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Using a dial bore gauge I got my measurements for the ‘52 Cadillac. I set it to be within factory standard specs and this is what I came up with. I can see out or round and I can see taper from the measurements. Is that all it was that caused my low compression and oil consumption? I’m assuming I need to bore these to .010 over now. Knowledge me up on this because this part is new to me.

T - Top (about an inch down)

M - Middle

B - Bottom (about 1.5-2 inches from the bottom)

H - Measuring rod was horizontal

V - Measuring rod was Vertical

80F394D7-30A3-4493-881E-82A249C10581.jpeg

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     I just got on the forums so I went back and read your story. Great project you have there.

     Your bores have some wear. Piston clearance is the whole story though. Get some long feeler blades or micrometers and check that. I've read that you can go as much as .003 per inch of bore diameter. That's a lot in my book but I know someone who's done it. Ring life will be shorter due to the pistons rocking in the bores. I tore down a Pontiac 421 once that had over .020" clearance and it had broken rings in 6 of the cylinders. Proper finish on the cylinders is important to get the rings seated so make sure you hone them as recommended by the ring manufacturer. Just sticking new rings in a worn bore won't work. 

     Usually I'd say that if compression is much below below 80-90 lbs, you have burnt or sticking valves (or those broken rings). Squirting oil into the chamber and re testing will narrow it down; if the compression goes up it's rings, no change means valves. You can also burn a lot of oil with bad valve seals and/or guides.

     I'd make sure the crank and bearing clearances are right. Don't skimp here. Normally when you spin a rod bearing the rod is junk. 

Hope this helps.

 

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It looks like you have enough taper that a machine shop would probably recommend boring it, but they'll probably go out to .030 over rather than .010, just to make sure it's cleaned up AND because that seems to be the most common replacement piston size.  Most of the manuals I've read have recommended no more than .005" of taper, but I'd guess that was pushing it.  You'd still have to hone it for the rings to seat, and even then those new rings would be getting a workout as they expand and contract to fit the bore taper.  It would be a short term fix for sure, but if you don't put on too many miles, it might get you by for a while.  I've certainly done things the "wrong way" to save a few dollars (temporarily) and just to see if it could be done, but I'm a weird guy.  :)

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I have overhauled Chevy v8s  with more taper than that and had a good running engine. My experience is that you will get 20-30,000 miles before it starts using oil seriously.  So.... its the budget versus perfection question again. 

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