OT: Seat-of-the-pants Crank Shaft Physics


I'll appreciate any attention and advice you knowledgeable folks might be able to offer on my diesel engine maintenance question at BoatDiesel.com.

(If the link stops working, go to boatdiesel.com, and look in the Engines / General Diesel discussion board for my plea entitled "Seat-of-the-pants Crank Shaft Physics".)



I was going to suggest Yelnats69's solution before I saw it - assuming you can disconnect the rods from the crankshaft.

Almost anything unsticks better when pounded rather than forced slowly!!

As to crankshaft physics, the longer the lever arm (i.e. the effective radius of the piston rod wrt the shaft), the better for getting something to move (assuming you don't bend something!).


Dave, you are right -- pounding worked fine, and was much simpler than rigging up a makeshift "piston press".

The physics of the whole arrangement were far less significant once I figured out that disconnecting one rod would enable me to push only the other one.

Thanks for offering your view!

Edited: 23 Mar 2008, 12:19 p.m.



I don't think this is going to be easy. Your method is trying to move all the pistons at one time unless you can unbolt them and separate them from the crank. Theoretically you would have the most leverage by working on a piston that is neither at the bottom or the top if you are really trying to turn the crank. Mid height places the lever arm furthest from the center of the crank at 90-degrees. But still you are trying to move all the pistons.

If you have the engine out and upside down, then I would try and unbolt the con-rods and the crank from the journals and see if you can get them all separated. You may be able to unbolt the pistons that are 90-degrees first and use a wooden hammer handle or something to reach up by the wrist pin and smack them to move them away from the crank.

You could also try some dry ice on each piston. Unfortunately you cannot see how they are stuck, but aluminium may have oxidixed in a manner that has swelled between the side of the piston and the sleeve. And you have no idea what type of rings are on the pistons and how bad they are rusted.

There are some penetrating oils sold by Kano Laboratories like Penephite that might help. Also, alternate heating and cooling might break some of the oxidation bonds.

Too bad someone didn't dump some thin oil on the top of each piston and lay the head back on it.

I dont know much about diesel engines but I assume the sleeves are installed with a heat/cool interference fit that would be near impossible to move.

Anyway, I think your best try is one at a time rather than all at once.



Forrest, I just now read your suggestions, and you were right on the money.

Behind all my ruminations (some of which I related in my original BoatDiesel.com post) was the assumption that I'd have to move both pistons simultaneously, as I couldn't remove the crank without at least moving the rods. After a few tentative tries with a rubber mallet & oak dowel, I realized that I could disconnect one of the pistons, and push on the other while it was still attached to the crank. Moving the second piston upward swung the other crank journal (is that the right word?) away from the first (disconnected) one. After getting the moved piston & rod out, I reattached the first and did the same. (It seemed safer to bang on a rod & crank as a connected unit, rather than upon one of a rod's disconnected ends.)

Worked like a charm -- both pistons out and overall, things look WAY better than I had a right to expect. (Not bad for a free 2nd engine!)

Also, FYI, after splintering a few sections, I found that a hose clamp around the business end of the oak dowel prevented splitting.


Edited: 23 Mar 2008, 12:21 p.m.



For some reason, I figured that this would be a four banger. Maybe I didn't pay enough attention to your other post. Anyway congratulations. If I had known it only had two pistons, I wouldn't have been as concerned.


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