I am working on an FD and swapping a good motor for a bad one. The used motor I bought didnt come with a flywheel nut, so Im trying to use the nut off my blown engine. It does begin to thread on nice and smooth, but then stops and will not thread more. I tried cranking it on by hand first, then using the impact to see if it will tighten, but it does not want to go farther. On inspection after taking it off again the threads dont look like they are crossing, and when I put the nut on the blown engine it will tighten with the impact.
Im worried now because Im not sure what to do. Im assuming that all engines for the FD have the same thread pattern back there, so I dont understand why it wouldnt work. The threads on the nut seem to be fine too. Im stumped.

 

you do realize the nut needs to be on there with ~300lb/ft of torque right? so to put it on the new motor with the flywheel in place is going to take a lot more than a hand tool or your impact wrench. usually takes some big boy tools. i used a 3/4 in drive breaker bar, 2 1/8 socket, and a ten foot long cheater bar. it takes a lot to get it on there tight enough

 

Agreed. Get yourself a cheater bar, a new flywheel nut, some loctite and make sure you hit that 300.

 

Examine the two engines carefully. Are the flywheels the same? Is there approximately the same amount of threads on both? Pictures might help.

 

If the nut is stopping before it bottoms out on the flywheel, STOP. There must be something wrong with the threads and using an impact probably just caused further damage. Pull off the flywheel and carefully examine the threads under bright light to check for issues. If there is damage, you may be able to fix it with a small file.

Also examine the threads in the nut for the same issues.

 

Check the key-way as well. Make sure the key is not sticking up somehow getting in the way.