Author Topic: CB750 Engine Case Interchangeable  (Read 1476 times)

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Offline JimmyJ

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CB750 Engine Case Interchangeable
« on: November 17, 2016, 07:58:27 am »
Hey all - I was able to find quite a few threads on interchangeability of SOHC 750 parts themselves between 69-78 however I wasn't able to find anything on the engine cases.  In the chart that identifies the changes among the years, there is a note that a new case was introduced in 76 for the following: CB750E-2434657 to CB750E-2439607

So with that all being said, I was curious if the engine cases are in fact interchangeable between 1974 internals and a 1978 case?  I currently have a cracked 74 CB750 motor with great internals and a 1978 cb750 case in great condition; so before spending the efforts I figured I would get confirmation before really diving into swapping the parts over.

It doesn't appear that the changes would have been significant but I just want to be sure I am not overlooking something.

As always, thanks in advance.

Offline chewbacca5000

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Re: CB750 Engine Case Interchangeable
« Reply #1 on: November 17, 2016, 08:18:44 am »
That will work.  just keep all the trans components the same.  The single row final drive will produce a little more power, but plugging the chain oiler is a bit of a pain.

If you have the 78 crank, and bearings that makes things a bit easier.  You are going to have to play around with the crank bearings a bit to make sure they are not too tight or loose.

Best to tighten each of the crank fasteners a little a time then turn the crank on the rotor to make sure nothing is binding up.  If just one bolt is binding you can loosen it a bit.  So long as you have 12ft on that bolt you will be ok.

Offline JimmyJ

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Re: CB750 Engine Case Interchangeable
« Reply #2 on: November 17, 2016, 10:25:52 am »
Perfect - thanks for the quick reply and additional info.  I do have the crank/bearings from the 78 however based on the limited view through the connecting rod holes, I am not expecting much of the 78 internals to be in great shape which is why I was leaning towards swapping the 74 internals in. In any regards, this is great info and will certainly help :)

Offline calj737

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Re: CB750 Engine Case Interchangeable
« Reply #3 on: November 17, 2016, 02:11:43 pm »
If you are swapping cranks, plastigauge the bearings, or at least compare the case markings from both sets.
'74 550 Build http://forums.sohc4.net/index.php?topic=126401.0
'73 500 Build http://forums.sohc4.net/index.php?topic=132935.0

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Offline scottly

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Re: CB750 Engine Case Interchangeable
« Reply #4 on: November 17, 2016, 05:53:42 pm »
The '78 case and it's counter-shaft mount the drive sprocket 10mm further out than the '74, so the chain alignment will be off. There are ways around this, such as an off-set rear sprocket.
It may be easier to seal the crack than to swap cases. ;)
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Offline 754

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Re: CB750 Engine Case Interchangeable
« Reply #5 on: November 17, 2016, 05:57:01 pm »
Pretty sure the shorter shaft fits in the late cases..make sure final drive shafts and gears match.
Maker of the WELDLESS 750 Frame Kit
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Kelowna B.C.       Canada

My next bike will be a ..ANFOB.....

It's All part of the ADVENTURE...

73 836cc.. Green, had it for 3 decades!!
Lost quite a few CB 750's along the way

Offline scottly

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Re: CB750 Engine Case Interchangeable
« Reply #6 on: November 17, 2016, 06:18:17 pm »
The case itself and the seal stick further out than the early case, so I don't know if the shorter shaft will work?
Don't fix it if it ain't broke!
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Offline madmtnmotors

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Re: CB750 Engine Case Interchangeable
« Reply #7 on: November 17, 2016, 06:20:25 pm »
I thought the later cases used a wider bearing at the output shaft as well?
TAMTF...


Wilbur



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Offline 754

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Re: CB750 Engine Case Interchangeable
« Reply #8 on: November 17, 2016, 06:39:16 pm »
73 and later , wide bearing i believe.
Years back  I put a 77K in a 72 frame..
After a while I noticed the sprocket shining one side only, and opposite on other end. So I put a straightedge on it and holey #*@+)$.  So since that point around 81 , I knew.
 My buddy had gotten a nice black F motor, and it was on display... Wanted to maybe use it someday..  When he found out about the shaft being longer, he was upset..I told him it was longer.
 He worried about it so much, he ordered a new short shaft, and split the cases and installed it.. The seal say sunken in, but I think it all works. He was not willng to grind seal boss down..
Maker of the WELDLESS 750 Frame Kit
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Kelowna B.C.       Canada

My next bike will be a ..ANFOB.....

