os 40fp question
12-02-2004, 03:39 PM
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os 40fp question
I have a used 40fp. The engine runs but when it idles it has a bad rattling noise. I took the engine apart and gave it an antifreeze cooking. The crankshaft has a lot of play in and out. None of the parts look like they have any wear. The connecting rod has a lot of movement on the crankshaft. Is this the reason the crank moves in and out alot or is there another problem. The engine only has a few internal parts but which one is the the culprit. The motor runs great at high speed but I think there is way too much play in the crankshaft.
12-02-2004, 05:19 PM
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RE: os 40fp question
The O.S. Max .40 FP has a bushing-supported crankshaft. It's normal for it to be able to move fore and aft. When the engine is running at high RPM, the thrust of the propeller keeps the crankshaft fully-forward. At moderate to low RPM, the crankshaft is free to travel fore and aft. When this happens, the rear of the crank will tap the inside of the case and produce the rattling noise you hear. It's completely normal for a bushing engine to make such a sound.
In the 1960's, when more engines had bushing-supported cranks, the rattling noise was common knowledge. Today, with many engines having ball bearing-supported cranks, the noise of bushing-supported cranks is not familiar, and is many times interpreted as indicating a problem. It's not.
Up to 0.010"-.012" is considered acceptable. If your engine has more play than that, you can use brass or steel shim stock to take up some of the space. This shim would go behind the drive washer. Clamp the shim between two pieces of wood to drill the hole, and then cut out the outside diameter. Surface it on a honing stone or 400 grit sandpaper to remove any burrs. You need some play, and 0.005"-0.006" is about the smallest amount of play. You need some play to make sure the crank is properly-lubricated inside the bushing.
In the 1960's, when more engines had bushing-supported cranks, the rattling noise was common knowledge. Today, with many engines having ball bearing-supported cranks, the noise of bushing-supported cranks is not familiar, and is many times interpreted as indicating a problem. It's not.
Up to 0.010"-.012" is considered acceptable. If your engine has more play than that, you can use brass or steel shim stock to take up some of the space. This shim would go behind the drive washer. Clamp the shim between two pieces of wood to drill the hole, and then cut out the outside diameter. Surface it on a honing stone or 400 grit sandpaper to remove any burrs. You need some play, and 0.005"-0.006" is about the smallest amount of play. You need some play to make sure the crank is properly-lubricated inside the bushing.
12-03-2004, 04:11 PM
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RE: os 40fp question
Thanks for the info Bill,
I think it has a little too much play so I will try the shim like you suggested. I have a 40la and it does'nt rattle as much as this one does. Thanks agin for the info.
I think it has a little too much play so I will try the shim like you suggested. I have a 40la and it does'nt rattle as much as this one does. Thanks agin for the info.
