Re: "Acceptable" bearing clearance

Posted by Owen_Dyneto On 2015/4/30 8:41:20

I've been down this road a few times back when a proper rebuild was financially out of the question. Short of a rebore and full engine rebuild there are some things to think about that can give serviceable results for perhaps 25,000 - 40,000 miles if the taper is moderate, say 0.007" or less. If the original pistons are in good condition they could be reused but check the ring grooves for wear, especially the top ring - worn grooves will allow the rings to "wobble" and wear the sharp edges rapidly and the solution to that is to recut the ring grooves to an oversize and fit with spacers; spacers should go above the new ring, not below! Piston ring suppliers at one time did supply special ring sets for worn but otherwise standard size bores though if no longer available even just a new set of standard rings should be quite satisfactory. If the ring grooves are recut, spacers are a commonly available item. You also might want to think about either installing piston expanders or having the skirts expanded (knurled) or peened to slightly increase the skirt diameter. Though there are those who frown on piston skirt knurling as being a short-lived solution, all the professional makers of engine rebuilding equipment offered tools for this and it is a viable option IMO. Again if reusing the existing pistons recut them for oversize wrist pins and new pin bushings for sure. And have the connecting rods straightened and aligned, and the big ends resized back to standard.

None of this will of course will match the result of a proper engine rebuild with new bores, but "in the day" engine overhauls like this were bread and butter for many a shop, I did a fair number myself.

But it's all for naught unless you deal properly with the rod and main bearing clearances.

Attachment from Glenn's outstanding book on engine service and rebuilding; though long out of print perhaps the very best book on auto service in general and engine rebuilding in particular.

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