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…

 

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4 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

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