I am replacing the harmonic balancer on my 89 bonneville and was wondering if anyone new the size of the bolt that holds it in place. Any other information on this project would be useful and greatly appreciated.

Not sure of the size but its torqued at 220 ft lbs of torque. You will need a BIG breaker bar or some kinda pipe to break this sucker loose.

I am looking in my FSM for the size and will post when i find it. Gimme a minute.

Gearheads! Where are yah?

They don't put it in the FSM At least not that I can find. I will continue to look though.

If I remember correctly, it took an 18mm socket to take that off. It has been a couple years, so I'm not positive. I'll go check later if no one else knows for sure.
If you take off the right front wheel and splash guard, you will be pretty close to the work. The best way to take off that bolt is with a pneumatic impact. If you don't have access to air tools, it may be best to pay someone else to do it. It shouldn't require a puller. If it is stuck, just wiggle it back and forth or tap it lightly with a rubber mallet and it should pull right off, once the bolt is out. Take care not to disturb the crank position sensor located behind the pulley unless you plan to replace it too.

I want to say 21mm, but its been awhile and cold out to boot. Biggest socket I own.

21mm sounds very right.

If you are going to buy a single socket to do the job, sounds like 21mm is the one to get. Any reputable tool seller will trade you if it isn't the right one and the trade in looks good enough to sell to someone else. You might find an electric impact to do the job. It would be cheaper to find a buddy with air tools like I did. Sometimes friends with air tools have full sets of metric impact sockets also.
I just went out to check mine. After I chipped the ice off the door and around the hood and finally got the hood up with its extra ice weight, the closest I got is a 15/16" socket. I know I went out and bought a single socket specifically to do that job, but I couldn't find it. I did find several old carburetors I forgot I had.
And my biggest socket is a 30mm deep impact that I bought to do the rear axle nuts on my bug.

Another pointer about doing the harmonic balancer swap: It'* a good time to put on a new accessory belt, since you'll have the old one off anyway. And while the belt is off, check the other pulleys for wobble.

I used a 15/16 impact socket, breaker bar and pipe to get it off. You will have to hold the flywheel with a screwdriver of clamp a visegrip on it.

I have seen two different head sizes used. One is 15/16" and the other isn't (think that one is 21 mm). The threads are the same. Important thing is that the 3800s before 1991 ("C" and "3" codes) use a slip fit balancer that slides on and off. "L" (later) engines use a press fit. The balancers do not interchange.

Yes, an impact gun is the best tool to break that sucker loose. Keep in mind that cheap tools with low torque will be less likely of getting the bolt loose. I'd say the minimum is 200 lb ft.

When that bolt is loose you'll need to thread it back in, and leave a nice gap about 1/2 inch or less. Use an impact happer on the bolt head while an assistant and you pry the balancer off. It might be slow or fast depending how tight its in there. when the balancer slides off and touches the bolt, losen the bolt more. Careful not to loosen it too lose to damaging the threads.

Thats the best and only proven method that works for me.