What will or won't a compression test tell me?

[Frankenstang]

Hello All,
Brand spanking newbie to the site! A quick background and I'm ready to bug you guys with my first question.

I've got a '66 200cid, pretty much stock. After the vehicle sat for some time, at least 5 years without firing it up (disgraceful), I tried to get her going. Long story short, a miss turned out to be a stuck valve (stuck closed). I used my favorite tool (a hammer), lightly working it loose, and after replacing a bent pushrod, I was back in business.

Working now on getting it road worth (mostly brake and carb issues from the long sit), I'm interested in doing a compression test to find out what kind of shape the engine is in at this point. I'm wondering if anyone can tell me what exactly will I learn from a compression test?

My brother-in-law (codename Bil) says that a compression test will tell me everything I want/need to know about the engine. Is this true? Can anyone give me a simplified version of the steps/tests that should be done (ie dry/wet...don't even know what that really means)?

My novice approach to knowing this engines condition in the past has been to make sure it's not burning any oil, checking how it does under load (pulling up a good incline) and such. Now I'm more concerned about the cylinder that had the stuck valve (#4), and what damage I might have done while it ran with that valve stuck closed (didn't put any significant mileage on it while valve was stuck). My first concern was something like stretching the timing chain, but that checked out with timing sitting at 12 degrees btdc and no fluctuation. Also am concerned about possible lifter or cam lobe damage on that cylinder. Will a compression test tell me that?

Let me also take just a minute to say I'm so very thankful to have found this forum. Already owe you guys a huge debt of gratitude for all your past posts that have enabled me to gleam everthing I needed to know in order to perform my upcoming T-5 swap. Thank you, thank you, thank you! One last thing, I have plunked down the cash for a new 'Vaporizer 1100' from Pony Carbs (as soon as they are ready...hopefully by the end of the month). I'll be sure to post some feedback and results once it and the new gas tank are installed.
Thanks,
Robert

 

[fordconvert]

A compression test is going to give you an idea of how well things are sealing. It could be valves or piston/ ring/ bore issues. With these engines its fairly easy and safe to do the test on a warmed up/hot motor. Some cars/engines its just too hard to get at stuff without getting burnt. Warmed up will give you the best readings because thats as close to 'normal' operating conditions you can get. What you are looking for is a reading above 100. If its pushing 200 its a high compression recently rebuilt motor. The more important factor is how close together all the readings are. If you say get 145,140,140,150,135,140 this would be good. If you had the same mix with a 100 thrown in there the cylinder with the low reading has a problem.

Some of the gauge sets have a way to hook up an air compressor. This is a handy way to try and identify what the problem is. By putting air into the questionable cylinder with the valves closed you can listen in the intake or exhaust to see if its a valve problem. If its quiet there but you hear a ton of air in the block it could be a ring issue. Its normal for the rings to leak a bit so you would have to compare the leak amount to a 'good' cylinder. If you have the rocker arms on you have to have the piston at TDC with the valves closed and be careful how much air you use because it will try to push the piston down. Easier way is to remove the rocker arm so all the valves are closed.

 

[rbohm]

8) you can learn about general engine condition with a compression check. you are looking for even compression pressures across the board, usually with in 20psi of each other. the first check you want to do dry, then you want to run some oil into the cylinders and see if anything changes. if you get a large increase in compression pressure, then you are looking at bad rings. if not then you can reasonably assume the bottom end is solid. if you have one or two cylinders that are down, and they are next to each other, you can figure on replacing the head gasket. if you have a low cylinder, and running some oil into the cylinder doesnt change things, then you can figure a leaking or sticking valve.

as for doing a compression check, follow this procedure;

remove the spark plugs.
set the throttle plate in the open position, a screw driver works nice.
plug the compression gauge into the number one spark plug hole and crank the engine over a few times until you get the maximum reading on the gauge. write this number down and repeat on each cylinder in turn.

 

[Patrick66]

edit: rbohm beat me to it

it wont tell you anything about lifters and cam lobes, but it will tell you if your rings are bad or if your valves arent sealing. dry would be just putting the tester on and doing a test, wet would be squirting a little motor oil in first then doing the test. the oil helps the rings seal against the cylinder wall for the test, so if you have bad compression dry, and good compression when its wet, you have bad rings. if its low for both dry and wet tests, its most likely a valve.

