CBX750 VS CB750 Seven Fifty. Engines comparing. Part1. Crankcases.

 So let’s begin engine comparing from crankcases.

To avoid misunderstanding:  on photos crankcase painted in black is CBX750 engine case and silver one – from CB750 Seven Fifty.

The first difference which one’s eye catches is front upper mounting points. As you see, Honda CBX750 crankcase has rubber bushing to cease vibration and Seven Fifty mounts are simply drilled. I can’t say how good those rubbers are for vibration but this difference causes a problem with engines swap. When you install CBX750 crankcase into Seven Fifty frame CB750 front upper mounting plates do not fit, nor fit CBX750 ones.  Same question you will have if decide to install Seven Fifty engine into CBX frame, I suppose.

Next difference is less visible at first glance but much more important. I’ve heard about CBX-750 engine with crankshaft dead after oil starvation and this particular one I bought for parts has same problem. Interesting, but fact: crankshaft is out, but cylinder head and camshaft are in good condition and have no traces of oil starvation.

The answer, I suppose, is in oil system. Engine has two oil circuits and so there are two chambers in oil pump. One feeds crankshaft  and camshafts through oil filter, another one works for oil cooling system and gearbox oiling.  In first circuit right after filter oil through filter gallery goes into crankshaft (main) oil gallery, from there it feeds all crankshaft bearings and sprays oil through connecting rods nozzles. But in the same time main oil gallery has a threaded hole to connect pipe trough which generator shaft and cylinder head components are fed with oil. And this hole is positioned right in front of second crankshaft main journal oil hole.

What does it may mean on practice? That’s an open hydraulic system and when it becomes dis-balanced for some reason (like low oil level for example or long run on full throttle) it is possible to get still normally- or over-fed  cylinder head but starving crankshaft in the same time.

So in fact such CBX750 oil system build may be a “bug” and a proof of this is how this issue fixed in Seven Fifty engine.

The Honda engineers decided to change location of head feeding hole, so in Seven Fifty engine connected to oil filter shaft and in addition it has lesser actual diameter. This solution fixes the question of oil distribution between crankshaft and cylinder head.  And in my opinion it is quite significant improvement.

One may say that with modern synthetic oil all in   engine (not in frame tubes partly) and Seven Fifty improved cooling system and oil pump with higher performance of “cooling contour” chamber there will not be a problem at all, but personally I decided not to use CBX750 crankcase in my project but ordered engine case from CB750 Seven Fifty.  Even if this decision is overkill, but I prefer to choose improvement where I can.

In fact that’s all about significant differences between Honda CBX750 and CB750 Seven Fifty engines, but there are also several small features which I may indicate.

Both Seven Fifty cases halves have grooves for gearbox main shaft bearing set ring while CBX750 has groove only on lower half of case.

That’s not significant, but one should use half set ring to install gearbox main shaft into engine case on CBX and  full ring may be used for this purpose on Seven fifty shaft bearing.

CBX750 manual advises to use special crankcase assembly pins to assemble crankcase halves in addition to two pins which position the case halves. And there are special holes for them in front part of crankcase halves.

There is no such feature in Seven  Fifty engine case but there is the third positioning pin in back part of cases:

That’s all differences I found, but anyhow my previous theory about Honda may of used supply of CBX engine cases to start Seven Fifty manufacturing is busted: all points I marked above are quite an evidence of Honda engineers work.

But now about crankcase covers. Clutch covers are obviously different,  but there is no problem with interchange of right and left (ignition) crankshaft covers.  Same with oil pans, they are simply identical:

I don’t know why Honda left oil filler on gear shift cover in Seven Fifty engine, but at least the cover itself has reinforcement all around.  That’s little but improvement of course.

And both, the CBX and CB covers are similar from inside.

So they are interchangeable too.

The next are gearbox covers. Seven Fifty is broken, but this does not prevent comparing. So Cb750 cover unlike to CBX one has reinforcement on lower edge, has no hole for clutch rod…

… whereas it still has place for rod seal on back side. Also oil nozzle in front of cover slightly bigger in Seven Fifty case.

That’s all about crankcases.

