Dry compression test

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k hutchins
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Dry compression test

Postby k hutchins » Sat Aug 15, 2020 12:32 pm

Before l start the tear down on my original '48, l decided to do a compression check on all cylinders just to get an idea of how deep l need to go, and how much l'd need to spend when the time comes.

I was actually surprised and pleased with the overall results with the exception of #1.
#1-90 #2-110 #3-113 #4-120
This was a dry test. Other than #1 l don't think it's too bad for a 72 yr old tractor that has never been rebuilt. The reason l have to tear it down is to replace the rear oil seal, and yes l've already been in contact with tst.
Originally l thought l'd be replacing bearings, rings, and valves, but now with these numbers l'm not sure that will all be neccessary. I one of those "if it ain't broke don't fix it" guys. Not because l'm cheap, but because l'm not a mechanic. It blows a bit of blue smoke when working, especially if it has sat idling for a bit, then throttle up and you get a cloud of blue smoke. Hence the reason l'd replace rings and probably hone the cylinder walls lightly. Now l'm thinking it may be more of a valve adjustment problem. Valves have been adjusted in 35 yrs.

I'll post a pic from last year when l changed the head gasket. I'm looking for opinions while l'm still in the prep/thinking about it phase. I do plan on doing other things while it's torn down, but want to keep the engine work to a minimum if l can because it runs and works very well.

Thanks for any input
Hutch
Why is there never enough time to do the job right, but always enough time to do it over. :?:

k hutchins
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Tractors Owned: 1948 Farmall Cub
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Re: Dry compression test

Postby k hutchins » Sat Aug 15, 2020 12:42 pm

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20171111_164757_resized.jpg (22.92 KiB) Viewed 640 times


Here is a picture l took last year. One of the block overall, and a close up of one of the valves. I'm hoping that if it is a valve problem l can just reseat the valves using new guides, springs, etc. Then hone the seats and lap the valves. I should mention l'm also a traditionalist and try to keep as much original as l can. If l can't, so be it, but if possible l'd like to.

Thanks again
Hutch
Why is there never enough time to do the job right, but always enough time to do it over. :?:

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Re: Dry compression test

Postby Mht » Sat Aug 15, 2020 2:44 pm

Those dry compression numbers look good. If it were mine I’d do a wet compression test also just to see what the numbers are. If wet test shows numbers the same or higher I don’t know that I would do rings and cylinder honing. If the oil pressure is good and I didn’t hear any unusual engine noises I would leave the bearings alone. Your valves look better than one I was just into for gaskets and seals and mine runs great. If your main problem is a leaking rear main seal I would just split the tractor, replace rear main, inspect adjust and or replace clutch if necessary put it back together and put it to work. A valve adjustment never hurts and they seem to need it if they haven’t been adjusted in a while. Im sure others will chime in with their thoughts and a little more info (oil pressure and wet compression numbers) from your end will get you some great advice.

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Re: Dry compression test

Postby Eugene » Sat Aug 15, 2020 2:45 pm

Conduct the wet compression test, making sure the loss of psi is from the valves.

I would use SeaFoam and run the tractor, as is.

If you are going to conduct valve work, might as well renew the rings, rod and main bearings - major overhaul.
I have an excuse. CRS.

k hutchins
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Re: Dry compression test

Postby k hutchins » Sat Aug 15, 2020 3:57 pm

Eugene
IF l do valves and rings, do the piston rods need to be replaced or just the bearings? I have heard a knocking once in a while on start up, but that goes away when it warms up.
I plan on a wet test once l get the hood off. I did todays test just for a bench mark to see where l'm at.
Why is there never enough time to do the job right, but always enough time to do it over. :?:

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Re: Dry compression test

Postby Eugene » Sat Aug 15, 2020 4:37 pm

k hutchins wrote:Eugene. If l do valves and rings, do the piston rods need to be replaced or just the bearings? I have heard a knocking once in a while on start up, but that goes away when it warms up.
During engine overhaul you need to check the piston wrist pins. Normally they don't need to be replaced.

The connecting rods are usually not replaced unless there is some damage. Normally good to go.

Rod and main bearing are replaced.

Engine overhaul. Place the engine on engine stand or work bench. During disassembly, measure, inspect, and record (write down) everything. Order no parts until everything is measured and inspected.

The knock on cold start up could be a bad rod bearing, or something else.
I have an excuse. CRS.

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Re: Dry compression test

Postby BigBill » Sat Aug 15, 2020 11:57 pm

Knocking on startup could be a spun rod bearing. The last rod bearing to get oil pressure is number 1. Try running the engine and remove the plug wire on each cylinder one at a time listening for the knock. It might show up. That will pin point your search.
I'm technically misunderstood at times i guess its been this way my whole life so why should it change now.

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Re: Dry compression test

Postby inairam » Sun Aug 16, 2020 6:47 am

I would adjust the valves and as Eugene said run some seafoam in the oil and gas for a few weeks and redo the test. Since your head is off may be worth doing the valves and seats first.

I would not worry about the bottom end and rings yet. I had one cylinder down and getting the head milled and doing the values and seats was all I needed.

Here is my valve job on a 49 viewtopic.php?f=1&t=93111&hilit=valves
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k hutchins
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Re: Dry compression test

Postby k hutchins » Sun Oct 25, 2020 12:59 pm

I know it's been a while but l finally got around to doing a wet compression test on all cylinders.