 

[Frankenstang]

Awesome guys! Just the kind of info I was looking for, thanks.

So just to make sure I'm clear, test the cylinders in sequence dry (record pressures looking for consistency), then put how much oil (~1 ounce?) in each cylinder (right before I test each?) and perform again, and how much of a pressure change would be bad (say they're coming in about 140 each as an example)? And why do you need the throttle plate open?

Just to be clear, its not like I don't know the history or maintenance of the engine. I put the rebuilt short block in ~20 years ago, last head job was about 14 years ago, and regretably it has mostly sat for the last 8 or so years.

Thanks again!
Robert

 

[page62]

I would drive the car a bit before thinking about a compression check. If there are any stuck rings, driving the car will help work them loose. It would also be a good idea to put some Marvel Mystery Oil in the gas and oil -- it really helps to loosen things up in there!

 

[rbohm]

8) hey patrick :rolflmao: :rolflmao:

robert, before you do anything you need to make sure the engine turns freely. it is very possibl that during the last 8 years the engine has been sitting, the rings may have rusted themselves to the cylinder walls, and breaking them free will be detrimental to the engine. follow this procedure;

1: try to turn the engine over by hand. you can put a socket on the crank bolt and try turning the engine. if it turns over freely then you are good to move on to the compression check.

2: if the engine doesnt turn over, start by soaking the cylinders with a good penetrating oil and let it sit overnight. if the next day the engine still wont turn over, soak the cylinders again and again let it sit overnight. repeat this until the engine does turn over by hand.

the other things you will need to do before starting the engine is to change the oil and filter, and pressurize the system by running the oil pump with a drill until you get a pressure reading on the gauge, or the indicator light goes out. put in a fresh set of plugs, points, wires, rebuild the carb, put in fresh fuel, and flush the cooling system. i would also put on new belts and hoses, as well as replacing the rubber fuel lines, and perhaps even put on a new fuel pump.

when you do go to start the engine, treat it like a new rebuild, and run the engine at between 1500-2000 rpm for about 15-20 minutes. this will get the oil splashing around the engine, and loosen things up like they should be. after that you can run your compression check.

 

[60s Refugee]

A compression test reading "0" can also mean a burnt piston. It's not as common with Fords, but I lost compression in my GM 350 and it was a penny sized hole burnt through a piston.

Another thing a compression guage can show is a bent rod. A rod can bend enough to lower compression but not hit the cylinder wall. In the case of a bent rod, you will get a consistant but lower compression reading in one cylinder.

And of course, a blown head gasket. If you get a "0" or other very low reading in two adjoining cylinders it indicates a bad gasket.

With what everyone else has said, that's pretty much all a compression guage can indicate.

 

[Frankenstang]

Thanks for the follow up rbohm and everyone else. Sorry about the disjointed timeline, but the 8 yr haitus ended about a year ago. That's when I found the bent push rod. I don't know if I lucked out or not, but apparently after the push rod bent it squirreled out from under the rocker arm (hoping that might have saved some wear on the cam lobe and/or lifter). For the last year I've not done much more than start it ocassionally, changed the oil, and run her around the block a few times. Probably less than 50 miles and less than 10 hrs run time in the last year (more idle time than driving time). She's up and running buth the old auto parts rebuilt 1100 is out of whack and missing parts.

Now that I'm trying to get her road worthy again my goal is to get at least an every other daily driver with good mpg. Funny (not haha) that I'm resorting to 40+ year old technology to save money over my '01 Dadge. The new carb is coming soon I hope, done is the new fuel tank to go with it, brake work/overhaul...only upgraded to early 70's maverick dual chamber w/new wheel cylinders all the way around, flushed cooling system and new aluminum radiator, next is the T-5, etc.

The air pressure test fordconvert mentioned, following a compression test sounds like a good idea to generally check the condition of the valves depending on my test readings. Bil gave me the impression the test would indicate valve problems, which it sounds like a bent, burnt or poorly seating valve could also be the cause of poor compression on any one cylinder just as piston or rod damage could also as Refugee mentioned. Sounds like I need some intial readings first. I plan on doing compression test this weekend and posting back with some numbers as I would welcome some objective input (eg need to do some additional tests, she's on her last leg, good to drive for now but be preparing for a rebuild, or good to go).