You may find additional photos here>>

In next part I’ll tell you about CBX750 and CB750 oil pumps, crankshafts, gearboxes and so  on…

Next part>>

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50 Comments

  1. Gareth

    Great info – can you use CBX 750 top end on seven fifty crankcase then? so swap pistons, barrel, head, cams etc on to seven fifty cases using crank and rods form seven fifty?

    Reply
    1. gazzz (Post author)

      Yes, there is no problem with such replacement, all CBX parts you counted fit Seven Fifty. I use CBX 750 cylinder head and camshafts in my current project, just yesterday I gave it to workshop for surface grinding and valve seats cutting.

      Reply
  2. Wesley

    Hi Gazzz, I recently purchased a CBX750 with a dead crank, also got a Seven Fifty motor to put in it. I noticed that the CB oil pan seems slightly deeper than the CBX one, didn’t measure it but possibly about a liter difference. I only noticed this after spending 2 hours cleaning the wrong one 🙁

    Reply
    1. gazzz (Post author)

      Hi Wesley. I didn’t notice any difference between CBX750 RC17 and CB750 RC42 oil pans, but I’ll check this question tomorrow and I’ll shot some photos.

      Reply
  3. gazzz (Post author)

    I checked the information. Dimensions of CBX750 and CB750 oil pans are same. See photos:

    http://www.gazzz-garage.com/wp-content/uploads/2017/08/IMG_4417.jpg
    http://www.gazzz-garage.com/wp-content/uploads/2017/08/IMG_4424.jpg
    http://www.gazzz-garage.com/wp-content/uploads/2017/08/IMG_4428.jpg

    What may cause misunderstanding is that mounting point on Seven Fifty oil pan are reinforced, therefore have larger heights.

    Reply
  4. Algirdas

    What do you think the main difference regarding power is? Different camshafts?

    Otherwise, excellent job comparing the two!

    Reply
    1. gazzz (Post author)

      Cylinder head make difference. CBX750 cylinder head has valves with larger diameter then Seven Fifty. Both, intake and exhaust. And CBX camshafts also different. later I’ll shoot photos and write article about cylinder heads.

      Reply
      1. Algirdas

        Might be interested in acquiring a CBX head myself then.

        Reply
      2. John wisor

        I have a 2003 nighthawk and would like to
        Use the cbx head and cams. Is it a straight swap? Also what would the side by side performance be like.
        Khrappom@yahoo.com
        Thanks John Wisor

        Reply
        1. gazzz (Post author)

          Hi John,

          Yes, it’s straight swap. Difference in specifications are: 8Nm and 17-18HP. The actual difference between engines that may cause this difference is cylinder head (larger valves) and camshafts (higher lobes). Plus exhaust, I assume.

          Kind regards,
          Nazar

          Reply
  5. Andy

    Could I just ask how difficult you found it to remove the gearbox cover on the CBX motor? The oil seal at the rear of the casting is weeping on mine. I saw a video on Youtube where two Germans were stripping the engine down. They really struggled to get it off. The video stopped and when it restarted the cover was off but they didn’t show how they did it.

    Reply
    1. gazzz (Post author)

      Hi Andy,

      Those cover sits tight on countershaft bearing bracket, but I didn’t recall any kind of difficulties in the process of removing. Just don’t forget unscrew all bolts. You may also use some kind of lever to pull cover off, just use it gentle, to avoid any damage.

      This my article with illustration may be useful for you. http://www.gazzz-garage.com/2017/09/06/seven-fifty-cafe-racer-engine-assembling-part-2/

      There you may find how it all is built.

      Kind regards,
      Nazar

      Reply
  6. Andy

    Hi Nazar, many thanks for that and it is very informative but I was more concerned with when it comes to initially removing the gearbox cover and whether it came off easily or whether you struggled to remove it because the oil seal was tight.

    Reply
  7. Kendy

    Hello, it’s my first time asking question about. you think the(top yoke) triple clamp seven fifty RC 42 will fit on a old cbx 750 p 1991?because i plan to change the all set.so what you suggest. thank for your time .

    Reply
    1. gazzz (Post author)

      P stands for Police (RC18) version as far, as I remember. I have triples for F model (RC17) so I may only guess about RC18 triples. But CB750 and both version of CBX750 use same steering stem bearings, so there is good chance to fit whole front end from RC42 in RC18.