It's All part of the ADVENTURE...

73 836cc.. Green, had it for 3 decades!!
Lost quite a few CB 750's along the way

Offline scottly

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Re: CB750 Engine Case Interchangeable
« Reply #9 on: November 17, 2016, 06:49:25 pm »
It still may be easier to seal the crack. ;)
Don't fix it if it ain't broke!
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Offline madmtnmotors

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Re: CB750 Engine Case Interchangeable
« Reply #10 on: November 17, 2016, 07:08:45 pm »
74 CB750K shows a full bearing retaining ring on the fiche while the 78 CB750K shows a half moon retaining ring.

74: http://www.manassashonda.com/oempartfinder.htm?aribrand=HOM&arian=MOTORCYCLE#/Honda/CB750K4A_MOTORCYCLE%2c_JPN%2c_VIN%23_CB750-2300001/FINAL_DRIVEN_SHAFT_%2b_DRIVE_SPROCKET/CB750K4-JPN-A/2Y143413AE14341321A

78: http://www.manassashonda.com/oempartfinder.htm?aribrand=HOM&arian=MOTORCYCLE#/Honda/CB750KA_(78)_750_FOUR_K%2c_JPN%2c_VIN%23_CB750-2800001/TRANSMISSION/CB750K-78-JPN-A/2Y134057AE13405717A

The bearings have a different part number as well. I'm pretty sure I've seen a thread where it has been done, I just don't recall the particulars.

Jimmy is wanting to swap out all of the internals. What you describe Frank is just swapping out the output shaft only. I would have to do some careful measuring before I could say one way or the other. Like I said, I think I've read about this scenario, I just don't remember exactly what was done. Maybe swap out all of the internals with the exception of the output shaft bearing?
TAMTF...


Wilbur



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Clean up that nasty harness: http://forums.sohc4.net/index.php?topic=137351.msg1549191#msg1549191
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Charging system diagnosis: http://forums.sohc4.net/index.php?topic=1012.msg8345#msg8345
Get the manuals: http://manuals.sohc4.net/cb750k/
The Dragon: http://forums.sohc4.net/index.php?topic=45183.msg1571675#msg1571675
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Engine Lifting Made Easy: http://forums.sohc4.net/index.php/topic,58210.msg1684742.html#msg1684742
                                      http://forums.sohc4.net/index.php/topic,100352.msg1675840.html#msg1675840
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Airbox Gasket Replacement: http://forums.sohc4.net/index.php/topic,114485.msg1290000.html#msg1290000
"Café" : http://forums.sohc4.net/index.php/topic,84697.msg953814.html#msg953814
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Offline 754

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Re: CB750 Engine Case Interchangeable
« Reply #11 on: November 17, 2016, 08:44:54 pm »
Your last sentence is what I meant , but you can use the shaft, one of the bearings, but i think the gear has to come off and switched to 78 style.
Maker of the WELDLESS 750 Frame Kit
dodogas99@gmail.com
Kelowna B.C.       Canada

My next bike will be a ..ANFOB.....

It's All part of the ADVENTURE...

73 836cc.. Green, had it for 3 decades!!
Lost quite a few CB 750's along the way

Offline Swoop

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Re: CB750 Engine Case Interchangeable
« Reply #12 on: November 18, 2016, 03:03:10 am »
Why not leave the 78 long output shaft in and just change the sprocket carrier?

Offline 754

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Re: CB750 Engine Case Interchangeable
« Reply #13 on: November 18, 2016, 08:28:12 am »
That is what I did back in the day, but
 Think the left side shock will be angled out a bit (can be shimmed)
Stock chainguard probably wont fit.

To be honest, I was surprised my buddy changed shafts, on a low miles spare motor sitting on the bench, i think he even sprung for OEM shaft.
Maker of the WELDLESS 750 Frame Kit
dodogas99@gmail.com
Kelowna B.C.       Canada

My next bike will be a ..ANFOB.....

It's All part of the ADVENTURE...

