Engine Rebuild #8 (Engine Case Mating)

CB750 Engine

Big Milestone! First real milestone on the old CB750.

After several months of purchasing parts, learning my way around the engine, and getting the confidence; I’m rejoining the two halves of the engine case. Putting together the case is the first thing in the rebuild that cannot easily be undone.

There is a short checklist of items to complete prior to connecting the two halves:

  1. Install and apply assembly lube all the crankshaft bearings.
  2. Razor blade the mating surface and clean it one more time with engine or break cleaner and wipe it down.
  3. Install the three dowels (one has an o-ring) in one half of the case. These are parts 30, 4, and 23 in the Parts Book.
  4. Use compressed air to clean out the case bolt threads to remove any debris prior to inserting the bolts and applying the correct torque.
  5. Have the torque order and specifications of the case bolts handy. Torque order can be found on page 52 of the Shop Manual.  I use this website for torque specs.  I used 10 ft/lbs for the 6mm bolts, 18 ft/lbs for the 8mm bolts, and 25 ft/lbs for the 10mm bolts.
  6. Separate the two sets of bolts into those that mount from the lower and upper half. Join all the nuts, bolts, and washers with their pairs.
  7. Have a quick way to flip the engine to install the lower bolts. You’ll probably need an engine stand to do this. I purchased the one from TC Bros (can also be found on Dime City Cycles) and bolted it to my shop table. As an aside, the Shop Manual suggests you place the many of the parts in the upper part of the case and install the bottom onto the top upside down. Hackaweek actually does this the same way. I assume it’s to extend the piston rods outward using gravity to make it easier to get them through the cylinder openings. You can see in Part 2 of the video for this post that I struggle with this. I found a trick that if you start with the front half of the engine you can guide the rods to the cylinder holes without doing the whole upside down thing which creates it’s own set of problems like joining the shifter forks to one of the transmission shafts with Hondabond all over the place and not being able to get a good look at it.
  8. Apply Hondabond (don’t have to use the Hi-Temp version for this particular application) to one of the halves of the case just prior to connecting. I had to use a trick to get it out of the tube. To get it out consistently, I used a pair of needle nose pliers to twist the bottom of the tube like toothpaste. This makes it much easier to consistently lay down the Hondabond on the mating surface.

At this point, you’re ready to connect the two halves, place the bolts in the case and torque them to the correct specification in the correct order. Good luck!

Here are the walkthrough videos.

Video Part 1

Video Part 2

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