Pilot Bearing Question

I am confused with the pilot bearing adapter.  The Porsche and LS engines have the same size shaft, 15mm (0.590"), so why is the adapter needed, and why can an LS pilot bearing not be used?

LS pilot bearing:
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And the Porsche pilot bearing is a 6202, 35mm OD, 15mm ID.

Or is the LS pilot bearing too tight on the Porsche shaft?  It seems at most it would be a half thousandth of interference.

EDIT:  Ok, still digging into this.  It seems that GM part number: 12557583 is the pilot bearing that has a 15mm ID.  It seems this is refered to as the "Big pilot bearing" as there are two bores in LS crankshafts.  Now, to add to my confusion, the "Big" pilot bearing is commonly a 6302DU bearing, which has an OD spec of 42mm.  But an LS crank shaft has an ID of 43.30mm.  Can't explain this part yet.  But a 6302DU has an ID of 15mm, so that confirms that an LS2/3/7 pilot bearing should work with a 944 shaft.

If someone has already tried this though, let me know.

Ok.... seems I have answered my own question. Looks like the adapter is mainly used to extend the pilot bearing past the end of the crankshaft.

The large GM pilot bearing 12557583 is perfect for the SBC and LT1 swap with a 0.61" TT adapter plate I discovered, and machined mine accordingly.

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It looks to me like the shaft extends far enough into the crank nose in this picture? Is the adapter suppose to keep the shaft from sliding forward?

The trans and its coupler keep it from moving forward or backward.

Ok, that makes sense. I didn't think a pilot bearing should have thrust load on it anyway.

Then it seems using the stock LS2/3/7 pilot bearing should work fine.  

Confirmed the fitment of a OEM pilot bearing on a LS engine?

superman22x wrote:Ok, that makes sense.  I didn't think a pilot bearing should have thrust load on it anyway.  

Then it seems using the stock LS2/3/7 pilot bearing should work fine.  

Stock LS2/3/7 pilot bearing. But I don't know if the porsche driveshaft is long enough to reach. The diameter should be perfect however.

This picture should explain why the adapter is needed with the LS engine; and you still need to move the drive shaft forward a 1/4" most of the time. Without the adapter the drive shaft would need to move forward almost a 1". The LS2 pilot bearing will work with the LT1/SBC engines only.

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Ah, ok, that makes sense! Thanks!