Bore Measurements 101

[51_Meadowbrook]

I know this isn’t exactly in the scope of Mopar forums but the knowledge I learn with this will help me when i rebuild my ‘51 Dodge Meadowbrook.

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

[50mech]

Looks like you're running about .005 taper...that'll do it. I'd expect though With cylinders with that kind of wear the rings were probably more worn and more at fault.

[51_Meadowbrook]

Thanks for responding. Prior to me having knowledge of clearances and measurements I had rebuilt the engine, well more just assembled it, with standard rings and pistons. I had low compression and oil consumption, blue smoke in the drivers side exhaust. A little history of the engine, it was removed in 1976 due to high oil consumption. When I got it, it was in the trunk and all of the parts were scattered so I cleaned it, bought a rebuild kit with standard rings and bearings. It ran pretty well for about 500 miles, with the exception of oil loss and overheating. I had low compression but the engine didn’t act up. Until a couple weeks ago when I got a rod knock because it ate through one of the bearings. I took the engine out and now I have to figure out what to do with the cylinders before I put it back together. 

[50mech]

Hmm, well with fresh rings .005 taper really shouldn't cause a very notable issue in normal driving. 

It's way beyond acceptable for something that  was just machined for rebuild but by itself probably not going to cause big issues.

I would be interested in the condition of the heads (warpage, cracks), valves, guides, and stem seals.

 

Regardless I guess you're mostly questioning what to do cylinder wise....if it's torn down this far and for the second time I guess I'd lean heavily towards having them bored. A lot of work and money to reassemble with sub par cylinder measurements.

But I suspect you have another major issue in the heads somewhere.

 

Also,I don't know about the other caddy 8s but the 346 block is known to develop hairline cracks.....

Edited August 15, 2020 by 50mech

[Adam H P15 D30]

What do the other rod and main bearings look like?

[51_Meadowbrook]

They looked pretty great. With the destroyed crankshaft from the spun bearing I am going to use the crankshaft from the parts car engine. But I need to understand these cylinders and what I should get them to measure like so I don’t waste my time with rebuilding the engine and still have low compression and oil consumption. 

[Adam H P15 D30]

.005 taper is max allowable for most engines but sometimes that taper closes up when the head is bolted on.  Also cast iron rings don't like that much taper and tend to crack.

Make sure to measure the donor crank for roundness and clearance before slapping it in.  If it were mine I'd ream the ridge, hone it, slap some new rings in and go.  How many miles are you planning to put on this car per year?

Edited August 24, 2020 by Adam H P15 D30