750c and 900c Project

B

bigiron59

CB750 Member
Messages
15
Reaction score
1
Points
3
Location
United States
Hey so I have an 81’ 750 Custom and long story short, my engine (at least the top end) is out of commission (deal gone wrong). The frame was kept stock besides a “hoop” done by a tech that worked at a Harley dealership near me and a lot of the hardware for shock mounts and wheels seems to be in a lot rougher of a shape than I was told. A guy I know is getting ready to part ways with his 900 Custom that has been rebuilt and is mostly stock. I was wondering if I should just get a 900 and (if possible) swap over some of the parts. I know that as long as you have the rubber mounts for the 900 engine then it can go in the 750 frame, but I would like to know I could swap over the shocks, hardware, and other components…or if I even should. Any advice helps.
 
Loose Chain said:
Why would you not just ride the 900?
Click to expand...
The 750 frame I have is powder coated, I have a nice seat for it, handlebars, new shocks, tires, and some other things as is. It was fairly cheap considering all I got for the price so I don’t have to keep it but I prefer the style. Obviously if I got a stock, clean 900 frame I’m not going to chop it, powder coat it, and weld a hoop on it. My plan was to see if I could do a clean swap so I could put most of the parts back on the 900 frame if I ever planned to sell. Some people I’ve talked to said that the 900 is much superior in form of overall technology, but I’m just trying to make use of what I got/like without ruining another bike frame for the sake of aesthetics. If that makes any sense. \_(^~^)_/
 
mrtwowheel said:
Powder coated frames are over rated. Enamel painted frames can be easily touched up with acrylic enamel and a paint brush, long lasting too.
Click to expand...
I bought it powder coated. I would prefer the longer lasting enamel paint but I’m just trying to work with what I have to piece together something moderately tasteful.
 
Loose Chain said:
Actually it doesn't
Click to expand...
Okay, I need a parts bike for 2 things: and engine, and hardware. Haven’t seen a running 750 recently that isn’t 4 grand. I’m not paying 4k for a bike from the 80s. I know a guy selling his 900 that has everything I’m looking for. I’m not going to redo the frame on it because it would be a waste of money if I had another frame with what I want in it. I just want to know if the parts are interchangeable. I don’t want to ride a stock 900. If the parts swap then I have a good condition 900 frame and a running 750 with a slightly bigger engine.
 
Loose Chain said:
Okay?
Click to expand...
So I’m trying to see if the hardware and parts swap. Are the frames different sizes? If so then do I need to worry about certain bolts being too long/short, thread counts, are the rim sizes different, how will it affect overall stability, etc. If I have to worry about a bunch of that stuff then I won’t bother trying to do the swap and I’m just going to have to wait for a reasonable 750. I really don’t see what is so hard to understand here.
 
mrtwowheel said:
I think what we don't understand is......................why not start with the 900 frame and engine etc. and use your best 750 parts on the 900? Title? Touch up the 900 frame without full disassembly? Sounds better to me.
Click to expand...
The 750 frame had the tail chopped, welded a hoop on, and had a custom seat made for it. I don’t know why the previous owner did cosmetic work before getting a new engine, wiring, tank, etc. but I like the style of the current 750 frame. If I bought the 900, to get it closer to what I want to build, I would have to go through the same work done on the 750 which means tearing down the 900 for paint, welding and adding more $$ to a new project when I could make use of my current painted and custom frame/parts. I never planned on looking for a 900, I just wanted to know if I could make use of its rebuilt engine and bolts without having to dig a money pit. I figured it would be better to buy a parts bike in my situation so I don’t break the bank buying 750 parts from a honda dealer, I just want to know if I could buy a 900 that I have a good deal on and successfully do a swap, or pass and just wait for a 750 to come my way. Titles on both the 750 and 900.
 
bigiron59 said:
my engine (at least the top end) is out of commission (deal gone wrong).
Click to expand...
What does this mean?

Is the 750 engine repairable? I'm not trying to discourage you from doing what you want to do, but, going by your questions it seems you may not have the know how to accomplish what you want to attempt. The job could be done but for a novice you might want to start small. Tell us what the problem is with the 750 engine you have.
 
