I need to check the compression on a 240d engine block that i have. I want to verify that the engine is good before i begin the task of replacing the worn engine in my 240. Right now it is missing the camshaft, timing chain, oil pump and chain, injection pump, and both manifolds. all the valves and springs are in place tho. If the answer is no, i guess i will install a chain and cam

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1979 240D with a 617 turbo diesel.

No.

While doing a traditional compression test would not be possible it is possible to do a leakdown test which would tell you if the rings and valves are any good. I guess that is what I would do if I were in your shoes. You could even test for leakdown at various points in each piston's travel (something you can't do with a cam installed). I would test each cylinder at TDC, BDC, and at two points in between. If they all passed then that engine will be OK.

Leakdown will tell you more than a compression test anyway...if there is a problem you will know if it is in the top or bottom end instantly.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz

Second nhdoc's post.

x 3.
Given that you can "preposition" the piston in each cylinder at multiple points you probably have more than you need to tell you regarding the compression.

If each cylinder in the the engine holds and doesn't leak initially, especially at multiple points, I believe you can safely assume it will compress air+fuel within the normal operating range.

BTW, How much air are you pumping in, when you make the test? do you have a way to ensure it's the same in each cylinder, like a gauge and a one-way valve on your air source?

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Strelnik
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Be sure you have a good way to stop the piston at those intermediate points. The leverage of the crankshaft will be much higher than at or near TDC.

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I have posted warnings about this in the past...

At BDC AND TDC leverage will be ZERO.

Those are the usual places a leakdown test would be run at..

but with us talking about checking at several places in the bore... which happens to correspond to the way in which measurements are taken when honing a cylinder to fit a particular piston we need to talk about having some way to LOCK THE CRANK.

lets take the position half way between TDC and BDC ....

Ignoring the fact that it is not exactly a straight line implementation of force... and I don't remember the trig involved to compute this slight difference...

and lets assume that a leak down test would start at 100 PSI...

and rounding off sizes to make the math easier....

a four inch piston has a surface area of 4 times 3. 14 for a total of 12.5 square inches.

With 100 PSI applied that is 1250 lbs of force.

if your crank has a 6 inch throw then you have 7500 lbs of force trying to turn the crank. This is the max it has ( at 90 degrees to the center line of the crank )... as it gets closer to BDC you have less and less until at BDC you have no turning force acting on the crank even though you still have 1250 lbs of pressure on the top of the piston ( assuming more air was put in at 100 PSI.

That does not mean the crank would stop at BDC because it is going to be moving at a reasonable speed and the momentum will carry it further. If your air supply allowed very fast introduction of that backup air then the piston would start to compress the air in the bore and stop at the point where the momentum was balanced by the increased pressure caused by the movement of the piston.

So , for the safety of hands, heads, windows and such... the crank should be clamped in some manner and nothing which can fly off should be attached.. like the breaker bar and socket you were using to position the crank by turning the crank bolt... because you are smart enough not to use the power steering bolt because the FSM warns against that due to the fact that that pulley is a tapered fit and can be damaged by excessive force...
LOL ..

Almost...that would actually be you have 7500 in-lbs of torque not force. In fact the offset of the crank is not 6" but shorter than that...so it would probably be closer to about 5000 in-lbs or 400 ft-lbs. Not insignificant but as static force you could counter it by putting the car in gear and setting the parking brake if it is a manual tranny or by devising a clever way to hold the nut on the end of the crank.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz

Thanks for the info guys. If i dont do the leak down, i might just throw a cam and chain on it. I am pretty sure that the compression is good, the block was allegedly rebuilt before i took ownership of it.

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1979 240D with a 617 turbo diesel.