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Will 1978 750k engine fit in 1976 750f frame?

gavoic92

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Hey guys, I just purchased a project bike with all the parts completely removed back to bare frame. The original motor on the bike was shot and the guy I bought it from found a nice good condition 78' 750k motor for it but he never got around to putting the bike together. I just want to know if the 1978 750k engine will fit in a 1976 750f frame? Newbie here. Sorry.
 
yes, all sohc 750 motors will interchange. Your only issue may be chain alignment. you may or may not have an issue. You will just have to bolt everything together and check and fix if necessary. Its easily fixed with finding and swapping correct sprockets.
 
yes, all sohc 750 motors will interchange. Your only issue may be chain alignment. you may or may not have an issue. You will just have to bolt everything together and check and fix if necessary. Its easily fixed with finding and swapping correct sprockets.

Thank you, I really appreciate the info. This is my first motorcycle project so I'm still new to this haha but I appreciate your response.
 
The 77/78 CB750's have an output shaft (front sprocket) that extends out an additional 10mm (3/8"). The 77/78 swingarm is 10mm wider to accommodate the 10mm thicker rear sprocket carrier. The left rear upper shock mount is also extended an additional 10mm outboard on the 77/78 models. You need to use the front sprocket cover from the 78' engine since it also has this additional 10mm of clearance. It's up to you as to how to address the additional 10mm of outboard alignment required at the rear wheel.
 
The 77/78 CB750's have an output shaft (front sprocket) that extends out an additional 10mm (3/8"). The 77/78 swingarm is 10mm wider to accommodate the 10mm thicker rear sprocket carrier. The left rear upper shock mount is also extended an additional 10mm outboard on the 77/78 models. You need to use the front sprocket cover from the 78' engine since it also has this additional 10mm of clearance. It's up to you as to how to address the additional 10mm of outboard alignment required at the rear wheel.


sorry I am hijacking this thread, I had the same question, I am putting a 74 motor in a 78 frame. I need to run a 530 chain as the 74 front sprocket only comes in 530 size and my 78 rear wheel is 630 size. What sprocket can I use to get the 78 rear wheel to accept 530 chain AND LINE UP with 74 motor!?

I emailed cycle x and they haven't got back to me.
 
sorry I am hijacking this thread, I had the same question, I am putting a 74 motor in a 78 frame. I need to run a 530 chain as the 74 front sprocket only comes in 530 size and my 78 rear wheel is 630 size. What sprocket can I use to get the 78 rear wheel to accept 530 chain AND LINE UP with 74 motor!?

I emailed cycle x and they haven't got back to me.


All you need is the sprocket carrier from the rear hub of a 76' or earlier K model. You may have to use a custom length spacer (10mm longer) on the outside of the sprocket carrier if you use the 78 swingarm, or use the earlier swingarm with the earlier sprocket hub and earlier spacers.
 
No tools required to remove the sprocket carrier. They can be a little tight, but once the wheel is off, just lay it brake side down and the sprocket carrier pulls off, work it back and forth and lift it off. Hint: put the sprocket back on for better leverage. If you use an offset front sprocket be sure to use the front sprocket cover from the 78' engine. The K7/K8/F2 were different to accommodate the extra 10mm outboard length of the output shaft:

I have attached some picts of both an early sprocket cover and a late sprocket cover. The most notable differences are at the top rear at both the opening for the chain itself as well as the depth of the upper mounting screw boss. You will also notice the early cover has what appears to be an extra "guide" post cast on the inside of the cover. I have never seen this extra "guide post" on a later model, but there may be early models that might not have it, so don't use that as the only identifier... be sure to measure too! Notice how the one screw boss recess is cutout from the outside face on the later model.

Early cover dimensions:

xCrW36s7JIBp2-6uZvLIsYSSMo1blVAwWStAg=w816-h612-no.jpg



z9PNf8JSlUVeHx2xAOE1lt_tGwGZSsJR7ey9g=w816-h612-no.jpg



And the later cover:

DfhQqSRXKW_PWtG_Kx3kq89u7KQL1zR0hOkN4=w816-h612-no.jpg



saMUvxJAmchqYxUJhNsFVldCa1Heg7RjVm5pI=w816-h612-no.jpg



Hope that helps!
 
No tools required to remove the sprocket carrier. They can be a little tight, but once the wheel is off, just lay it brake side down and the sprocket carrier pulls off, work it back and forth and lift it off. Hint: put the sprocket back on for better leverage. If you use an offset front sprocket be sure to use the front sprocket cover from the 78' engine. The K7/K8/F2 were different to accommodate the extra 10mm outboard length of the output shaft:

I have attached some picts of both an early sprocket cover and a late sprocket cover. The most notable differences are at the top rear at both the opening for the chain itself as well as the depth of the upper mounting screw boss. You will also notice the early cover has what appears to be an extra "guide" post cast on the inside of the cover. I have never seen this extra "guide post" on a later model, but there may be early models that might not have it, so don't use that as the only identifier... be sure to measure too! Notice how the one screw boss recess is cutout from the outside face on the later model.

Early cover dimensions:

xCrW36s7JIBp2-6uZvLIsYSSMo1blVAwWStAg=w816-h612-no.jpg



z9PNf8JSlUVeHx2xAOE1lt_tGwGZSsJR7ey9g=w816-h612-no.jpg



And the later cover:

DfhQqSRXKW_PWtG_Kx3kq89u7KQL1zR0hOkN4=w816-h612-no.jpg



saMUvxJAmchqYxUJhNsFVldCa1Heg7RjVm5pI=w816-h612-no.jpg



Hope that helps!


THANKS! I was able to remove it after your instruction, just banged it with a rubber mallet and wiggled it. So what are these 4 holes for anyways? I read another thread where a guy was saying you had to get a pin wrench or make a tool to unscrew this and it is a reverse thread apparently? Just out of curiosity what does that open up??

IMG_2198.jpg
 

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THANKS! I was able to remove it after your instruction, just banged it with a rubber mallet and wiggled it. So what are these 4 holes for anyways? I read another thread where a guy was saying you had to get a pin wrench or make a tool to unscrew this and it is a reverse thread apparently? Just out of curiosity what does that open up??

View attachment 7297


Those four holes do indeed require a "pin wrench", either store bought or homemade. There is a bearing behind that aluminum collar. Look closely and you will see punch marks (or "stakes") around the outer edge that help hold it in place. The stakes have to be drilled out to a depth of about 3/16" before attempting removal of this bearing collar or you will destroy the threads on the collar, if you were able to get it to move at all.
 
great info! I shouldn't need to take out that bearing anyhow, they all seem fine. So the end of my story for now is that I have removed both sprocket carriers, I have a new sprocket as well. Chain alignment should be good however any tips on which swingarm I should use? Not sure if I may also have to run the 74 swingarm instead of the 78 since the chain will be closer to the inside of the frame. Also will there be shock mounting issues since the 78 frame has an extended shock mount post and the 74 swingarm may not line up then. aye aye aye..
 
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