What makes an engine spin a main bearing?
 

My 930 case spun a bearing (PO tells me). what makes an engine spin a main bearing?

Andy

lack of lubricantion.

Right. Low oil means that the friction between the spinning parts (rod or main bearings) goes up, and the bearing lets loose in the case and starts to spin in its bore.

This explains your bore problem. The case needs to be machined in order to correct this, prior to assembly...

-Wayne

The only other possible cause is overloading, which would have to be pretty extreme on a 930.

Or an improper fit of the bearing in the bore...

-Wayne

Ok, so is it something that really only occurs through misuse and neglect? im assuming that if the cars on the track for a lot of its life then risk of oil starvation could induce a spun bearing but how about a street car thats well maintained? (by the way wayne, I am going to line bore my case, im just interested to know the reason that im having to do it!)

Andy

It could be a number of things. Most of the time it's running the oil low. Sometimes this could be caused by a seal failing, and the owner not realizing the engine is out of oil.

Ironically, track cars very rarely run out of oil because the people who design their configurations for the track typically make sure there's enough oil everywhere. The dry-sump design of the 911, in fact, does make it somewhat difficult to run out of oil, compared to other types of engines.

-Wayne

Which main bearing spun?

don't know, the machine shop just told me that there were two mains out by .001

Andy

I assume that there are no fragments inside your motor and someone has found this by using a bore gage. Make sure the Nose #8 bearing is in the case and they have tensioned the bolts properly. Also the Factory spec gives a + and - range. Ask how far out of the range you are before you make the cut. I believe that the spec alows about .0007 or .0177mm on the plus side.

Kindest regards

racing 97, the tolerence for the main bearings on the 3.3 engine (according to waynes book) is 65-65.019mm. my machine shop says that the mains are bigger by half a thou. are you saying that i can be over 65.019 by up to .0177mm?

Thanks,

Andy

No I am saying that there is a range in this case it is .0177mm which is .0007 ths or just 3 tenths sort of a full thou. If the case if you had journals that are 65mm and a few that were one thou over 65.019 which is .0007 thou. additional and in range, you would be out of tolerance by just 3 tenths of one thousands. It all depends what problem you were having when you took the motor apart.
Best regards

the case was fine, the guy said that it had spun one bearing. this was evident on the crank (i had to throw it) but thatas all i know about the case. everything else on the case was ok.

racing97, do you think my case needs line boring?

Thanks,

Andy

Yes it does if it spun on the case side.


Best regards

I think racing97 means that the spec is +.019mm not .0177mm.

My book has the same information as the Porsche Spec books, and the spec on this dimension is 65.000 + .019mm (I rechecked the spec).

You need to line bore, or machine the case back to std/std if your main bore is out of this spec. Very common for a case with a spun bearing.

-Wayne

thanks racing97 and wayne. Im going to get it machined and be done with it. (no sleepless nights thinking 'what if...') Have a good xmas!

Andy