      So the main question is steering head heights. I’ll measure CB750 RC42 steering head heights and post result here thus you’ll may compare it with the steering head height of your bike.

      The last question is compatibility of steering stoppers, but sure, even if they are not compatible, the question may be easily solved by cutting old stoppers from frame and welding new ones.

      Reply
  8. John Dahl

    Hey Gazzz.
    It seems that you know a lot about CB750/CBX750.
    Do you have any idea about the ignition coils, are they similare ?
    When I find them there is a big difference in price.

    Reply
    1. gazzz (Post author)

      Hi John,

      Ignition coils of CB750 and CBX750 are compatible so CB750 coils may be used with CBX750 ignition unit and wiring. However I don’t now if their mountings are same.

      Kind regards
      Nazar

      Reply
  9. Mister_T

    Hello Nazar, greetings from Australia.
    Great job on your website. I have recommended your site to the members of the TeamRC17 Australia Google Group. We are a small band of crazy people that own (or have owned in the past) the CBX750F and related bikes.

    One minor difference Honda made on the CBX750FG and F2G models was to fit a longer oil dipstick to raise the oil level. See http://teamrc17.net/technical/34/basic-maintenance for a pic.
    I guess this was a quick (cheap) fix by Honda. If a RC17 throws a rod out of the case, it is a safe bet that it would be #2 rod.
    If you have a CBX750FE engine and use the original (long) dipstick, you should overfill the oil.
    Also, the FG and F2G used the one piece cylinder head oil feed pipe as used on the RC42.

    I assume racing experience with the FE engine led to the oil system improvements for the FG and later the RC38/RC42 production bikes.
    Have a look at the 1984 Suzuka 8-Hour race results, lots of CBX750F there.

    Reply
  10. Mister_T

    Oops, I meant to write “shorter dipstick to raise the oil level”. Silly me.

    Reply
    1. gazzz (Post author)

      Hi there RC17 team!

      Sorry for late answer, I had a busy weekend in my garage.
      Thank you for good words! I really appreciate your information; it was interesting to learn that CBX got quite impressive results in races and how Honda tried to fix an issue with oil starving of second rod.
      In my cafe-racer project I decided to use best parts (in my opinion, of course) from both, RC17 and RC42 engines: crankcases, oil system, crankshaft, alternator chain and tensioner from RC42 and cylinder head with camshafts from RC17. I also chose RC17 gearbox and clutch, as I more used for six gear gearboxes. And I am excited to test my solution of 815ccm piston kit for these engins. Sure, I’ll share its receipt after testing.

      Kind regards,
      Nazar

      Reply
  11. Arthur

    Great website with valuable information!

    Reply
    1. gazzz (Post author)

      Thank you, Arthur!

      Reply
  12. Gary

    Hi, thanks for sharing your great info on this project. Does it matter what year CBX head you use, or are they all compatible?

    Cheers
    Gary

    Reply
    1. gazzz (Post author)

      Hi Gary,

      For sure I could say “yes” only about CXB750 RC17 1984-1986 heads and camshafts.

      Kind regards,
      Nazar

      Reply
  13. Andrew

    Hello

    I have a CBX head on a CB750 bottom end, looking at these casings would it be possible to use a CBX750 gearbox eg 6 speed on the RC42 mechanical clutch and basket?

    Mnay thanks Andrew

    Reply
  14. gazzz (Post author)

    Hello Andrew,

    Yes, you may do it, you only have to use CB750 main shaft with CBX750 gears.

    Here you may find more information:

    https://www.gazzz-garage.com/2017/11/27/cbx750-vs-cb750-seven-fifty-engines-comparing-part1-crankcase-internals/

    Kind regards,
    Nazar

    Reply
  15. Brandon

    I know this is kind of old, but amazing. I am curious if parts from the American (rc18, cb700sc), such as cams, head will fit similarly to the rc17’s. This is something I’ve wondered for a long time. I know the crank and rods differ, but if everything else is more or less equal, it should be possible to make a similar modification in the US. The US version gives the number of teeth on the trans, but not the cbx, the primary reductions are the same. Even if the gearing differs greatly, if the cassette fits, then I’d assume this can be made up for with proper final reduction gearing. The primary reduction should be noted as the same on all these bikes. Technically the US version’s 5th and 6th are overdrives, keeping the crankcase and shaft may be required, but the oil pans swap and the barrels should as well from my research. My question is this, is a similar frankenhawk possible using RC18 parts with the rc42?

