and do a full rebuild. Didn't expect to find this!!
I'm glad I took the extra time to check everything.

I found out that the engine has been apart once before, but I don't know why. It's an 1980 3.0 (euro).

What could have caused that kind of wear on only one rod bearing? A blocked oil passage to the bearing? :confused:

Ove,

WOW, count your lucky stars. That is as close to disaster as anyone can come. That should show up in the sump screen.

Now why? It is going to take some careful investigation on your part.

The big end of the rod could be mis-sized.
If the crank was ground, was it improper?
Could that cylinder have been hydrauliced at some point.
Is the rod bent?
Yes, there could be an oiling problem.
Could there be a single-cylinder detonation problem?
What do you know about the prior rebuild?

Best,
Grady

Grady,

Lucky? You bet! Looks like it would have failed the next time the engine was started!

I'm sending the crank and the rods to a machine shop to have them checked/measured. The crankshaft is of course scratched in the bearing area. I guess it will have to be reground, not just polished.. And since regrinding a crank is not recommended I will have to look for a used one in good condition.

Unfortunately I don't know anything about the last rebuild.

I'm thinking that the crank could have been bent and/or the rod could have been damaged as a result of the engine failure(?) that happened prior to the rebuild.

I have now checked the inside of the case closely, and found some damage. Material is chipped off by the oil passage (see picture). Just like if it was hit with a big screwdriver. But it doesn't cut through the metal there. Must have been a rod that made that damage. What do you think?

http://forums.pelicanparts.com/uploa...1082250181.jpg

Just an overall question on the circled picture. This is the second time in the past 6 months I've seen the chipped oil passage. What's making that tube fail?

My case didn't have trouble there, but I just repaired the only leak remaining on my rebuild- this same oilway tube showing on the outside of the case. I JB'd the slots. One was outside and the other was adjacent to it on the outside behind the oil cooler.

Lee78SC.

Hallå Ove!

Which of bearings is showing copper? Could you take a sharper picture of the bearing?

Talking about getting used crank: they are quite expensive, are you sure you want to buy new? I don't see any problem with re-grinding the one you have and using oversize bearing shells.

Goran,

I'll see if I can get a better picture of the bearing surface.
Here's a picture showing the damage to the case and the location of the damaged bearing. Both are in the same area.

http://forums.pelicanparts.com/uploa...1082292492.jpg

The reason why I mentioned getting a used crankshaft is that Wayne recommends against having it reground in his engine rebuild book. The reason being that the crank is hardened in a special process. After a regrind the crank must be rehardened but the quality of the original hardening process cannot be duplicated by any machine shop.

I really must compare the cost of a regrind + hardening to getting a good used crank and having it polished.
There are groves in the bearing area of the one I have so I don't think a polishing job will be enough.

And then there is the question of what made the bearing fail. I'll have to have the crank and the rods checked by a machine shop to see if there is any damage. If the crank is bent I don't have a choice, I must get a used one to replace it.

A bearing worn out like that is usually due to some type of oil starvation problem. In a properly running engine that is never turned off, the bearings can last forever, as there is no friction (they ride on an oil film).

Getting a used crank is a good idea, although the costs of the 3.0 cranks are much more than the earlier ones...

-Wayne

The damage to the case you are looking at has been covered in a previous post. This is actually common to a lot of 3 litre cases, and it's not actually any damage. It's where the oil tube runs up from the pressure relief valve in the bottom of the case. Porsche sometimes cleans this area up when making the case. Other times it's just left with dags of alloy hanging off it. My 3 litre turbo case has this same "damage". Nothing to worry about.

Thanks Paul,
Thats good news :)

All things considered I'm not too depressed by what I've found.
Splitting the case at this time has saved me a LOT of time and money. Just imagine what would have happened if I decided not to check the inside and just put it back in the car after replacing the head studs :eek:

I agree those aluminum chips are very disconcerting, I posted some pics of mine awhile back. A couple of mine actually came loose (the chips) with a fingernail. I've seen this before on other makes of engines and in those engines the steel oil passage is assembled by driving that steel pipe into the unassembled case halves. When it is forced through the aluminum case those chips are created and if they aren't cleaned off they just wait for enough vibration to shake them loose. I would expect more from Porsche -- poor QC for sure.

Hmm...as far as I know those bearings glide on oil film so I don't see the problem with crank re-hardening as bearing shells are made of very soft material...they are first to go if mechanical contact occurs.

Porsche sells original undersize bearing shells, so I wonder why they do that if crank regrind is so bad? I have seen many reground cranks, no problems with "hardening" at all?

I say measure it and regrind just big end rod surface if possible. Purchase undersize bearing shells (exist in 0.25, 0.5, 0.75 and 1.00mm sizes) and go for it. It's much cheaper than bying new crank. On the happy side, it will even make your crank lighter ;-)

I also recommend to torroughly inspect case and all oil-paths for debris so you don't suffer oil-starvation/spun bearing again.


P.S. I'm moving to Oslo this fall, (BI studies) so I can lend you a hand if you are not done by then ;)

Damomma,
That's exactly how my case was, I got several alloy chips off using only my fingers. Not impressed by the factory workmanship here.

Goran,
I will know more next week when the crank has been checked by my machine shop. But I agree with what you said about the regrind. Anyway I'll see if I can find some std/std cranks in good condition to compare the costs.

So you're moving? Sounds good! :)
I'll send you a PM later.