Swapping A Mower Engine – Part 2

As I worked through the various alignment issues I think I finally got it ready to go.  The problem was the shaft on the new engine is not only longer but it isn’t milled down to 1″ all the way to the mower housing.  The pulleys need to line up or the belts will lose power and could be damaged.

According to a friend that is a small gas engine repairman the shafts are cut off all the time if they are too long.  The only problem is if your application gets something screwed into the end.  If you cut it off you might not have enough threads left or the hole could be too shallow.  In my case I’m using pulleys with a key-way and a set screw, so cutting the shaft was no problem.

However I did have a problem with the air supply to the cut-off tool I was using.  My air compressor just couldn’t supply volume to run the tool, so I had to pause cutting and wait for the tank to fill back up with air.  A better solution (other than a much bigger air compressor) is a cut off disk for my 4-1/2″ angle grinder.  These cut of disks are very thin, which is a good thing as that means the cut is narrower so less material is removed which means less heat is generated.  With any kind of metal-cutting heat is always a concern, if you were to just use a grinding disk it would cut a much wider slot and generate a lot more heat.  It will also take more power from the tool so if you are at the limits of what it can do you might find your tool stalling out and maybe burning up the motor.  If the piece you are working gets too how it can ruin the heat treatment.

Next step testing it out.

Swapping A Mower Engine – Part 1

I have a DR Field & Brush Mower from the early 1980’s.  This is an amazing tool for clearing land and property maintenance.  However I’ve been having problems with the engine for the last two years and finally it seized up this month.  I considered trying to fix the engine, but after 35 years of use I’m not sure how worn out this engine might be.  A replacement Briggs was something over $500, however the 8HP Harbor Freight Predator brand was a bit less than $250 so I decided to give that a try.

The first step is removing the old engine and making sure the replacement is as close as possible.  In this case the old engine was an 8HP horizontal shaft engine with a 1″ keyed shaft.  I needed the pulleys from the output shaft and if I couldn’t get them off I was going to take a saw to the shaft and then drive it out.  However the puller worked easily.

You do have to be careful with the 3 jaw pullers and they can exert a tremendous force on the parts involved.  If you get to the point where you are really putting some force on the screw you have to start considering which part you might have to sacrifice.  If you need to save the shaft you can cut the pulleys of if the engine is bad anyway cut the shaft.

Another trick is to heat the parts with a torch.  if most of the heat is directed on one of the two parts stuck together you will get expansion that could be enough to break the connection between the two.  You will not be able to see it with your eyes, but the differential rate of expansion is there and you only need a small amount of movement to free up a joint or connection that is stuck.

Next mounting the new engine.