73 836cc.. Green, had it for 3 decades!!
Lost quite a few CB 750's along the way

Offline Swoop

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Re: CB750 Engine Case Interchangeable
« Reply #14 on: November 18, 2016, 08:59:52 am »
That is what I did back in the day, but
 Think the left side shock will be angled out a bit (can be shimmed)
Stock chainguard probably wont fit.
Pretty sure the angle issue with the rear shock is if you use a 78 swingarm. Agree with the chainguard.......chain comes closer to the left shock, but doesnt touch on mine. May depend on your shocks.

Offline madmtnmotors

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Re: CB750 Engine Case Interchangeable
« Reply #15 on: November 18, 2016, 09:01:42 am »
Pretty sure the angle issue with the rear shock is if you use a 78 swingarm. Agree with the chainguard.......chain comes closer to the left shock, but doesnt touch on mine. May depend on your shocks.

Correct.
TAMTF...


Wilbur



My most recent project "Evolution": http://forums.sohc4.net/index.php?topic=100352.0
F2/F3 O-rings: http://forums.sohc4.net/index.php?topic=113672.msg1300721#msg1300721
Cam Tower Studs: https://www.mcmaster.com/#93210a017/=t19sgp
Clean up that nasty harness: http://forums.sohc4.net/index.php?topic=137351.msg1549191#msg1549191
                                           http://forums.sohc4.net/index.php/topic,148188.msg1688494.html#msg1688494
                                           http://forums.sohc4.net/index.php/topic,139544.msg1579364.html#msg1579364
                                           http://oldmanhonda.com/MC/wiring750K1.html
Charging system diagnosis: http://forums.sohc4.net/index.php?topic=1012.msg8345#msg8345
Get the manuals: http://manuals.sohc4.net/cb750k/
The Dragon: http://forums.sohc4.net/index.php?topic=45183.msg1571675#msg1571675
Headlight Switch: http://forums.sohc4.net/index.php?topic=113986.msg1283236#msg1283236
Branden's leak free top end thread: http://forums.sohc4.net/index.php?topic=107040.0
Engine Lifting Made Easy: http://forums.sohc4.net/index.php/topic,58210.msg1684742.html#msg1684742
                                      http://forums.sohc4.net/index.php/topic,100352.msg1675840.html#msg1675840
Static and Dynamic Timing: http://www.hondachopper.com/garage/carb_info/timing/timing1.html
Airbox Gasket Replacement: http://forums.sohc4.net/index.php/topic,114485.msg1290000.html#msg1290000
"Café" : http://forums.sohc4.net/index.php/topic,84697.msg953814.html#msg953814
PD Carb Choke Linkage: http://forums.sohc4.net/index.php/topic,100352.msg1669248.html#msg1669248
                                    http://forums.sohc4.net/index.php/topic,110931.msg1248354.html#msg1248354
Follow up on your damn posts: http://forums.sohc4.net/index.php/topic,144305.msg1791605.html#msg1791605
Taiwanese Cam Chain Tensioners:  http://forums.sohc4.net/index.php/topic,155043.msg1774841.html#msg1774841
Gumtwo Seat Cover: http://forums.sohc4.net/index.php/topic,164440.msg1897366.html#msg1897366
Primary Drive: http://forums.sohc4.net/index.php/topic,166063.msg1919278.html#msg1919278
Tank Latch: http://forums.sohc4.net/index.php/topic,165975.msg1919495.html#msg1919495
Shorten your forks: http://vintage-and-classic-honda-s.456789.n3.nabble.com/How-to-shorten-forks-td4042465.html DO NOT CUT THE SPRINGS!
Clutch How To: http://vintage-and-classic-honda-s.456789.n3.nabble.com/How-to-change-and-adjust-a-clutch-SOHC-td4040391.html
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Offline kmb69

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Re: CB750 Engine Case Interchangeable
« Reply #16 on: November 18, 2016, 09:05:07 am »
Pretty sure the angle issue with the rear shock is if you use a 78 swingarm. Agree with the chainguard.......chain comes closer to the left shock, but doesnt touch on mine. May depend on your shocks.

Correct.

Maybe ya'll can get Spanky to post pictures of the top shock extension kit I made for him. Works very nice IMHO.

