The Stovebolt Site Forums The Shop Area Engine Shop cylinder compression

the motor is on the engine stand yet and i can turn it over with jumper cables , my question is , what should compression readings ,be at this point (non running ) 6 or 7 revolutions ?

depends on the engine size and condition, you might generally want to see about 110-120 if you want to use it - the real important point is that the cylinders be very close to the same, if there's more than ~20# difference something needs attention

Bill

Check all 6 the same amount of rotations with the carb wired full open or removed. All 6 should be with in 10% or so of each other and the 100 to 120 range would be about right. Its the one or two way off that are a problem, even all six being lower then normal would still let it run with out much problem.

Joe

ok this is a new rebuild , carb is off , they are all 90 to 95 ,so theirs no big difference within the 8 , i'm just concerned that it's to low , what do you think , will compression levels increase once i get it running ? thanks again

It will run. Yes the comp will probably rise when running temp.

When I take compression tests, I pull all the plugs and turn it over so there are 3 compression jumps in the guage. Watch how far each goes up. The higher it is on the first jump the better.

example: 60-75-95 is better than 45-70-95. (NOTE: these numbers are just examples and not to be confused with what the compression numbers should read)

Try this and see how consistent the numbers are. I would also assume that you have a manual for your truck which will be the source of most of the information you need. If not, try
www.gregsonline.com
for a manual.

Camshaft timing makes a big difference when checking compression. If the cam has a lot of valve overlap, then some of the compression goes out the exhaust before the intake gets closed all the way.

Once you get the engine broke in good, recheck the compression if you are concerned about.

I would really caution you about checking a new engine before the cam has been broke in, its a really good way to wipe out a cam! Read up on oils and break in procedures.

Joe

i would hope the cam could'nt be ruined (that easily) by checking compression , before i install motor , didn't want to get in all in and put together then find out I have a problem , and have to start over , but thanks for all the advice , also should i do the cam break in before install , for the same reasons , ? and I did pick up a manual, workbench- how to rebuild a sbc ,has anyone else noticed a misprint with lining up the timing gear marks on the sprockets ? every other source and web site i've check shows just the opposite , I hope mine is right though , the round dimple on the rim of the cam pully is at 6 o'clock , while the crank dimple is at 12 o' Clock ,
9at least i hope its a misprint , can anyone tell me the correct setup , thanks

i would hope the cam could'nt be ruined (that easily) by checking compression , before i install motor , didn't want to get in all in and put together then find out I have a problem , and have to start over , but thanks for all the advice , also should i do the cam break in before install , for the same reasons , ? and I did pick up a manual, workbench- how to rebuild a sbc ,has anyone else noticed a misprint with lining up the timing gear marks on the sprockets ? every other source and web site i've check shows just the opposite , I hope mine is right though , the round dimple on the rim of the cam pully is at 6 o'clock , while the crank dimple is at 12 o' Clock ,
9at least i hope its a misprint , can anyone tell me the correct setup , thanks

Is this a new (rebuilt) engine? Or is this an engine with miles on it?

If it is rebuilt, I would already believe that my settings on assembly were correct and expect it to run. I wouldn't even do a compression check unless I felt something was amiss, or until after my initial first start and run at least.

I'm a little confused Joe. Why would anyone be checking an engine without the cam timing correct? Wouldn't that all be buttoned up by the time one got to that point?

And, the cam would already be prelubed with the cam lube when assembled. Why would spinning the engine over with the starter hurt the cam any more than starting it the first time?

What does the supposed "misprint" call for in the way of timing mark alignment? The timing marks will align opposite each other (cam sprocket at 6:00, crank sprocket at 12:00) once every two turns of the crankshaft. On the next turn, the crank will be at 12:00 and the cam will also be at 12:00. Nothing wrong with either situation, it just takes two turns of the crankshaft to make the marks align again.

I rebuilt a Ford 3000 3-cylinder farm tractor once, with a very complex gear-drive timing system. There were several idler gears, accessory drives, etc., all with timing marks. I did the "turn it twice" check, and some of the marks were out of synch. Tear it back down, reassemble, recheck- - - - -repeatedly! Finally, I kept turning the crankshaft, to discover that the idlers had a "hunting" tooth pattern, and all the timing marks finally re-aligned every 96 turns of the crankshaft! That's like mechanical clock gearing, so the gears don't develop a repetitive wear pattern!
Jerry

so your saying put it in and hook everything up , and break in the cam , first then check for problems ? also i have heard of guys cutting a slot in an old valve cover to adjust final lash , should this be done right at start up , during start up or after ? thanks again ,( oh and by the way I know setting the timming is first and formost at startup)

Valve "lash" on hydraulic lifters is set with the engine stopped. Trying to do it while the engine is running just makes a horrible mess and never gets the adjustment done accurately. By using the "companion cylinder" adjustment method, all 16 valves are adjusted in two turns of the crankshaft.
Jerry

If you read anything about cam break in, the first thing they say is, get the rpm up to 1800-2000 as soon as the engine fires, no messing around with tune up or adjustment, ASAP. You must have everything as close as possible before you start.
If the engine fires up, get the rpm up, don't mess with the timing, its close enough if its running. Read up at the Comp Cams site or any of the other cam makers sites on break-in procedures, its not like it used to be 20 years ago! After the 15 to 20 minutes of break-in, then set your timing and carb settings. Also, don't start out with a new or untried carburetor, it just adds more worries and unknowns.

The first few rpm's are the most critical, once up to speed, the crank throughs enough oil to lube the cam/lifter contact point. Before that, its the cam lube you have installed, and by now, thats been wiped off and is laying in the oil pan. Pull the intake and look, the cam will not be dry, but it won't be greased either. There is no oil system that lubes the cam/lifter contact point, is all splash from the crank and rods. High rpm is the only way to supply enough for break-in. Cam lube is only good for the first few rotations and the heavier the valve spring, the fewer rotations it stays on the lobe.

And, yes, just a few turns of the crank before break in can indeed hurt the camshaft if every surface is not 100% correct. Did you knock the edge off each lobe and check each lifter for a crown on the face? Was the oil system primed? does it have oil pressure? Do you have break-in oil or additives in the oil?

Pre-run in is not the time to be checking compression. That should have been figured out on paper before this point. And afterwards only if it doesn't run right.

I have started a lot of expensive engines over the last few years with out any problems, one thing I always do is start them with in 24 hours of installing the cam, and only turning them over 2 revolutions to set the valve train, any more then that just wipes the cam grease off. I install the intake after adjusting the valve train so I can add more grease or break in lube back on the cam lobes. Also waiting days or weeks to start lets the cam grease drip off.

As far as compression at cranking and how much it should have all depends on the camshaft timing, how it was ground, not so much as how it was installed. The cam card will tell you how much overlap it has. You can also figure it out with a degree wheel and dial indicator. As for timing marks aligning, the gears are 2-1, they will be right every other rotation but will still be in alignment using a straight edge each rotation. Once at TDC valves closed, once at TDC valves open.

You also have to consider the compression test invalid if the lifters were not full of oil. You will not get full valve lift with a empty lifter.

Joe

I can concur with that Joe. I get the cam lube part.I missed that part about the engine being on an engine stand. For some reason I thought this was an engine that had been running already. My bad.