Advice on compression numbers.

[Max10]

Hi everyone. Thanks in advance for any advice.

I am planning to pick up a 1969 A-100. The seller did compression numbers and they are as follows:
1 - 145
2- 125
3- 150
4 - 145
5 - 145
6 - 140

I am curious about the difference in cylinder 2. Is it 'high enough' or does the difference between it and the others seem like a big issue? I am just trying to get a sense of whether the engine will need to be opened up soon or whether I might reasonably expect a decent amount of driving left before that's necessary.

I have had a few older vehicles, by thus far my hands-on experience has been things like changing radiators, starters, etc. I have cracked an engine open yet.

 

[Demonracer]

The number 2 cylinder rings may have taken a set due to sitting. If it were mine, I'd drive a few hundred miles & check it again. The first things I'd due would be to change all of the vital fluids, check the plugs & fire it up. After that, drive it & see how it performs. Most likely the cylinder pressures will even out after said few miles of driving it.

 

[Max10]

I appreciate the response. The seller has driven the van pretty on some long (several thousand miles) road trips over the past few years, but in the last year or so I am not sure, it may have sat mostly.

 

[AJ/FormS]

125/150 is 83%. So, the low cylinder has lost 17%, and that is bad. It could be because of
bad rings, or it could be a
bad valve, or it could just be
carbon stuck on the seat, or a
too-tight valve lash.
But it could be a few other things, none of which would be cheap or easy to fix.

So, the first thing I would do is;
Take the valve cover off, go to that cylinder, turn the crank over to TDC compression, then turn it over so the piston is down a half inch or more. Then, bop the valves a few times to see what they sound like when they land back on the seat, and to crush any carbon that may have got trapped on the seat. Repeat about 5 times. Then do a compression test.
If still low, I would reset the valves on that cylinder, then retest.
If still low, inject a lil oil into the cylinder. Let it sit for 5 minutes, then pump it out thru the plug hole. Now repeat the test.
If the pressure comes up, then the rings were just dry, but there is no way to know why or if it will happen again. But;
If still low; I would do a LeakDown test on one of the higher cylinders, and then on the low cylinder, then compare them.
Less than 4% is an excellent number
More than 20% is super lousy.
10% is typical of a well-worn engine.
6 to 8% is a reasonable target.
I would not buy it as it is, except as a core.
Good luck

Btw: 150 psi is an excellent number; are you close to sealevel down there?

 

[SIX225]

What are you expectations out of the vehicle? Recently check mine for the first time on my '68 and they ranged for 110 to about 135. A little disappointed, but runs good enough to keep up with traffic at 70mph. It uses a bit of oil (needs valve stem seals), but for most of the around town cruising I do, it's not coming apart unless something else major happens. Lucky if I put 1500 miles a year on it. A MAJOR tuneup (rebuild carb, free weighs in distributor, free heat riser, etc) did wonders to mine. When I first got it I was so far out of tune, pulling out in traffic was a concern. I'd be more concerned about oil pressure.

 

[RustyRatRod]

"NOW" is not a good time to worry about this. Get it running on a regular basis first. Drive it some. Put it in the back of your mind and fuggetaboutit for a while. Then after a couple months, as long as it hasn't started smoking badly revisit the compression test, but make sure the engine is warm. I "just bet" the numbers even out some. 125 PSI is PLENTY enough cylinder pressure to fire on, you'll never notice a difference in running. So drive it some and see if it levels out after awhile. It could just be some surface rust on the cylinders and or rings. It'll come off in no time if you just drive it. It's a slant six. Keep oil and water in it and it won't die. It probably won't die anyway.

 

[MOPAROFFICIAL]

Check valve lash, you may want to do a leakdown test for bad exh valve.

Slants ,ime. Fall victim to burnt exh valves from poor tune/carb jetting.

 

[Max10]

Thanks for these responses. I've taken note of the suggestions.

Expectations for the van are hopefully that it can run reasonably well and be capable of some half day road trips without major engine work needed soon. Obviously I understand anything can happen in mechanical systems, particularly those many decades in age. Not being that familiar with compression reading and internal engine work I just want to get a basic sense of whether cylinder 2 is a real cause for concern or an 'eh, just let it ride' type thing. And of course, I know there is no sure way to know.

The compression numbers were taken at or close to sea level.

Running A-100s in my area, in this price range ($2,500) seem hard to find. The van does have some rust in the rain gutter areas, and a few other spots, but nothing structural. I'm willing to experiment with my sheet metal patching for the time being. Overall, it's a decent looker.