CBX750 VS CB750 Seven Fifty. Engines comparing. Part1. Crankcase internals.

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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.

 

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