993 case line boring?

[burgermeister]

I am in the process of rebuilding a 993 engine. I checked the case bores to their upper limit "spec" of 65.018mm, and got the following results (.0001"'s greater than 65.018mm ... yes I am mixing units - it's what I have on hand)

9-3 is across the case halves, the other locations are either side of the parting line.

ID \ Location 9-3 1-7 5-11
Nose 26 -8 53
#1 10 -5 40
#2 13 7 31
#3 16 10 22
#4 17 10 22
#5 16 13 19
#6 13 18 18
#7 15 20 8

Since most dimensions are >.001" over the max, and especially 1 and 2 have considerable ovality, I assume line boring is indicated? I do wonder what would cause this in an aluminum case. Part of me wonders if it came out of spec from the factory.

This all came about because the new Glyco std main bearing shells plastigaged with way too much clearance, at least partly because they are considerably thinner than the old ones that were in the engine, but apparently also because the case has issues.

[burgermeister]

I keep forgetting that the forum deletes extra spaces one might use to improve readability of tables of information.

[Fever]

Mever seen an out of spec 993 case in those areas!

[dannobee]

Did you bolt the two case halves together using all of the through bolts before you checked? Were they torqued to spec? It makes a difference.

[stownsen914]

Did you use something in place of the nose bearing when you measured? The nose bearing aligns the case, so it can be off if you try to meaure without it.

[burgermeister]

Every fastener holding the case together is torqued to spec.

Interesting about the nose bearing - no, I didn't bolt a nose bearing in, as I wanted to measure the bore - also I don't think I could get the bore gage through the nose bearing, and it's too short to get to #1-3 mains from the other side. But that might explain the weird numbers next to the parting line at the front of the engine. I will see if I can re-bolt everything together with the nose bearing at least partially in there.

Though there is still the issue of all the bores being .001 to .002 oversize. Or is that not an issue? It does seem that the case has probably always been this way - I cannot imagine all the bores somehow deforming oversize in a uniform manner.

And there is the bearing problem. The quality new Glyco mains add about .0015 of clearance over the old worn shells (guessing 200K), which brings main bearing clearance into the .004 range - seems a bit excessive...

[Mixed76]

If the old bearings aren't worn you could just reuse them. They don't age if they aren't being abraded.

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[dannobee]

Yup, if the old bearings still look good and give you the right clearance, you could always reuse them.

0.001" over on main clearance won't hurt anything. In fact, many race engine builders prefer main clearances 0.001"over. I think it was Pro Stock engine builder Darin Morgan who said, "If your clearances are too big, you'll know. If your clearances are too small, EVERYBODY will know."

Another option is buying coated bearings. That'll reduce clearance by about 0.0007". If the bore size is big by a thou, there's your difference.

[stownsen914]

You need something in the nose bearing spot, or the case won't be aligned properly and your measurements will be off. I got the tip to use a buy a set of bearings from a VW bug engine - one of them is a full circumference bearing with the same OD as a 911 nose bearing, but with a much larger ID that allows a bore gauge to fit through.

[mikedsilva]

Originally Posted by burgermeister

Every fastener holding the case together is torqued to spec.

Interesting about the nose bearing - no, I didn't bolt a nose bearing in, as I wanted to measure the bore - also I don't think I could get the bore gage through the nose bearing, and it's too short to get to #1-3 mains from the other side. But that might explain the weird numbers next to the parting line at the front of the engine. I will see if I can re-bolt everything together with the nose bearing at least partially in there.

Though there is still the issue of all the bores being .001 to .002 oversize. Or is that not an issue? It does seem that the case has probably always been this way - I cannot imagine all the bores somehow deforming oversize in a uniform manner.

And there is the bearing problem. The quality new Glyco mains add about .0015 of clearance over the old worn shells (guessing 200K), which brings main bearing clearance into the .004 range - seems a bit excessive...

If the nose bearing was not installed, then no, you cannot trust any of those measurements. Install the bearing without the dowel, lightly torque the through bolts, then either tap out the bearing with a long rod, or use a slide puller with a hook to pull it. Then continue torquing the case.

[burgermeister]

I cut an old nose bearing a bit shorter and bored it out on the lathe so I can pass the bore gauge, and repeated the whole process. Results are slightly better, but not much. Looks like the case halves may be shifted by .003-.004 at the front of the case - the i-shaft bores look to have issues as well.