I opened the valve cover to to re-adjust the gasket because whoever took it apart last for maintenance didn’t put it on right. I noticed that when I took the valve cover off, the inside looked like it had been gone through with a wire brush which gave me some concern. I looked around for further damage and noticed some loose bolts on the bearing caps, like loose loose, loose enough to take out with your fingers. There were 5 bolts that pulled right out of the bearing caps with what appeared to be the threads from the block coming out with them. A bad tap, thread inserts, helicoils, I have no clue. The top of the bearing cap where the bolt hole starts is only large enough for a size 8-10 bolt. A local machine shop told me that tapping through a bearing cap to get to the block would be unwise. Thread inserts aren’t a good idea because of the oil running through the system, and helicoils might not be able to handle the heat or would be a short term solution to a long term problem. I figured if I was going to replace the top end, it would be better just to go ahead and find a running engine in good condition. Just a bad deal from the guy I bought from.
 
Loose Chain said:
What does this mean?

Is the 750 engine repairable? I'm not trying to discourage you from doing what you want to do, but, going by your questions it seems you may not have the know how to accomplish what you want to attempt. The job could be done but for a novice you might want to start small. Tell us what the problem is with the 750 engine you have.
Click to expand...
The bottom most bearing cap just to the left of the timing chain has the same issue with the top bolt it’s just not labeled in the picture.
 

Attachments

  • 2F47D6D9-627D-4370-B8BA-694911973D15.jpeg
    2F47D6D9-627D-4370-B8BA-694911973D15.jpeg
    383.2 KB · Views: 39
If the head is still on the engine do a leak down test on the cylinders. If that goes good then I would suggest repairing or replacing the head. A good machine shop should be able to fix it, if I'm understanding what you are trying to say. Heli coils actually work good in aluminum. If done properly a heli coiled repair in aluminum is actually stronger than the original threaded hole in the aluminum. How many miles are on the engine?
 
I wasn’t given a specific number but it was estimated to be between 25-30k. Though that was in his words so I expect it could be more.
 
Loose Chain said:
25-30k for what exactly? When you say "his words" who are you talking about?
Click to expand...
Miles on the engine. The guy I bought the bike from told me that the guy before him (so 2 owners ago) said the bike had around 25-30k miles on the engine but apparently the gauges were swapped some time in the process of switching ownership between the previous couple of guys (before the person I bought it from) so I am assuming it has more miles on it.
 
Last edited:
Loose Chain said:
If the head is still on the engine do a leak down test on the cylinders. If that goes good then I would suggest repairing or replacing the head. A good machine shop should be able to fix it, if I'm understanding what you are trying to say. Heli coils actually work good in aluminum. If done properly a heli coiled repair in aluminum is actually stronger than the original threaded hole in the aluminum. How many miles are on the engine?
Click to expand...
The question still remains the same in my opinion. I’m not trying to hold out on necessary purchases but is it worth going through the process of repairing the head or replacing the head (which would mean buying a parts engine), buying and replacing the gaskets, taking apart a whole engine to clean and get everything within spec, or just buying a whole parts bike with a running motor that has already been rebuilt? The engine is just the heart of it, the wiring harness needs new components and wiring, I need new bolts for most of the stuff to mount, etc. Time in my case is something I don’t necessarily have very much of and I think buying a parts bike with everything ready saves the hassle of reviving an engine which in my case may need a lot of work done to it. What’s a $200-$300 difference (the cb900 would cost around $200-$300 more than what the estimate of rebuilding my engine costs) if I can’t even get the thing running by fall?
 
Last edited:

Similar Threads

blueoctahedron
82 cb750c build plan and questions.
Replies
8
Views
922
jasonscb750build
J
twotone_ra64
'81 CB750C - Can a Feller Get It Running and Riding After 18 Years?
2
Replies
23
Views
2K
mrtwowheel
mrtwowheel
B
erratic idle, carb farts :(
Replies
17
Views
1K
bretgs
B
deeveeance
Almost done with project 1982 CB750C, looking for final elements/advice
Replies
2
Views
688
Walter
W
N
Upcoming project
Replies
3
Views
227
Loose Chain
Loose Chain
Back
Top