Offline madmtnmotors

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Re: CB750 Engine Case Interchangeable
« Reply #17 on: November 18, 2016, 09:43:39 am »
Pretty sure the angle issue with the rear shock is if you use a 78 swingarm. Agree with the chainguard.......chain comes closer to the left shock, but doesnt touch on mine. May depend on your shocks.

Correct.

Maybe ya'll can get Spanky to post pictures of the top shock extension kit I made for him. Works very nice IMHO.



Yes, indeed it does. That requires swapping out the swingarm, the rear sprocket carrier, the chain guard, and the wheel spacers, provided the longer output shaft from the 78' motor is used. I get the impression that Jimmy is just looking for a way to swap the 74' engine internals into a 78 engine case with minimal fuss (i.e. additional parts and $$$) and bolting it back into the 74' frame.

If the shorter output shaft will work with the 78' output shaft bearing, then that's probably the least painful, provided there are no other deviations between the engine case internals...

Depending on where you are located Jimmy, maybe someone in your area can swap a 74' case for your 78'?
TAMTF...


Wilbur



My most recent project "Evolution": http://forums.sohc4.net/index.php?topic=100352.0
F2/F3 O-rings: http://forums.sohc4.net/index.php?topic=113672.msg1300721#msg1300721
Cam Tower Studs: https://www.mcmaster.com/#93210a017/=t19sgp
Clean up that nasty harness: http://forums.sohc4.net/index.php?topic=137351.msg1549191#msg1549191
                                           http://forums.sohc4.net/index.php/topic,148188.msg1688494.html#msg1688494
                                           http://forums.sohc4.net/index.php/topic,139544.msg1579364.html#msg1579364
                                           http://oldmanhonda.com/MC/wiring750K1.html
Charging system diagnosis: http://forums.sohc4.net/index.php?topic=1012.msg8345#msg8345
Get the manuals: http://manuals.sohc4.net/cb750k/
The Dragon: http://forums.sohc4.net/index.php?topic=45183.msg1571675#msg1571675
Headlight Switch: http://forums.sohc4.net/index.php?topic=113986.msg1283236#msg1283236
Branden's leak free top end thread: http://forums.sohc4.net/index.php?topic=107040.0
Engine Lifting Made Easy: http://forums.sohc4.net/index.php/topic,58210.msg1684742.html#msg1684742
                                      http://forums.sohc4.net/index.php/topic,100352.msg1675840.html#msg1675840
Static and Dynamic Timing: http://www.hondachopper.com/garage/carb_info/timing/timing1.html
Airbox Gasket Replacement: http://forums.sohc4.net/index.php/topic,114485.msg1290000.html#msg1290000
"Café" : http://forums.sohc4.net/index.php/topic,84697.msg953814.html#msg953814
PD Carb Choke Linkage: http://forums.sohc4.net/index.php/topic,100352.msg1669248.html#msg1669248
                                    http://forums.sohc4.net/index.php/topic,110931.msg1248354.html#msg1248354
Follow up on your damn posts: http://forums.sohc4.net/index.php/topic,144305.msg1791605.html#msg1791605
Taiwanese Cam Chain Tensioners:  http://forums.sohc4.net/index.php/topic,155043.msg1774841.html#msg1774841
Gumtwo Seat Cover: http://forums.sohc4.net/index.php/topic,164440.msg1897366.html#msg1897366
Primary Drive: http://forums.sohc4.net/index.php/topic,166063.msg1919278.html#msg1919278
Tank Latch: http://forums.sohc4.net/index.php/topic,165975.msg1919495.html#msg1919495
Shorten your forks: http://vintage-and-classic-honda-s.456789.n3.nabble.com/How-to-shorten-forks-td4042465.html DO NOT CUT THE SPRINGS!
Clutch How To: http://vintage-and-classic-honda-s.456789.n3.nabble.com/How-to-change-and-adjust-a-clutch-SOHC-td4040391.html
Sent from my Tandy TRS-80!

Offline JimmyJ

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Re: CB750 Engine Case Interchangeable
« Reply #18 on: November 18, 2016, 12:34:41 pm »
Pretty sure the angle issue with the rear shock is if you use a 78 swingarm. Agree with the chainguard.......chain comes closer to the left shock, but doesnt touch on mine. May depend on your shocks.

Correct.

Maybe ya'll can get Spanky to post pictures of the top shock extension kit I made for him. Works very nice IMHO.



