bebbetufs

Can't find it in the manual.

KuHL 951

It's not in the manual. PM sent

944Ross

I don't think those specs indicate the ovality spec on an individual journal?

KuHL 951

Take multiple radial readings on each journal and compare to limits? If it's that out of round it would fall below the lower allowance and require machining right? The mains on these engines are incredibly stout unless they've taken debris damage from a RB failure, milkshake, or funky oil. Have you ever heard of a MB failure independent of some other lubrication or mechanical problem except for the thrust bearing?

944Ross

I agree it would be unusual to find an oval journal without other major damage, and such damage doesn't usually make a journal oval. More likely that a journal isn't ground perfectly round during a rebuild. I'm surprised there isn't a spec to guide the machinist.

bebbetufs

Sorry guys, I have not been clear. I did not mean the journals, I meant the bearings/bore....

87 944 C

your main bearings? that should be in the shop manual somewhere. usually once you start seeing brass or copper color the bearing is at or beyond it's wear limit.

944Ross

Bearing shells are not supposed to be perfectly circular. An oval shape helps build the oil wedge.

Some good metrics in this article:
http://www.aera.org/engine-professio...gine-bearings/

bebbetufs

I know, but how much can the main bores be out of round before the block needs to be machined?

944Ross

So, are you asking when the caps need to be line-bored?

bebbetufs

That's it. I should have posed the question like this; what is the max permissible out of round for the main bearing bore?

KuHL 951

I have to ask "Why would the bores be out of round unless they came that way from the factory?" The only wear they could ever possibly see that is from damage if you completely spin and toast a main bearing which is rare. I don't think any company even makes a bearing set for over sized line bores and shimming the back side of the bearings would be a bad idea. Have you actually measured yours and found them to be out of round? If they are out of round I feel the block would be useless since you can't get bearings for it. Maybe I'm reading all this wrong, I'm old and confuse easily!

67King

No, you are correct. Roundness is a measurement of manufacturing capability, and I would be surprised if many machine shops had CMM's that could accurately measure roundness.

bebbetufs

I can't agree that a block is toast because the bore is slightly out of round. Everything has tolerances. The roundness of the bores can be affected by several things, the most obvious would be how well the two halves of the crank case actually line up. Add to this that the bores probably weren't perfectly round from the factory as well as the creep in the girdle which leads to the troublesome need to oil stone the front of the block/oil pump surface. Given all this I would not be surprised to find imperfections in the bore. If there are no Porsche specific tolerances I will go by generic recommendations. I have the necessary measuring equipment.

67King

You should be able to line-bore it. If necessary, cut a few hundred microns off of the surface of either the bottom of the block or the girdle, reassamble to torque, then bore.