964 main bearing clearance

[SilentPorsche]

This is my first 964/993 (3.8) build and the main bearing checks out to be .003 all the way to the rear main which check out to be .002, using the plastigage. Based on my previous job as journeyman toolmaker that deals with assembly drawings with tight tolerance, and I learned that Porsche factory set the max wear limit to .0028". It will be using mainly on the track with 11.5 CR and aggressive camshaft and expect to push to 7,500 rpm, my question is that .003 clearance safe to go or recommend to use different bearings than Glycol? Rod bearings check out to be in good shape, .0015. Any suggestion or experience with loose crank clearance will be greatly appreciated.

[txhokie4life]

964 Main Bearing Thickness - Clearance

[mikedsilva]

I've only built 1 of these motors and just to stock specification.

Which bearings are you using? the Glyco bearings which can be bought anywhere, or the Glyco bearings that are sold from a Porsche dealer? You will find that the Porsche genuine bearings, which are also made by glyco, are all measured individually and have tighter clearances than the Glyco bearings that you buy from online vendors such as our host. (lots of threads about this).

Also, if you contact someone like EBS, they can supply you 3 different versions of bearings with different clearances.. I believe they are labelled as red, yellow or blue. I only had a brief conversation with them, but I believe they say most race builders aim for an extra thou or so of clearance, so after mic'ing your crankshaft, you choose the bearings accordingly.

My car had done low mileage, and I rebuilt it only to fix leaks, and basically to learn. Using plastigauge on my stock bearings (which were all in perfect condition) I was getting approximately 1.5-2 thou clearance. (plastigauge is not as accurate as mic'ing everything).
When I used the generic Glyco bearings I was getting 3 thou everywhere..
Then I stumped up the big dollars and went for genuine Porsche (made by glyco but individually measured) and got back my 1.5-2 thou... probably didnt need to change, but I had it apart, so why not?

Anyhow, I think a call to EBS will give you all the information you need.

[boosted79]

Ditch the "maybe gauge", get out your Mitu's. Pay the extra and get the motorsport bearings and run wider than stock for a race engine.

[txhokie4life]

Is there an alternative to Glyco other than OEM?

I'm using Clevitte 77's for my rod bearings.

thanks,

Mike

[m42racer]

What do the crank journals measure? What is the main housing bore size? The main housing bore sets the crush on the bearing shells which in turn makes them round and to size. If the main housing is not to spec, the bearings will be off size as well. These two factors have a huge effect on the oil clearance.

Late main shells come in 3 sizes. Yellow is std , Red are +0.100 and Blue are - 0..001". You may have top mix and match them to get the clearances you want and all the same.

As suggested use a more accurate way of measuring the sizes involved. If you are using plastic gauge, forget about been accurate and go with the std sizes. You will be off +-0.001" and maybe that 0.003" you have measured is not 0.003".

Luckily, in these engines the #1 main and the #7 main control the oiling pressure and volume to the rod bearings. You can run mains a little wider if you have a good pump and control the oil temps.

It starts with the case and the crank before you even think about the bearings.