351C with 35,000 miles - Need to rebuild?

I started the engine today on the test stand and the results are good thus far. Oil pressure is 75+ PSI even when the engine began to warm up. Oil is Valvoline VR1 20/50. The valve train sounds good. Compression on cylinder #1 read 180 or so. But my compression gauge is old and it is best to start over with a new one to complete the test. The only issue thus far is smoke (black I think?) from the right exhaust on restart. The exhaust looks and sounds good when the engine is running. I plan to complete the compression test tomorrow and will post the results.

why 20-50?

if your bearing clearances are not set purposefully large for 20-50 you're risking insufficient oil flow through the rod bearings and a bad rest of the day

not to mention what it's doing to the dizzy cam drive gears

If this is an original engine, never rebuilt, it was designed for 20W40 oil.

Most of this engine's life, the most popular oil, high performance or transportation duty, was 20W50.

I will switch to a lighter weight oil, probably 10/40.

But wait. Aside from single viscosity offerings, Valvoline VR1 only comes in 20/50 or 10/30. Stock Pantera engines need zinc for the flat tappet cam and VR1 is one of the few oils made today that still contains zinc (they took zinc out of oil because it fouls catalytic converters). George noted that 20/50 has been the most commonly used oil in stock Pantera motors. It looks like I may just continue with 20/50. I have been using it for a long time without issues.

Here are the compression test results. I'm not sure about the accuracy of the gauge but the following is good for comparison between cylinders. There are two readings for each cylinder, dry and wet. Dry was taken with the engine warm. Wet was followed by the dry test but a teaspoon of oil was squirted into each spark plug hole.

Are these readings healthy for a stock 1971 351C? If not, should it get new rings?

i thought we were in the 10-30, 10-40, 15-40 zone with factory clearances?

i don't see anything alarming enough to tear it down tonight, for more technical insight do a cylinder leak-down test.

did you have all the plugs out and the throttle wedged open the whole time?

The spark plugs were out for both the dry and wet tests but the throttle was sitting at idle. Today I warmed the engine up again, pulled the spark plugs, and used a spring to hold the throttle wide open and then retested. Today's compression test yielded the same results as yesterday.

I will proceed with the leak-down and find the pressure loss for each cylinder.

not only psi loss but where it's going, listen for the air at the carb, exhaust and valve cover.

Here are the leak down test results. I could only find air escaping through the oil cap breather.

Can anyone provide guidance on interpreting the results of the leak down test and compression test. The engine runs well but now is the time to work on it since it is out of the car and on a test stand.

sorry i forget where you're at with the whole project, this engine still has factory vales in the heads or you've replaced them?

I have not replaced them. But when the heads were off I could see that some were Ford valves and others were unmarked. I pulled one of the valves and the seat looked good. And its seal looked good as well. The engine may have had a valve job.

i don't know what to say, running them is risking it all.

Got it.

I will get the cylinder heads serviced including new valves (not Ford or Chinese as per George). The engine will get new piston rings and cylinders honed, and this should bring compression test results up to the 175-180 PSI range. The leak down test results should also improve. The rod nuts will get replaced with ARP. Since oil pressure is good I plan to keep the bearing unless I can find issues during the complete disassembly. I don't plan on any other changes since the engine is currently running nicely on the test stand.

The bearings, mains and rods, are really not expensive, it would be a shame not to replace them.

i don't see the cranking cylinder PSI coming up that far w/o a cam timing change. i suspect the timing set has the cam in the emissions era retarded position and is lowering your Dynamic Compression Ratio. during tear-down, set up the degree wheel and find where the cam is and compare to desired spec. advancing the cam will close the intake earlier and generate more DCR

The compression and leak down tests look pretty good to me, an even spread within 10%. Presumably on a 71 you have closed chamber heads?

I would replace the bearings and probably the oil pump as part of any tear down. If this is an original engine get a decent (non Chinese) timing chain set.

Note there are various ways of doing a leak-down test and interpreting same: mine is to use a dual-gauge system starting with 100psi in the chamber. Using 100psi to start gives you % leakage directly. Run the test cylinder's piston to TDC, put the car in 1st gear and check the gauge reading after exactly 120 seconds (2 min). Note-100 psi in a cylinder may cause an unrestrained engine to move the crank slightly, spoiling the test.

Rule of Thumb for my personal leak-down tests: 2-5%: darn good engine (there are no zero-leak engines, even newly broken in with gapless rings);

5-10%: well broken in, typical street engine;

10-15%: worn but usable for more miles if you're careful with rpms;

Over 15%: there is metal missing somewhere in the combustion chamber; a general overhaul will soon be needed.

Repeat for all cylinders and also check for big variations between cylinders as with a compression test.

Some shops specify only a hot engine, some take a gauge reading right away for % leakakage or after some other duration, and some always wear their 'lucky' socks. You can do a quick diagnosis on potential trouble spots by listening for air flow in carb (intake valve seal), air flow in tailpipe (exhaust valve seal) or air or bubbles showing in rad tank or in valve cover breather cap (head gasket problems).