I thought I had saved one of those write ups but I guess not.
Anyway, you bore out the bottom bushing to take an "F" Series needle bearing. A search of these archives or simply putting a search for Lawn Boy Bottom Bearing Replacement in your search engine, Google or whatever, should turn up a thread on it.

Walt Conner

As Walt said, but don't run the mower again until after you replace it. As the crankshaft slips lower, it puts the connecting rod in a bind, which will ruin your day. It's also possible for the crank to eat it's way into the block there.

Lawn Boy "D" crankcase repair.

Sometime back there was a post on the RCM newsgroup about repairing the Lawn Boy
"D" engine block when the lower bushing wears out. When this bushing goes, the
bottom seal fails and the gas tends to leak out of the crankcase instead of being
forced in to the combustion chamber, (this tends to make the mower somewhat "wimpy").

Lawnboy1.JPG shows 2 "D" engine crankcases after boring for roller bearings, the
case at the right is bored for a Torrington BH1412 bearing (which is the "F" engine
replacement), the case at the left is bored to accept a smaller OD Torrington roller
bearing (that's what I was given).

Lawnboy2.JPG shows what a typical setup might look like in a horizontal spindle
machine, (although the crankcase will fit quite well in a vise or on parallels in a
vertical mill).

The fix involves removing the lower seal and boring the crankcase to accept a roller
bearing like the "F" engine has. The bushing in the "D" crankcase is cast into the
case during the die-casting process and is longer than the replacement bearing. The
existing bushing must either be bored .015" oversize, or completely bored out because
the process of counterboring may loosen the bushing and allow it to bind in the crank
bearing area causing the motor to seize.

After boring the bushing oversize, the case is counterbored to accept a roller
bearing, (as there are several bearings out there with a 7/8" ID I won't give
dimensions here). After machining, the bearing and seal are pressed in and you will
have a good crankcase again, (I asked the repair shop if they had ever seen the top
bushing go bad and the answer was "never").

Again the bearing for the "F" block is a Torrington BH1412, the seal is a CR 8624,
type CRW1 with an "R" lip code.

The pictures mentioned are not included in this post.

Here are the links to this article with pictures as referenced by tinkerer

http://www.metalworking.com/Dropbox/_1998_retired_files/Lawnboy.txt
http://www.metalworking.com/Dropbox/_1998_retired_files/Lawnboy1.jpg
http://www.metalworking.com/Dropbox/_1998_retired_files/Lawnboy2.jpg

From this site http://www.metalworking.com/Dropbox/_1998_retired_files/

You can get to the article by clicking below, then searching for lawnboy

Here is a link that might be useful: Source

lawnboy lower bushing repair

There are at least two ways of doing this. One is to just bore to a shoulder from the bottom (blade end) and press the bearing in place (allow at least .010 gap to shoulder). Most of the old bronze bushing will remain in place, but the remaining part must be bored about1/64 oversize so it will no longer contact the crankshaft. By locating the needle bearing here it keeps it clear of (just below) the crankshaft keyway. This is the method Lawn Boy used on the last D model they made in 1982 the 5006. The bearing they used was their 679277 (trade number B-148)
A second way and the one I decided on is to bore a straight through bore completely removing the old bronze bushing and then counter bore a shoulder in the floor of the crankcase for a new bronze thrust bearing that is pressed into the same bore and pinned at the flange to insure it never turns. This thrust bearing is wider and gives full contact with the face of the crankshaft. In the lower part of the bore, at the same place Lawn Boy located their bearing I pressed in a Lawn Boy 678443 (trade number BH-1412)
One thing to note with installing a needle bearing in a D is the fact that the D crankshaft is sized (both ends ground) for the full compliment needle bearing making it a slightly loose fit compared with the F engine which used a caged needle bearing that can and is fitted with very little play. The full compliment needle bearing requires this fit to function properly. One caveat is that you don’t want the new needle bearing running against a worn grooved crankshaft which will defeat the whole purpose, so unless your crankshaft is in excellent condition you may need to find another one.

I’m always amazed how long gas mowers last, 1974? I bet it’s still running, wouldn’t pass an emissions test but... my 1st experience was mowing my neighbours lawn with an old Toro. There were problems with it but he had 2 of them and he would just swap them out and get them repaired.

When I co-owned a landscaping company we would by Toro’s used, tune them up and then abuse them. The engine was still good so they were always worth fixing and maintaining.

I use a Honda engine now but those Briggs & Stratton engines would last a long time. A bit loud but very reliable