VCCA Home Forums TECHNICAL - GENERAL Mechanical - Engine, Trans, etc checking compression

As long as there is no dumb questions here goes. I have 3 motors on the floor, a '31, a'49, and a '52. Can one check the compression by taking all the spark plugs out and then turning it over with a full charged battery or does the motor have to be running on it's own?

Yes, Jerry, you can check them right where they are.

You might want to turn them over a few times (20 or 30 seconds) so that they are loose before you check them. The pressure that you see should be within 10% of actual.

It might change a little once the engine is running for a while, and after everything gets oiled up a lot, the rings get freed up and the engine is warm.

Compression test can be done in or out of the chassis but you do need to secure the starter in position and all spark plugs removed.
I would look for readings within 20psi from low to high and all above 70psi.
Tony

and if a carb is installed the butterfly should be held wide open while cranking

If the engines have been laying around for years without being run the compression reading will not be very accurate. There can be rust on the valve stems and seats, cylinder walls, and the rings could be "stuck". All or any of these things could cause the compression to be low on SOME cylinders.

I was always under the impression that the engine should be warmed up to operating temperature to get the most accurate compression figure. I've always done it that way.

To find out if there's any major problems, checking it cold should give you a general idea of the engine's condition.

Thanks guys, this is interesting. Now that there isn't anything electrical hooked up, could a guy use a 12 volt to get a faster spin with out hurting the starter?

Sure! But just make sure that you only run the starter for 10 to 12 seconds at a time.

Using a 12 V battery will spin the engine faster than with a 6 V.....Since the factory compression figures are for carnking speed (of a 6 V) your reading may be a little higher than the book says. As long as the cylinders are about even is the main thing.

OK. Thanks again.

put a couple of drops of engine oil in each pot and wind over then check compression

OK. Took '52 engine out of cold garage and put it in my heated shop. Was 20 above in shop at 8AM so got it up to 75 above by 8:30. Took out spark plugs and put in a little MMO in each piston and turned over with crank. At 3:30PM we took a compression test using a not quite full charge 12 volt battery and it turned over real nice. #1, 2, & 3 read 105#. #4, 5, & 6 read 100#. Forgot to check if the butterfly was open. When we put in the spark plugs and tried turning it over without any wiring hooked up it had a tough time getting over one piston and then would stop. Needless to say we only tried this 2 or 3 times so as not to ruin the starter. It could of been a poor connection so will try a full charged 6 volt tomorrow. According to the '49 to '53 Shop Manual they should be 110#'s or better and within 5#'s of each. I'm also thinking mine could riase a little after it warms up and is driven for a while. Was suppose to be overhauled about 5,000 miles ago and looking at the oil on the dip stick it looks like it is ready for an oil change. Any comments?

Sounds OK to me. The main thing is that there is only 5 pound difference. If the engine were to be driven for a few miles I am sure the compression would come up a bit. There could be a little light rust on the valve seats and cylinder walls. Also the compression gauge could be a little "off".

Thanks Gene,
That's the one that will go into my '49 then.
More questions will be coming.

I would go with that engine since the compression readings sound fine to me as well.

Thanks Dawg.
Here's one to you too.

Update:
Lifted motor and put 2 tires underneath and set lightly on tires. No spark plugs in but the weak 6 volt wouldn't turn it over. Tried 12 volt. No dice. Tried turning with both crank and pipe wrench at same time. Turned over but back of motor started to walk. Whoa. Starter ring gear is on tire. Lifted off of tire and it turned with 6 volt. Wouldn't draw gas to fuel pump from jar. Lifted plastic hose and let it gravity feed. Put jar back down and now draws. Put plugs in but weak 6 volt wouldn't turn it over. 12 volt did. Put gas in carb and presto it took off and sounded good. Only let run for 6 or 7 seconds. Good thing as smoke from MMO chased us out. Now I can sleep tonite knowing it works.

If you havenn't done so I would suggest draining out the old oil and filling with just 3 Qts. of 5W-30 for now. Then when your ready to do some driving drain it again as you should have run it more times for adjustement , etc, by then.

That I will do and change the oil filter after I drain out the 3 qts of 5W-30 and start driving. Thanks again.

OK. Has been over a year but I do have the '52 motor in my '49. Had to change the guts from the '52 transmission to the '49 and have it at the local repair shop right now getting greesed and brakes and so fort double checked.
Brought the '49 motor, which has not been run for 12 yrs according to the previos owner, into the shop the day before yesterday and put Seafoam in each cylinder and turned it over by the fan blade. Same thing yesterday. Put fire in the shop this morning and this afternoon gave it a compression check. #1- 88 lbs, #2- 88 lbs, #3- 92 lbs, #4- 90 lbs, #5- 90 lbs, #6- 40 lbs. Found out #6 exaust was set too tight and did not close. Second check. 1-90, 2-90, 3-92, 4-90, 5-90, 6-93. I know the book says 110 lbs but they are all even so would this mean I could be OK with this? Quite sure I'll gain compression once it's run awhile. The previous owner said he didn't recall it using much oil. Plan on putting in 3 qts. of 5w30 oil and try starting it up tomorrow to see if I hear any knocks. Then I'll have more to go on.

Your compression readings are within limits without the motor being run.

Suggest you add at least one more quart of oil if you intend to start it.

Ray. Gonna try it tomorrow.

Finally started it up today after checking out fuel pump and carbruator. Used a jar for the gas tank. Sounded pretty good but let it run for just a short time as I don't have a radiator for it and it's just on a stand. Got it hooked up to run a water hose through it so I can run it longer tomorrow and give it a good check out. I think I had 10 lbs of oil pressure but will double check tomorrow. Also put sea foam in the spare '31 motor through the spark plugs for the last couple of days and turned over by crank and then by battery today. Will take a compression test on that also which I have on a stand.

Started the '49 motor up yesterday and let it run for about 10 minuets and everything sounded great. Oil pressure started at 10# cold at idle speed and went down to about 2# when it warmed up. When I reved the motor up after being warmed up it went up to about 8#. Took another compression test today and gained on each cylinder. #1- 88# to 98# #2- 88# to 106# #3-92# to 104 #4- 90# to 106 #5- 90# to 108 & #6- 93# to 102. I'll run it off and on and check later on. It really does idle and sound nice with no knocks but not sure about the oil pressure yet. ???? I'm thinking it should be OK on a '49. Wow, Just read Gene's last post and got my answers I think. I put in 5W30.