Re: Hard vs. soft engine blocks

Posted by Owen_Dyneto On 2013/4/16 9:00:29

I'm not sure what your definition of "high mileage" is and there should be a different definition for engines from the Babbitt bearing era.

My 54 Patrician went about 122,000 miles before I chose to rebuild it. It had not had a prior rebuild. Cylinder wall wear and taper was about 0.008 so I didn't bore it, pistons were knurled and re-ringed. Mains were replaced with std, rods with 0.001 undersize though if the engine were out of the car I probably would have had the crank turned. Timing chain was very sloppy and one cam lobe was wearing. It was still running well enough before but smoking a bit, and oil pressure at hot idle was marginal. I probably could have driven it considerably further but decided I wanted the experience of rebuilding it.

My 48 Custom 8 was also over 120,000 at the time I decided to rebuild it. It was running well enough but a bit of a smoker, and it too had not had a prior rebuild by all accounts. It likewise had minimal bore wear; I replaced the pistons with new (knurled) STD pistons and re-ringed plus new rod, main and cam bearings, rods straightened, etc. The major factor that lead to the rebuild was a dumb mistake; on removing the spark plugs I forgot to blow out the plug wells first and a small pebble fell into the cylinder which I couldn't remove despite trying every trick I could think of. So I (foolishly) started the engine hoping it would pass thru the exhaust valve but it didn't, it blew a hole in the top of the piston and blew the oil filler cap into the adjacent yard. But - it was rebuild time nonetheless. New bearings were replaced with Std butr again as with the 54, if I had done the engine rebuild with the engine out of the car I probably would have had the crank turned. Likewise as with the 54, if the engine were out of the car I would no doubt have rebored it, but these were the kinds of engine "rebuilds" we did as youngsters when funds were tight.

My 34 Eight made about 83,000 miles before it developed a light rod knock which turned out to be babbitt failure on one rod, and the others were likewise showing fatique. It required boring 0.030" oversize and the crank journals needed turning to 0.010" undersize.

My 56 Carib has about 98,000 miles, no engine work yet though 3 cylinders have somewhat lowered compression, about 110 psi which is probably more valves than rings as it uses no oil between changes. Oil pressure is still excellent, just above 40 psi at road speeds when hot, and about 20 psi at the filter inlet. All I've found that needed attention was the oil pump inlet screen which was pretty choked with sludge - original oil pump still in place.