    Reply
  16. gazzz (Post author)

    Hi Brandon,

    Sorry for late reply.

    I think it might be possible as engines have much in common. You may also find some information here: http://teamrc17.net/articles/120/cb700sc-nighthawk-s-and-co-produced-motorcycles

    Kind regards,
    Nazar

    Reply
  17. mohsen

    Hi gazzz I have a HONDA CBX 750P. Can you explain how to set up and assemble the camshafts?

    Reply
    1. gazzz (Post author)

      Hi Mohsen,

      The procedure of camshaft installation and timing adjusting is clearly described in service manual. If you have no such manual, just copy “CBX750 service manual” into google search line and you’ll find it in no time.

      You also may find useful information (and what is more important, clear pictures) about camshafts installation in CB750 Seven Fifty or in CB750 Night Hawk service manuals. They are also available for download in net.

      Here is some info about how hydraulic tappet might be cleaned and how installed camshafts should look.

      Kind regards,
      Nazar

      Reply
      1. Mohsen

        Thanks dear Nazar
        With your guide could solve the problem
        Best regards
        Good luck

        Reply
  18. Jack Perkins

    I do not believe your theory of honda using CBX crankcase’s for Nighthawk production is bust, Today i bought an engine from a CB750 nighthawk that had a RC17E Engine identification number….. yet the remainder of the engine was CB750, such as cable operated clutch yet to see what the front mounts look lke…

    Reply
    1. Jack Perkins

      I believe I may be mixing a few things up in that statement haha, I have been looking at a CB750 Nighthawk engine, not a CB750 ‘SevenFifty’. Damn you Honda….

      Reply
    2. gazzz (Post author)

      Hi Jack,

      I based my theory on facts. The first fact, as I mentioned in this very article is that CB750 (Seven Fifty and Nighthawk) crankcase is “updated” and “patched” version of CBX750 crankcase. Another fact is that early models of CB750 Seven Fifty (and as you said Nighthawk) crankcases have serial numbers that starts with RC17 while Seven Fifty cases of late production has RC42 on them and Nighthawk has RC38.

      I doubt that Honda have no RC42 stamps when manufacturing of Seven Fifty was launched in 1992. And I doubt that they stamped early cases for RC42 with RC17 for nothing. So I just worked out most likely theory that explains facts in most simple way: there was a pull of “upgraded” cases stamped with RC17 And sure, even if my theory is wrong , the facts remain true.

      Reply
  19. RAINERS

    Hi i hawe honda cbx750f clutch sliping i change all plates metal and friction new, but clutch slips again, im planing to bay new clutch spring washer, but it is not available for rc17 1984, there iz one witch is for cbx750p 1998 spring washer do that waser will fit to my rc17 clutch?

    Reply
    1. gazzz (Post author)

      Hi,

      Yes, 22441-MB0-771 would fit.

      Reply
  20. Szymon Modelski

    Hi,

    excellent information! Do you have any experience in fitting aftermarket exhausts dedicated for Seven Fifty to rc17 bikes e.g cbx750f? From all the similarities I would expect exactly the same header pipes/cylinder connection but you never know…
    Selection for Seven Fifty is much better than for rc17 bikes.

    Thanks for sharing!

    Reply
    1. gazzz (Post author)

      Hi Szymon,

      Exhaust ports on CBX750 and Seven Fifty have the same spacing, so exhaust headers from Seven Fifty will fit CBX750. The only thing is that CBX750 has 8to6mm exhaust studs while Seven Fifty has 8to7mm studs. However as both heads have 8mm thread for studs, exhaust studs from Seven Fifty may be installed in CBX750 head. In my project I used CBX750 cylinder head with Seven Fifty aftermarket exhaust header.

      Reply
  21. ivan

    Hi,

    I am wondering if you know whether the carburetor from a RC42 can be used with the CBX750P engine.. the part numbers are different (16100-MW3-670 vs 16100-MJ1-724). The RC42 uses the Keihin VE66BA carbs while the CBX750P uses the VE65BD ones.

