The Stovebolt Site Forums The Shop Area Engine Shop Low Compression on Rebuilt Engine

I finally got the chance to perform a first start and break-in run on a rebuilt 1954 Chevy 235.

After the 20 minute breakin run and another 40 minutes at varuous rpms making carb and timing adjustments i ran a compression check on the cylinders and got the results below.

Cylinder 1 was 108 psi
Cylinder 2, 3, 4, and 5 were all was 120-122
Cylinder 6 was 110 psi.

These numbers were much lower than the 130ish psi I was expecting and had a much wider variance than it sounds like I should be seeing.

Should I be concerned enough to pull the oil pan? Will the compression numbers improve with time? I assume they will only drop as it wears.

I had a few issues at the first several minutes of the breakin run causing big jumps up and down in the rpm but no other issues during the build and breaking run.

The engine seems to start easily and run ok.

Thanks

If its rings that are causing your lower numbers, then they will probably improve with more run time.
You're within 10% variation, which isn't terrible. I would give it some more run time.
BTW, pulling the pan won't do anything to help compression.

Did you check the piston fit in each cylinder when you put the engine together? Did you check the ring end-to-end gap, getting the ring set correctly in each cylinder? Did you use cast iron or chrome rings? I've seen some very sloppy machine work done where end gaps were nearly .030" on brand new rings and that also means the pistons will be too loose in their bores. Ideally the ring gap should be .008~.009". Chrome rings are always more difficult to break-in than cast iron rings, by the way and it may be a long, long time before they seat correctly.

To seat your rings, you might go from 30 to 60mph about 6 or 7 times. That should take care of that.

I would not jump to the conclusion the rings are the issue.

Was there work done on the head and valve train ? Is the valve lash set correctly ?

I used cast iron rings. I did put the rings in the cylinder bores and check the ring gaps. They all were within spec and I did not file any. I made sure they were offset per the instructions.

I measured the bores before I installed the pistons and I believe they were all at the max diameter end of the spec. I built this a year or so ago and I haven't found my notes yet.

I probably had the valves adjusted too tight during the breakin as I set the to zero lash and then went another 1/4 turn while they were cold. I re adjusted the valves after everything was hot and the compression number stayed the same.

I didn't get a chance to put a few squirts of oil in the cylinder to see if that increased the numbers.

" . . rebuilt 1954 Chevy 235"

Car or truck 235?
Hydraulic or mechanical lifters?
What valve adjustment procedure did you use?

The machine shop went through the head and assembled it. They ground the valves and put in new valve seats

It is a car 235 with new hydraulic lifters. To adjust the valves I let the engine warm up, tightened the adjuster until I couldn't feel any slop in the rocker, then went another 1/4 turn tighter. I did this with #1 cylinder at TDC and with #6 at TDC so I could do the appropriate valves.

Sounds better. Cast iron rings on new pistons which fit correctly in the cylinders are your best possible combination. With just what you've done, those rings will have already seated somewhat, and more use will be a good thing. Your valve adjustment should not be the problem unless there's some reason they aren't seating/closing right. Did they do anything to the guides? Probably obvious, but the only two cylinders that get my attention are 1 and 6. New cam or used? New valve lifters? 1954 head?

The head is a 1954. I found the machine shop bill it looks like the machine shop ground the seats, ground the valves, and surfaced the head.

They did not do anything with the guides.

I have new hydraulic lifters on the original/old cam.

The machine shop did order all parts for a 1955+ engine, so I had to get a different oil pump and bearings for my 1954 model

How is the vacuum? What is the reading at idle with engine at normal operating temperature? Is the needle relatively steady?

Vacuum gage needle was relatively steady at around 16 inches Hg at around 550 RPM.

To help seat the rings, lug the engine to put a load on the rings by slowing down then stepping on the gas until you reach speed, then do it again several times.

Thanks for the tip. I should clarify. The engine is currently on a stand. I have not installed it in the truck so it has not had a load on it.

Have you done a cylinder leakdown test, where the crankshaft is locked and air pressure is put into the cylinder through the spark plug hole? That allows you to listen at the carburetor, the tailpipe, and the crankcase to see where the pressure loss is coming from. You can also find a leaky head gasket by looking for bubbles in the radiator.

Until you have 100 miles or more on the engine under normal driving conditions, and at least 10 accelerations in high gear from 30 to 50 MPH, you're probably not going to see the kind of numbers you're looking for, and looking for 130 pounds of pressure on a compression test is a pipe dream.
Jerry

Thanks for the input Jerry. I have not done a leakdown test. I read one of your other posts where you shared your spark plug leak tester adapter trick and I intend to find an old spark plug and try it. I'm not necessarily looking for 130 psi, that's just a number I read on the forums somewhere. I was more concerned with the variance between cylinders with 1 and 6 being lower than the rest but I have no experience.

Being the first engine I have ever assembled I mainly wanted to get some direction from those who know better if I should run it as is or if that sounds like something serious enough that I should tear it apart and look it over.

Sounds like I should do a leakdown test and if I only hear noise through the crankcase I should assume it's the rings and run it as is for a while.

Until the engine can be put under a driving load, the rings won't exert enough pressure against the cylinder walls to burnish themselves into a proper fit. The old idea of needing to position compression ring gaps in a certain way has no merit at all, since the rings rotate at random around the piston as the engine runs, and the gaps will align themselves with each other momentarily. At 2000 RPM, the pistons are traveling the full length of the cylinder 60 times a second. Even if the gaps align at times, there's very little time for any serious leakage to happen. Ring gaps are also unimportant, as long as they're not so close as to let the ring ends butt together as the engine heats up. If that happens, severe cylinder scoring will happen. Two or even three times the minimum gap will have virtually no effect on compression.

Rings actually seal against the cylinder walls by compression and combustion pressure getting behind the ring and forcing it against the cylinder wall, and down against the bottom of the ring lands on the piston. The spring tension of the ring helps a little, but the dynamic seal caused by combustion pressure does most of the work.
Jerry

"Too tight" makes no difference unless/until the valve does not close completely.
If the valve does not close that cylinder stops working.

I am with Jerry on this. You need to get that motor into a car and get a load on it. The initial break in is primarily for the cam shaft. To get the rings seated you need a little heat and a varying load (by driving). I would do a leak down test if you have the ability.

One of these days, I'm going to have to get my water brake engine dyno back into operation. I can simulate a really long uphill run for ring seating with that little toy! I'm tinkering with the idea of making it portable, on a 16 foot trailer with a 500 gallon plastic tank for the load cell water, and an 18 wheeler radiator to keep the water cooled down. It might be fun to tow that rig to a car show or a swap meet as a "Lie Detector"!
Jerry