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turning a crankshaft

Practical Engine Building

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hey i got a question about how freely should a crankshaft be able to spin when piston are installed?

this video show about how much effort it takes to turn my crankshaft

thats not me in the video so dont take offense im just using the video since its about how mine feel like


Well, that guy is just taking the piss!

Assuming you've done the basic pre-assembly checks on tunnel straightness and crank straightness, it should get progressively more difficult to turn the crank over as more piston/rod assemblies are added. It isn't the bearings that provide most of the resistance, but the piston rings being dragged along the bores.

How difficult should it be - going to depend on the engine, the bigger it is the more drag there will be, the rings used, the lubrication used, etc. As I said, should get more difficult by about the same amount as each is added. Some builders will even check the torque required at each step with a dial torque wrench. Crank on it's own should rotate very freely. You'd be surprised just how much drag there is before the initial running in is done.

would i be able to turn it by hand if i grab end of the crankshaft and turn it? or would i need a ratchet?

i can turn my rotating assembly the way the guy in the video is doing it and it is able to turn 360 degree with the same pressure all around

im asking because i seen another video of someone turning their rotating assembly with minimum effort

Hi Tan,

yeah the guy in the video is umm... yeah

I would never be afraid to turn a crank by hand or using a ratchet; as you assemble the engine it will get harder to turn by hand as the friction of the rotating assembly increases.

once the pistons have been installed it’s always a good idea to put a torque wrench on the crank and record what torque it takes to rotate the assembly. What the figure will be changes per engine and build and even what fluid you have used on the bearings; ie if it’s the engine oil or an assembly lube. I have found in the past thick assembly lubes give you a much higher turning torque than with an engine oil.

If you record this number you can use this as a check on future builds. Turning the engine over also gives you the feel of the engine; this should flag up if there is anything wrong before you go further with the build.

ok i guess its because im using assembly lube thats making it tougher to turn

Is there any videos you could take of yourself... when I say tougher, not by a considerable amount but it is noticeable; there’s a drag if the assembly lube is particularly sticky.

my camera is messed up so i cant but i put a torque wrench on it to turn it and it took approximately 6.5 lbs to turn

With the torque wrench on; was it a smooth rotation?

8.8Nm (6.5lbft)? Isn’t that much tbh and I would say your happy to proceed; when I’m turning an engine over in looking for it to be smooth without any tight spots; sound uniform and I’m inspecting everything during the rotation for anything glaringly obvious.

cool and yeah it turns smoothly thanks

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