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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23 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
  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

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