• Enter the March CB750 Supply gift certificate giveaway! It's easy... Click here, post something, and you're entered into the drawing!

#3 rod destroyed 82' CB750F Supersport

Wookie

CB750 Member
Messages
6
Reaction score
0
Points
1
Location
Angels Camp, CA
New to forum and 1st post. Bought a project bike and got burned. Found the #3 piston not moving while turning it over by hand, so I began to tear down the motor. Found what was left of the rod and the piston skirt in the oil pan. After spitting the cases I found gouging from the rod on the inside of the lower case. The #3 rod journal is marked up (you can physically feel it) and you can see the heat discoloration in the crank around the #3 rod journal from what would appear to be it getting very hot. I found the rod cap with the bolts and nuts intact, so not sure if the rod broke apart somewhere else or if it spun a bearing, got hot, then came apart. You can see the damage in the photos. Rest of the crankshaft looks good, although I still need to make sure it is in spec. I have since bought two 82 750's supersports as parts bikes, but they are all apart. One of the bikes was involved in a collision and had bent forks and looks like some case damage on the clutch housing and alternator area. I was told that one of the motors was running. Not sure of the condition of the other donor motor. Will be checking on those two motors shortly. Just trying to wrap my head around salvaging a running motor out of everything I have.

Question:

#1 Are these cases destined for the trash pile? I am pretty sure the cases are toast for obvious reasons. Although there was no physical hole in the case, there are what appear to be stress cracks on the exterior of the case right were the gouging is on the opposite side. My gut is telling me finished

#2 Is the crank destroyed?

20190116_173815.jpg20190116_173835.jpg20190116_173853.jpg20190116_173922.jpg20190116_173947.jpg20190116_174008.jpg20190116_174025.jpg20190116_174032.jpg

I was trying to get two running motors, but I think I'll be lucky to get one out of this mess. My goal is a cafe racer style bike. I wanted to hopefully go with a 836 kit, for a fun street machine not a racer. Just was looking for some direction as I begin to peace things together. Thanks in advance for your help.
 

Attachments

  • 20190116_173912.jpg
    20190116_173912.jpg
    231.4 KB · Views: 165
Take apart the other motors and see what you got. The crank is junk, it will cost more to repair then what you could buy a used one for off of ebay but you have other engine so no real point in that. Same with the case, if you have a good set then throw this in the scrap metal. Not many parts available for these dohc anymore so not many rebuild them, if something lets go just scrap it and buy another one. The 836 is a sohc bore size...not the same as this motor, think they are a 823 for these. But you will have a hard time finding chain tensioner parts for these motors as honda no longer makes them. You need to do some serious looking at what parts are going to be needed for your build and if what you need is even available.
 
Thanks dirtdigger. I was hoping you'd chime in. That's kinda what I was thinking. I'll see what I have in the other motors. Your right on the performance kit. Although Dynoman makes a performance kit for the 79'-82' 750F's with Wiseco 823 pistons or DP835 pistons. They make them in 3 stages, increasing in performance, parts and price. I'm guessing either the Stage 1 or 2. Stage 3 is overkill for me. What do you think of those kits? Will the stock rods and rod bolts be OK with either of those kits? This is going to be a street machine not a track bike. Oh and another question, are the stock rod bolts reusable or are they the torque to yield kind which means I need new ones. Connecting rods are very difficult to find for the DOHC bikes.
 
Stock rods are fine. They are not torque to yield but is always a good practice to replace rod bolts. Unless you have kept track of the stretch from new you have no idea how close they are to breaking. Aftermarket rods can be ordered but they are expensive and not necessary for street motor. I would contact Buzz at Dynoman or Ken at Cyclex and ask for some rod bolts. Cyclex also has pistons and some other parts for the dohc motor and they do machine work in house. Keep in mind the stock carbs are garbage and will be next to impossible to get to run good with some increased performance. I would put most of my money into a good set of Keihin cr or mikuni rs carbs.
 
Back
Top