Crank- turn it or not?

[Peteinjp]

I'm close to max specs on my 964 crank- BUT not over spec. I did check (and will double) with plastigauge and it came out good with the new bearings on one con rod but measuring the journal. The crank looks great in terms of grooving etc and has about 60,000 miles. Just looking at it you would think its fine.

Sample measures are very consistent between journals- all are JUST within spec by .03mm and the finish - not mirror polish mind you- but looks very good with no scratches or grooves. For example #6 con rod journal measured at 54.96mm Spec is 54.971-54.990mm. #8 main is also .03mm within spec.

So my question is how much time would I be buying on my next rebuild by turning for oversized bearings? Or another way to put it is will the bearings outlast the seals and other components until the next tear down? This is with new rods, pistons/cylinders, camshafts, valves, rockers, chains, ramps etc.

Pete

[Flat6pac]

Sent you a PM
Bruce

[r lane]

Just my opinion, no way I would turn a standard Porsche crank that did not lose a bearing. I am thinking the range is no more than .03mm. According to your measurements, how much journal is missing. The less cutting on anything the better unless absolutely necessary. Define necessary I guess.

[targa72e]

I would have the crank inspected for cracks, straightness and polished. If it is still within specs I would use it. If you wanted to reduce the clearance a little you could use coated bearings.

John

[Peteinjp]

Good points all- and thanks Bruce for the recommendation. I'm in Japan so its hard to get a feel for what shops can be easily trusted and which can not. I did another run with the platigauge and got under .051mm which is well within spec- which kind of baffled me- until I realized that the clearance can be met even if the journal is out of spec. For example a 30.96 #8 bearing shaft ( out of spec by .02mm) can still be with the max clearance of .104mm gap IF the bearing ID measures 31.064 which seems likely with a new bearing as that is well within spec.

Anyway- my mind tells me to get measured and my gut tells me that its totally fine. Again it looks beautiful very good. My mechanic is telling me that I'm thinking about it too much.

How much can the clearance be reduced with bearing coatings? What are the coatings for and do the go on top of the babbit coating?

[targa72e]

Generally the coatings are to reduce friction in instances when crank contacts bearings or improve oil retention/flow. It goes on top of the bearing. Some manufactures use coated bearings as OEM. Clevite 77 Porsche bearings from automotive associates are coated out of the box.
Here is a discussion on pelican that covers a lot of coating information,
Just got my engine bearings coated...looks great

john

[Peteinjp]

That was an interesting read- Thanks. I haven't bought my rod bearings yet- just one set (Porsche) for measuring up. I'll definitely consider the clevite set if there is an advantage to be had. According to that thread you listed it seems debatable- but no harm done kinda thing....


Pete

[Peteinjp]

My current plan with the crank is to install it win the case and Plastigauge the mains, install a piston and cylinder/rod and measure again with Plastigauge and see where I am exactly. I just have not been able to get what I consider a perfect reading with the rod just dangling around. No matter how hard I try it always moves a smidge when cranking it down so I'll do it in the case with a piston holding it. If that all looks good I'm going to run it.

The crank has a nice ring to it and appears to have lead a pampered life in its 60,000 miles. My local factory trained, indy mechanic took one look at the crank and said he would just pop it right back in if it were his- also because he knows the car. Not a scratch on it.


I should probably have just reinstalled the original bearings- or at least measured them. Lesson learned.

[spyerx]

high performance build or stock-ish build?

if higher performance build, it might be worth the modest cost of having it fully inspected and balanced. Even just for peace of mind...

coming from a non-engine builder :-)

[Peteinjp]

3.8 9m build- 993 rsr P/C's, custom grind schrick cams, motec with a sold base map (will need tweaking of course) arrow rods, stock redline, stock intake, No cat +cup pipe. Nothing too extreme but should look like this 3.6 when done (blue line) plus some extra shove across the range from the 3.8:





I know you're right - and I may end up doing just that.

P

[Neil Harvey]

Originally Posted by Peteinjp

My current plan with the crank is to install it win the case and Plastigauge the mains, install a piston and cylinder/rod and measure again with Plastigauge and see where I am exactly. I just have not been able to get what I consider a perfect reading with the rod just dangling around. No matter how hard I try it always moves a smidge when cranking it down so I'll do it in the case with a piston holding it. If that all looks good I'm going to run it.

The crank has a nice ring to it and appears to have lead a pampered life in its 60,000 miles. My local factory trained, indy mechanic took one look at the crank and said he would just pop it right back in if it were his- also because he knows the car. Not a scratch on it.


I should probably have just reinstalled the original bearings- or at least measured them. Lesson learned.

If I can give my 02C worth here.

When measuring clearances with Plastigauge, you have to expect some variance. You are using your eyeballs to measure the width of the crushed material against a line on a piece of paper.

Two things to mention here. There is no set in stone clearance you need to have. The factory will give a tolerance to go by as to make sure you have good oil pressure. Its not uncommon to run wider clearances without issue. All you are adding is a thicker oil film. Many times we will fit shells that are 0.0005" thinner to gain an extra 0.001" clearance. Or, we may add a thinner shell to one side to get the clearance we need. 0.03mm is 0.0012". This will not give you any issues. As for life span, if everything works as it should, the bearing life and journal life should last forever. Its only dirt and debris that usually messes up your day.

The best and most accurate way to measure the clearances is to mic the crank journal, fit a bearing et to the rods, Torque the bolts ( new bolts) and measure the bearing ID. Then do the math.

There are a lot more important factors to be concerned about.

[Peteinjp]

Thanks for the input Neil. I actually went to the trouble of adding the 993 console and filter after seeing how many ways a piece of sand could get through with something as simple as a hose or oil cooler change so hopefully that will keep the bearings nice and clean smooth.

Yes its true that you are relying on your eye but the delineation between .002 and .003. if hard to read on the green type is easier on the red. Based on your thoughts I'd say its more about consistency in the reading and across readings then just getting an exact number. The engine had great oil pressure when I tore it down (only tore it down to fix leaks and just to see how it all works really)

Anyway- if you care to expound on the "more important factors" I'm all ears. I'm in the thick of weighing and sorting rings, pistons, etc. Paring the heaviest piston with all the lightest rings,wrist pins etc.and the lightest piston with the heaviest rings etc leaves a .67gram difference.

Pete

[Neil Harvey]

I guess my point was to say, bearing clearances are "not" absolutely critical any more than other areas of an engine.

Yes they need to be measured and known what they are, but you can run tighter or looser if required.

Where to start to say other factors?

Case not cracked.
All threaded bores in great shape
Main housing bore straight and round
Crank straight
Crank not cracked
Journals in good shape
Oil pump clearances good
Rods bores parallel
Cylinders round and no taper


It goes on and on. This is about rebuilding not just assembling. I'm often amazed when I'm told the engine was rebuilt by a Porsche trained mechanic and nothing was ever measured.