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frankr 10-07-2019 06:34 AM

8.1s Reluctor/crankshaft
I have a 2004 8.1 Mercruiser BBC with 150hrs. It started to run a little ruff at idle then hard starting causing starter replacements.Then no run. I changed out crank shaft sensor still no run. Examining end of crankshaft sensor i noticed it was chewed up.I put a camera in crankshaft hole and noticed the re-luctor was not turning true (wobbling). Engine was pulled out and we found the rear bearing was chewed up on the crank. It appeared the crank has play in it causing the reluctor to rub against bearing.Speaking with local mechanics 1 said the harmonic balancer was not torqued down right from factory. Before i put back together i wanted to see if anyone has ran into this issue before. Any input would be appreciated.

boatnt 10-07-2019 09:36 AM

Nope that is not correct, you need a crankshaft, the thrust bearings keeps the crankshaft from walking back-and-forth

frankr 10-07-2019 10:01 AM

Thanks for reply Is the thrust bearing part of the crankshaft? Are the reluctors part of crank? I don't see any crankshafts out there that have either of the 2? Hate to be a but its all new to me.

BUP 10-07-2019 02:41 PM

the reluctor is pressed on the crank at GM and tech speaking it is part of the crank because of this tight press fitment. Also the crankshaft is not balanced so if the harmonic balancer was not pressed / installed correctly - I could see in due time that causing some sort of issue and or even a broken snout of the crankshaft. But with that said / timewise one would have started to feel the engine vibrating or at least a not so smooth idle conditions ( however) for an unbalance of the harmonic balancer not installed properly.

Also one would prob start to hear noise of some sort from an unbalance effect from an improper installed harmonic balancer

BillK 10-07-2019 06:10 PM


Where are you located ? I suggest you find a good machine shop that can help you with this.

You will need to find out what caused the thrust bearing to fail. Something pushing the crankshaft forward would do it. Drive / coupler etc not installed properly or misaligned ? The harmonic damper would not affect this in any way. Depending on how badly the bearing was damaged the block may also have a problem.

At the very least if the bearing was worn badly the entire engine should be disassembled so that all of the metal from the bearing can be cleaned out of the oil passages etc.

Sydwayz 10-07-2019 06:31 PM

Edited/corrected below.

frankr 10-07-2019 06:35 PM

I was afraid of that not 100% sure until i remove crank from block to see if there is any damage to it meaning the block.I suspect engine coupler was out of line when engine was installed. Thanks for this info and i will let you no more when i remove

BUP 10-07-2019 07:04 PM

Brian -- the cam sensor has been change min of 4 different times and NOT interchangeable . The crankshaft sensor was changed and the external portion of it. It is interchangeable. There was the gen 1 crank sensor ribbed design and the gen 2 design NON ribbed designed. The crank sensor is only good for 3 pull outs after 3 pull outs and reinstalls - it needs to changed.

Sydwayz 10-08-2019 07:23 AM

Ah yes, you are completely right BUP. I mixed them up the cam and crank sensor. Thanks for catching that!!

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