#1 -150 , #2 -150 , #3 -135 , #4 -148

If you look above you'll see the dry test numbers. The #1 cylinder is the one l was worried about because it was the lowest at 90, but wet it's 150. Now it appears that #3 may have a problem.

Originally l was thinking about leaving well enough alone and just replacing valve springs, guides, and retainers, then cleaning and lapping the old valves, along with checking adjustments. I know for a fact that the valve adjustment cover hasn't even been off in 35+ years.
Now l'm wondering if l shouldn't just go ahead and throw a new set of rings on the cylinders as long as l've got it torn apart. I'm already planning on replacing bearings since l'll have the crank out to replace front and rear oil seals.

Trying to be efficient yet frugal. I hate to do too much since the real reason for the tear down is the rear seal leaking like it's nonexistant. This is my work horse and it starts and runs like it's 25 yrs old and not 72. (You older guys know what l mean). I hate to do anything that might cause more problems that l don't have now.

Looking for input and suggestions.
Thanks in advance
Hutch
Why is there never enough time to do the job right, but always enough time to do it over. :?:

Clemsonfor
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Re: Dry compression test

Postby Clemsonfor » Sun Oct 25, 2020 1:17 pm

Adjust the valves. Run a ton of sea foam and work it hard. I personally would leave it. You want 10% +/- between them and your close enough and overall compression is high enough on all. 10% of 150 is 15. So 135psi + 15psi = 150PSI which is what your high numbers are. I personally would leave well enough alone.

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Re: Dry compression test

Postby Eugene » Sun Oct 25, 2020 1:17 pm

Do you have the engine apart or just removed from the tractor?

If you have the engine apart, do a major overhaul. Might as well while you are there.

If the engine is not apart and you are just doing the rear main seal. --My thoughts--. The wet compession test results 150 psi - 135 psi are close enough to the 10% difference range that I would adjust the valve tappet clearance, run the engine some more.
I have an excuse. CRS.

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Re: Dry compression test

Postby BigBill » Sun Oct 25, 2020 1:55 pm

How’s the oil pressure? Knocking from where? Put a ratchet long extension to your ear when it’s running along side each cylinder to see which one is the problem? Low oil pressure can be too much bearing clearances. The rear main seal tells me it might be mains.

If the head is off I’d lap the valves in after decarboning them. Does it burn oil?

If your going to rebuild it do it right.
I'm technically misunderstood at times i guess its been this way my whole life so why should it change now.

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Re: Dry compression test

Postby ricky racer » Sun Oct 25, 2020 5:48 pm

If I'm reading correctly, the only thing you're not planning on doing that would make it a complete rebuild is a bore job and new pistons (with the possibility of turning the crank if needed). Only you can make the decision on what would be the best use of your time and money. An old engine can only benefit from new bearings, rings and seals but by reboring the cylinders the engine will be brought back to "new" specs. Is the rest of the tractor in good shape where it makes sense to put money into the engine or is a tired old girl that needs constant attention to keep it working?
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k hutchins
10+ Years
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Posts: 630
Joined: Tue Mar 27, 2012 5:48 pm
Zip Code: 48843
Tractors Owned: 1948 Farmall Cub
193 plow
1948 snow/grading blade
Woods 59 C3
Cub 144 cultivator
Cub 22 mower
Cub 172 one row planter
Original manuals for all the above
Circle of Safety: Y

Re: Dry compression test

Postby k hutchins » Sun Oct 25, 2020 6:01 pm

Thanks guys.
Eugene, the engine isn't apart yet, but will be soon. I already plan on doing front and rear seals. So while l had it apart that far l figured l would put in new mains.
Bill, the knock l hear is on start up and coming from the bottom end, which is why l planned on replacing mains. Just torn on whether l should do piston bearings as well. As far as burning oil, just a bit. Hard to tell how much with the rear leaking the way it was. If it's running at low idle for a few l'll get a cloud of blue smoke when l throttle up. Other than that l don't get alot of blue smoke.

I'm happy compression is good. You all just confirmed what l was thinking.
This tractor has never been rebuilt. The rear seal has been previously replaced about 45 yrs ago, and l replaced the throw out bearing 10 yrs ago. Those are the only times it's even been split. I replaced the head 40 yrs ago (original was cracked and had been brazed) and the head gasket 2 yrs ago.
Thanks again for the responses, but as far as pistons and rings go, l'll let the be. If it ain't broke don't fix it right?
Why is there never enough time to do the job right, but always enough time to do it over. :?:

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Re: Dry compression test

Postby Rick Spivey » Sun Oct 25, 2020 7:18 pm

k hutchins wrote: I'm already planning on replacing bearings since l'll have the crank out to replace front and rear oil seals.


Just want to be sure you don't do anything unnecessary. You don't have to remove the crankshaft in order to replace front and rear seals; you don't even have to remove the oil pan if you don't want, and are careful not to drop anything in it while at the rear seal.

The difference from replacing seals to a full rebuild is on the order of $100 going to $1200. If your only concern is the leaking of the seals, and your engine runs well with normal (Cub) power, then I wouldn't suggest you perform a rebuild. Your compression numbers are great, the only thing I read in all this is the possible engine knock on occasion. I'd probably try to pinpoint the cause of that, then decide what to do beyond seals.
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