    Reply
    1. gazzz (Post author)

      Hi Ivan,

      Sorry for the late reply. I know little about CBX750P. However carburettors RC42 could be installed CBX750 RC17 which in its turn was equipped with VE64B (16100-MJ0-602). It seems to me, since all these engines are tightly related to each other  their carburettors are basically 34mm Keihins,  they are all interchangeable. And definitely, main jets, pilot jets and jet needles  have to be set to specs of motorcycle on which carburettors have to be installed.

      Reply
  22. Nicolas Stas

    Hi guys,

    I would like to install a Delkevic exhaust on a CBX 750 from 1984. I looked on Delkevic website and it looks like they do have a model for Honda CB750 but not for Honda CBX.

    My question is therefore, is it possible to swap exhaust between Honda CB750 and Honda CBX 750 ?

    The delkevic exhaust that i would like to install on my CBX is the following one :

    https://delkevic.co.uk/honda-cb750c-f-s-sport-80-83-stainless-steel-4-1-header-exhaust-downpipes.html?search=honda%20CB%20750&description=true

    Thanks for your help !

    Reply
    1. gazzz (Post author)

      Hi Nicolas,

      I doubt that you could install that exhaust header on your CBX750. It is for the previous generation of CB750 engines, which likely has different exhaust ports spacing. Try to find exhaust for CB750 RC42 Seven Fifty (1992-….). It will fit a CBX750 cylinder head.

      Kind regards,
      Nazar

      Reply
  23. David Jones

    Is a clutch cover from an rc17e inter changeable with rc42

    Reply
    1. gazzz (Post author)

      Clutch cover from RC42 could be installed on RC17 with the clutch actuator removed and its hole plugged. RC17 cover couldn’t be installed on RC42 since RC42 cover has oil filler with oil dipstick. RC42 oil level is higher than those for RC17. That’s why RC42 has a plug with “do not open” writing on in place of RC17 has oil filler cap/dipstick.

      https://www.gazzz-garage.com/2016/12/14/seven-fifty-cafe-racer-engine-works-started/

      https://www.gazzz-garage.com/2018/04/16/seven-fifty-cafe-racer-preparation-for-assembling/

      Reply
  24. Juan Jose Paramo

    Hi Gazzz,

    I have a 1985 CB700sc which has the same piston bore 67mm as de CBX, but with with different stroke. Can I use pistons and rings from RC42?
    Thanks for all your hard work.
    Kind regards,
    Juan

    Reply
    1. gazzz (Post author)

      Hi Juan,

      I do believe that CB750 RC42/RC38 rings could be used in CB700. As for the pistons, the answer is, unfortunately, no. Even as bores of CB700 and CB750 are identical and pistons look to be mostly similar, they are different. The compression (crown) height of the CB700 piston is larger than that of the CB750 piston. So if you use CB750 piston (RC42 or RC38) in a CB700 engine the compression ratio will drop significantly.

      If you take a look into the photos of CB750 and CB700 pistons you will notice that the upper segment of the CB700 piston crown is higher than that of CB750 piston. This difference could be calculated.

      Compression ratio of both engines are the same. Bores too. I have information on CB700 and CB750 connecting rods: CB700 connecting rod is 2mm longer (eye to eye) than CB750 connecting rod.

      CB750 stroke is 53mm, CB700 stroke is 49.6mm, this gives us 3.4mm of difference between strokes, while difference between connecting rods is only 2mm. This means that the CB700 piston crown is around 1.4mm higher than the crown of CB750 piston. It’s a big difference.

      Kind regards,
      Nazar

      Reply
      1. Juan Jose Paramo

        Hi Nazar,

        I was just waiting for your answer to get a new ones from a site who happen to have RC38 rings .

        I really appreciate your time and kindness to answer my question and all your hard work on your site. It’s really impressive.

        Kind regards,

        Juan

        Reply
  25. Vic

    hey guys, do you think a carb repair kit of 1980 honda cb750 will fit on 1984 cbx750 carb??

    Reply
    1. gazzz (Post author)

      Hi Vic,

      I think it will not fit.

      Kind regards,
      Nazar

      Reply

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