It is late ... I will take some pictures & post data tomorrow.

[burgermeister]

Here is the rather modified nose bearing:


Main Bore dimensions and clearance to crank with the old bearing shells. The new Glycos would add .0010 to those numbers. Every one is out of spec in every direction.


And just for reference, the i-shaft bearings that came out of the engine.



Part of me is wondering if I should just get the old main bearings coated and put it all back together again...

[Neil Harvey]

Maybe i can give some advice here. First though, am I reading the numbers correctly?

You are showing in some directions the diameters are 0.0038" over the max size? Main housing bore should be 65.00mm - 65.019mm, I think.

Here is what you could do. Hone the bore to get it round, but it will be oversize at a minimum of 0.0038' over if the person doing this is careful. This will make the bore round and give less but equal crush on the bearing shells. It will increase the clearances too. But, honing can get you ito lots of trouble too. The hone follows the alignment you have. Measuring the main bores shows their ovality and size, not the alignment.

In the past, a slightly thicker and thinner mains was offered by Porsche. I think it was the yellow and blue shells. I cannot remember exactly but I think it gave +/-0.0005" per side. I don't think these are available anymore.

If the bores are out of round and misaligned, it all will show up on the bearings. It never gets better, It comes down to what you accept. We would bore this oversize and fit 1st over bearings and make sure the clearances are what we want them to be.

As for the 1st motion shaft bearings showing all that wear. Quite common and typically caused by the oil pump misalignment when fitted. Make sure when the pump is bolted to the case the coupler shaft can move back and forth. If not, you will tend to twist the shaft in the shells loading one side more than the other.

[burgermeister]

Yes, you are reading the numbers correctly. That is what the bore gauge says.

I have decided to have a car free year and send the case to Ollies. They will take 4 months, and the next 6 months after that it will be too cold to do anything. Probably a good thing. This junkyard 993 motor, emphasis on junk, appears to need literally everything. I have become quite frustrated, and it certainly is long past where it was fun.

[Neil Harvey]

Originally Posted by burgermeister

Yes, you are reading the numbers correctly. That is what the bore gauge says.

I have decided to have a car free year and send the case to Ollies. They will take 4 months, and the next 6 months after that it will be too cold to do anything. Probably a good thing. This junkyard 993 motor, emphasis on junk, appears to need literally everything. I have become quite frustrated, and it certainly is long past where it was fun.

Ok, good that you have a plan.

But 4 months!!!! Is this how long it takes??? Did you mean 4 weeks??? Maybe we need to get into the line bore business more. We turn these around in 2 weeks typically.

[burgermeister]

They quoted 14 weeks ... so yes, you should get into the line boring business.

Porsche machine work is a funny thing. In the past I have had some difficulty finding a place that does heads, as the shops everyone uses tend to have a huge backlog. But the places I used, which did not have a backlog, ended up causing me to rebuild the engine every 2000-15000 miles when something went wrong with the valve guides. Which is strange - heads are not that difficult to DIY. After rebuild #4 I was fed up and I did them myself, and so far so good.

I did consider attempting to bore the case myself - it should be doable to construct a boring setup like the old VW mag case dealer tools (of course DIY failure is a distinct possibility and would be really expensive). But a set of .25 over outside mains is $1700; the cost to surface the case halves and bore to std is less than that, and I cannot surface case halves.

If you do this sort of thing, you should advertise it a bit. Some folks on this bbs have made statements that only 2 places in the world are qualified to do Porsche cases. I find that unlikely, but then I have no experience in the matter. Ollies was recommended by a friend who rebuilds Porsche engines for a living - he stated he would not consider sending a case anywhere else. So there might be a bit of a wall to climb breaking into the business.

[Neil Harvey]

Interesting.

I'm currently in Florida on business. I will look into this when I get back.

We are a fully equipped machine shop including CNC tools. Our wheel house work is prototyping and building race and custom engines. We are not a production machine shop.

We have been doing Porsche work for years, mostly for ourselves and other shops. We do do machine work for DIY customers but often we are disqualified because of the cost. Our costs are typically higher, but we do work at a higher level. Because we do not have huge backlogs, we manage to get work out a lot quicker. Its not for all, but some do like the quicker turnaround.

This Porsche work is not difficult. Sometimes you are faced with situations that require some thought. But usually attention to detail, patience and care are all that is required.

To line bore at home, I'd be careful. You get one chance and the chances of making a mistake are big.