I have unexpected delay in painting works of my Honda CB750 Seven Fifty cafe-racer project and other processes are not as much spectacular, so there is some lack of fresh material about project. But every cloud has a silver lining, so I decided to fill the gap with another part of CBX750 RC17 and CB750 RC42 engine comparison. This time I compare engine cases’ guts.
Clutch baskets are completely different, so there is no reason to compare them. They are even not interchangeable, cause main shafts of CBX and CB have different lengths of splines. But complete gearboxes are interchangeable, as well as their shafts. Main shafts’ interchangeability means that one may not only use complete 5 gear CB750 RC42 gearbox with its clutch (mechanical drive system) in CBX750 or 6 gear CBX750 RC17 gearbox with its clutch (hydraulic drive system) in CB750, but also may use 6 gear CBX750 gearbox with CB750 clutch as well as 5 gear CB750 gearbox with CBX750 clutch, by just swapping main shafts.
As you may see, CB750 RC42 gearbox is not more than simplified version of CBX750 gearbox, so all gears may be swapped. Difference in length between shaft spines are quite visible even without measuring:
Obviously, gearshift drums have different patterns, but gearshift forks are identical, as well as gearshift spindle and other small parts:
Next point of comparison is crankshaft. The difference is in generator chain drive sprocket. Quite easy to notice that CB750 Seven Fifty has wider sprocket than CBX750 has.
Generator chain was a weak place of CBX750 engine so for Seven Fifty Honda engineers designed reinforced chain:
And sure, alternator and starter clutch packs have only one difference, the chain driven sprocket:
CBX750 and CB750 Seven Fifty alternator chain’ tensioners are different, but Seven Fifty’ tensioner may be used in CBX750 engine together with CB750 crankshaft and alternator driven sprocket. Unfortunately I have no photos of tensioners, as CBX750 one which I got with other lot of parts was broken.
The oil pump of Seven Fifty comparing to CXB750 pump was also improved. Again, I shot no photos of pumps to show difference, but it simple enough to explain by the words. Both oil system are similarly organized and both oil pumps has two chambers:
- one chamber pumps oil through oil cooler and feeds gearbox;
- another one work for crankshaft and cylinder head feeding (through oil filter).
So, to solve the question of crankshaft oil starving Honda engineers not only modified the oil channels in crankcase (see chapter one of the story) but also increased the performance of chamber which feeds crankshaft and cylinder head. They simply designed oil pump with same intakes and outputs but with larger chamber and therefore, rotors. Thus, Seven Fifty oil pump may be easily used in CBX750 engine and that will be quite a good update for it.
And last difference which is insignificant, just a matter of timing chains design. CBX chain has quite unordinary pattern of links:
When I’ll have some spare moments next time I’ll write about difference between CB and CBX cylinder heads and camshafts.
First off great article on the two engines I was wondering if
You went any further with the comparison of the top end
Eg connecting rods, stroke of the engines and differences
Between the pistons and cylinder bores?? I have a 1984 cbx750f and am curious to see if the pistons and connecting rods match up! Again great read thank you for the information keep up the great work!
Thank yo for good words.
Bore and stroke are same for both Seven Fifty and CBX750 engines, and their pistons, connecting rods and cylinder barrels are compatible.
Hi is the rc17 engine top head with rc 17 cam shafts and rc42 sprockets on them fit to rc42engine top? And do that give more hp to rc42 engine than stok head?
I used RC17 cylinder head with RC17 camshafts on RC42. Cam sprockets of RC17 and RC42 is similar. RC17 has bigger valves and higher camshafts, so yes, it gives some additional performance.
Thenkyou , i hawe another question , do the clutch side cover from rc17- not fit to rc42 bicose of mechanikal cluch or or bolt hole paterns? I would like to use in my project Rc42 crak case + cranksaft + rc17 top head+ rc17 gearbox and + rc17 hidraulic cluch and Cluch cover. Thanks wery good web page , alot of important info !
Clutch covers are interchangeable. You could use RC17 clutch cover with RC17 gearbox and RC17 clutch in RC42 crankcase. Just don’t forget left engine cover from RC17 as it has a hole with seal for clutch rod.
Hello Gazzz, Im doing something similar to Rainers, i Bought a RC17 Engine from a cb750 Nighthawk to replace my rc17 cbx750 engine. i want to use my hydraulic clutch but could i install clutch plates, basket etc, and cover but use the cb750 gearbox ?, thanks!
CB750 Nighthawk has same gearbox as CB750 Seven Fifty. CBX750 and CB750 clutch baskets are not compatible.
So, if you wish to use CBX750 clutch with CB750 gearbox you have to repack CBX750 main shaft with gears from CB750 main shaft.
Thankyou for that reply Gazzz, i know what to do now!. oh its all so confusing that honda had so many CB750/CBX750’s. Do you know if the CB750 Nighthawk engine is the same as a CB750 seven fifty ? or possibly a direct swap for the CBX750? minus the clutch im worried about the firing order and having to replace the loom and CDI with a CB750 nighthawk set. cheers!
I do believe that Honda CB750 RC38 Night Hawk engine is identical to CB750 RC42 Seven Fifty engine.
I suppose you could put CB750 RC38/RC42 engine into RC17 frame. The greatest issue would be to manufacture new front engine mounts. I see no problem with keeping original CBX750 oil cooler and “oil in frame” system as long as you fill oil to the level on CB750 (not CBX) oil dipstick.
As for firing order and ignition system: the myth about different firing sequence of CBX750 and CB750 caused by markings on CBX750 ignition coils and therefore misleading statement in CBX750 manual. In fact CBX750 and CB750 have identical positioning of camshaft lobes, timing and firing sequence. I ran RC42 engine with electrical system from RC17 without slightest issue.
Congrats on the great article.
I am looking forward to by ’85 CBX 750. However, the bike is with out the exhaust system. It is gonna be a project bike so I can modify mounting brackets etc. Do you think that CB750 header pipes will fit?
CB750 Seven Fifty header pipes fit CBX750 head. The only difference is exhaust studs. CBX750 RX17 studs have bodies with reduction from 8mm to 6mm, while Seven Fifty studs’ bodies are reduced from 8mm to 7mm. Seven Fifty studs could be installed in CBX750 head.
Thanks for the explanations, it helps us a lot! Could you tell me if the oil pump of the cb750 is compatible with the cbx750? Thank you !!!
As far as I could remember, Seven Fifty oil pump is compatible with CBX750.