Yes, indeed it does. That requires swapping out the swingarm, the rear sprocket carrier, the chain guard, and the wheel spacers, provided the longer output shaft from the 78' motor is used. I get the impression that Jimmy is just looking for a way to swap the 74' engine internals into a 78 engine case with minimal fuss (i.e. additional parts and $$$) and bolting it back into the 74' frame.

If the shorter output shaft will work with the 78' output shaft bearing, then that's probably the least painful, provided there are no other deviations between the engine case internals...

Depending on where you are located Jimmy, maybe someone in your area can swap a 74' case for your 78'?

Wow this thread certainly took off - but madmtnmotors nailed it on the head.  Essentially I'm looking to eliminate as much fuss as possible with what I have on hand (if something pops up locally (NY) I would certainly entertain the thought).  Unfortunately the 74 case at some point was welded (or I should say an attempt at welding was made) by a p.o.   The result? A mound of material, a complete eye sore, and something that still leaks which is why I wanted to try and start fresh.  With that, I had hoped that the swap would be painless but from the sounds of it based on everyone's contributions, it sounds that I may have a couple different options to toy with and see where everything lines up.

The other changes with the motors from the research I have done seem to be negligible.  Some shifting improvements and other tweaks but nothing that should introduce major implications.

Offline scottly

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Re: CB750 Engine Case Interchangeable
« Reply #19 on: November 18, 2016, 06:27:07 pm »
Don't discount the option of merely sealing the leak, especially if the motor is currently assembled and running. ;) My motor had a leaky welded repair from chain damage, and I fixed it with a piece of aluminum glued on with RTV. Please post some pics of the area in question..
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Offline JimmyJ

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CB750 Engine Case Interchangeable
« Reply #20 on: October 23, 2017, 07:50:43 am »
Nothing like digging up an old thread almost a year later... I finally got to tearing into the motor. As of right now, it looks like the 74 final drive put into the 78 case won’t work as is. I’ll have to try swapping the bearings as others have mentioned and see how that works out. As others mentioned, it seems that the 78 case is built out further than the earlier cases and may not allow for the chain to run without rubbing the side. In any case (no pun intended), I’ve included some shots of the fantastic fix someone had done on the 74 case previously....










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« Last Edit: October 23, 2017, 07:53:30 am by JimmyJ »

Offline BradM

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Re: CB750 Engine Case Interchangeable
« Reply #21 on: January 27, 2019, 12:25:56 pm »
I'm wondering if I would have the same issues trying to use 74 cases on my 72. I read through this whole thread and didn't quite get my answer. If anyone knows, please let me know.

Offline ekpent

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Re: CB750 Engine Case Interchangeable
« Reply #22 on: January 27, 2019, 01:33:17 pm »
I'm wondering if I would have the same issues trying to use 74 cases on my 72. I read through this whole thread and didn't quite get my answer. If anyone knows, please let me know.
  That would be a good straightforward swap.

Offline HondaMan

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Re: CB750 Engine Case Interchangeable
« Reply #23 on: January 27, 2019, 06:52:08 pm »
I'm wondering if I would have the same issues trying to use 74 cases on my 72. I read through this whole thread and didn't quite get my answer. If anyone knows, please let me know.

There are 'early' and 'late' K2 engines. The 'early' ones (mostly before 3/72 production) had a single-row output shaft bearing and the bearing seat in the case is narrower than in the K4 cases. For a brief while in the late K2 and Old Factory K3 cases, this bearing seat was made a little wider than in the K4 and 2 grooves for the 2 different types of final-drive bearing retainers were cut into the cases, presumably so the production techs could install either one that day, depending on which parts they had on hand.

If you have the late K2 with the wide 2-row output bearing, it will drop right into the K4 cases. If not, you will have to obtain either the K4 final drive shaft or the late K2/K3 output shafts that resembled the K4 version, but had a spacer installed on it when the case had the narrow bearing seat. Some have gone so far as to take the K4 shaft and make their own spacer, when the case had the narrow bearing.

If you have my book, take a look at the final-drive section(s) and you'll see the differences.
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Offline BradM

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Re: CB750 Engine Case Interchangeable
« Reply #24 on: January 30, 2019, 02:45:08 am »
Would you be able to tell by these pictures which one I have? I don’t have your book but I’m ordering it now.

 

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