I can't really help with the numbers but you removed all the plugs when you took your readings, right?

My 54 Twin H hydro used to get 14.7 MPG city / highway (mostly city) I think the best highway miles I got was 16.5.

Hi Ed - yes, I took out all the plugs before running the starter to turn over the engine. All the plugs looked fine, nice and tan and still holding their original gap of 0.035". Thank for posting., Mike

The results on that 3rd cylinder indicate the compression loss is in the piston area vs. a leaky valve. i.e., broken ring(s) etc.

Hi Park - thanks. I was thinking the same thing, but wondering if it was really the case. How do I say this and leave the site g-rated: ... oh gosh darn? Maybe gee willikers. I'm really not ready for a ring job. :ohmy:

Michael,

Oil is cheap...relatively speaking.
As soon as you fire up a completely new rebuilt engine, it becomes "used" and the level of wear and life goes down from there.
Yours just happens to have more "experience" than some, but is still serviceable. When you see a heavy blue haze in the rear view mirror, and the car is consuming a quart of oil every 50 miles, then you'll probably need to think about rings. (Based on normal ring wear and not something broken, and even then, maybe)

Rob

Michael,

I basically agree with Rob! Does the engine idle on all six cylinders when warmed up?

Per

Hi Per - yes. The idle is a little shaky - I can't balance a nickel on it's edge on the engine while its idling, but other than that, it's relatively smooth. I guess I could try shorting out #3 plug to see if the RPM drops considerably. I'm also wondering if the maybe this is just a situation wherein the gaps in the rings all lined up, giving poor compression until the engine gets hot. I definitely did not see this behavior last fall. Thanks, Mike

Hi, Michael. I'm certainly no expert on this stuff, especially when compared to some of the guys around here, but I also had/have a cylinder that's low on compression. Actually all of my cylinders are low on compression -- quite low -- but this one pumped up to a whopping 10 lbs (hot!) and then dropped before my eyes! I had the head off just over a year ago to try a ring job to fix my blow-by problem (with very limited effect), and none of my valves were burned at that time. I believe that I have a sticky valve on that cylinder (engine noise seems to concur), and am currently treating with Marvel Mystery Oil and whatever other cylinder lube gas additives I have lying around. So far, the noise has gotten better but not gone away, and my idle has improved, but isn't as smooth as others seem to be experiencing. My engine also seems smoother when warmed up, which might well be due to increased compression on that cylinder, or possibly the amazing placebo effect. Or even confirmation bias! Anyway, I'm well aware my engine needs a rebuild, but if you're hearing valve clatter, you might want to try a few products before opening up the motor to do the rings. It probably won't work, and definitely won't work fully, but it might help some.

Good luck!

Hi Michael,

Have you shorted out the plug to see if #3 is contributing to idling? I would suggest that you drive it a while to see if it improves. Sometimes a situation will occur for a while after an engine has not been for a season or more. Then it may just go away.

Chevrolets up to about 1952 tended to need valve jobs if driven a little bit fast. Frequently a cylinder or two would not be firing consistently at idling after the car got warmed up. Often it would be a long time until a valve job got done. After the valve job was done, the engine would idle perfectly, but oil consumption would go up! The junk that had been produced by the poor idling of the affected cylinder (or cylinders) would start to gum up the rings. Eventually this would become an abnormal amount of gases blowing by the piston into the crankcase. This abnormal amount of crankcase vapor would gradually affect nearby cylinders, causing ring problems in these cylinders also.

So, if the situation does not "heal" after a few thousand miles, I guess something should be done.

Per

Michael, something doesn't really make sense here?. Does your head gasket look like it is leaking down the side of the block?. This is usually noticeable by drooling lines coming from between the cyl.head and the deck of the block. It could be the head gasket is seaping between 3 and 4, which is very common. It swells up and seals much better when the engine is warmed up. Also, was/has your engine been running with a contaminated fuel system anytime recently?. An engine running on stale gas will cause heavy build up around the valve stems, and cause sticking of the valves. This applies to all engines of all makes. Once the engine warms up, it allows everything to work free. If continued running on stale fuel, it will eventually stick the valves open, and stay there, causing a no start situation and cause a back-fire thru the exhaust and/or intake. Also, have you checked your timing chain and/or oil pump drive gear?. All these questions are factors in performance .

I'll take a look tomorrow when it's light out. I should say that I put in new valves and guides in 2014 - and a new head gasket. I could recheck the torque on the head bolts too.

Update - 8/8/2016 - head bolts all at proper torque, no obvious leaks by the head gasket./