Sevenfifty 02 VS seventies CB750 - Wrist Twisters
 
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post #1 of 11 Old 12-07-2015, 02:49 PM Thread Starter
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Sevenfifty 02 VS seventies CB750

Planning a Cafe Racer from a CB750 frame, came across a cheap 2002 sevenfifty (Nighthawk in the US?)
I thought that the newer sevenfifty had the same frame as the older CB but guess I was wrong

But what about the sevenfifty motor?, will it fit the older CB750 frame?
Does the SOHC and DOHC motors have different frames?

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post #2 of 11 Old 12-07-2015, 02:51 PM
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I'd hit up a site like ronayers and compare part #s between the major components (i.e. engine cases)

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post #3 of 11 Old 12-07-2015, 03:25 PM
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Yes, the frames are different between the SOHC and DOHC bikes. No, you can't interchange the two different motor types without cutting and welding. What's more, the 02 'sevenfifty' doesn't even use the same motor as the first generation DOHC 750s of the late 70s, early 80s. It's actually completely redesigned with a completely different layout; this engine premiered in my other bike, the 1984-1986 Nighthawk 700S/750S/CBX750 (I have an 86 Nighthawk 700S.) They decontented that engine for use in the 91-on Nighthawk 750, but it's still essentially the same. It does *not* fit in any of the older bike frames - for one thing, the engine casing is much longer and much taller due to the reorganization of parts compared to the old SOHC or DOHC motors. So even if you did have a DOHC frame, you couldn't use this motor in it. It is yet a third type of '750' air cooled motor.

1986 Honda Nighthawk 700S
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post #4 of 11 Old 12-07-2015, 09:17 PM Thread Starter
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Wow, thanks for a very explanatory answer, you really seem to know your stuff around these bikes!
I have compared a lot of pictures of frames and motors and the older DOHC seem very close to the newer sevenfifty and I was under the impression that it would be an easy swap, but hey nothing is impossible, I am quite handy with the TIG

However I think I will run with the 02 frame and motor, it still has that retro look I am going for even if the frame will cause some problems I need to solve.

I have bought a Triumph ST Sprint single sided swingarm, the goal is a retro cafe racer with a modern touch, now I cant decide if I should go for a newer USD fork or keep the stock one and lower it.
Also want to fit the 70s CB fuel tank.

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post #5 of 11 Old 12-08-2015, 06:50 AM
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The older DOHC *looks* kind of like it would fit, but it doesn't. Honda actually made two Nighthawk 650s during the 1980s - one with the last SOHC motor and one with an early version of that third type that's in my 86 and your 02 750. People have tried interchanging them and there's no way. Look at a cutaway diagram of one and you can see how different it is - look at where the alternator is, for example.

(Image courtesy of Honda - yes, the mothership is proud of this one)


The Nighthawk S type engine is much narrower, longer, taller has much different mounts and one *enormous* difference to the earlier bikes over and above the dimensional differences - it *requires* an oil cooler. There are short hoses on the lower sides of the motor that connect to the frame on the earlier version and from there, they connect to the frame mounted oil cooler. Yes, they're oil-in-frame bikes. The later 91-03 750 uses a hose setup for simpler assembly, but it still has the oil cooler mounted in the same place - right up front, below the steering head on the downpipes.



If you don't have an oil cooler, the Nighthawk S family of 700-750 motors *will* overheat and die in short order. The air cooling fins are only a little more than vestigial.

However, it's more than possible to cafe out a Nighthawk 750 without having to put the engine in a new frame. It's been a somewhat popular conversion. Here's a well done one, but it doesn't have a dustbin fairing.
Back To The Future: custom Honda Nighthawk | Bike EXIF

1986 Honda Nighthawk 700S
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post #6 of 11 Old 12-08-2015, 08:13 AM Thread Starter
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Super informative and highly interesting, thank you yet again. The newer frame will work fine, the only part I dont like is that big central pipe that follows the tank, it prohibits the use of pods on the carbs.
I plan to build a separate subframe in aluminum and this constructions makes that a bit harder.
Hopefully I can fit an oldschool fuel tank to this frame as well.

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post #7 of 11 Old 12-08-2015, 08:56 AM
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Seriously, check out the second link. They kept the front part of the frame the same and didn't have problems fitting pods. Don't be surprised if it runs worse with pods on, though.

They also don't seem to have had problems fitting another tank.

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post #8 of 11 Old 12-08-2015, 08:57 AM
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post #9 of 11 Old 12-08-2015, 10:25 AM Thread Starter
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Looks great, but I have studied closeups and it seem to get pretty crowded with the intakes for cyl 2 and 3, well that is later problems, first I need to strip down the sevenfifty and start chopping it
Btw do you know if they use taperd needle bearings or traditional triple clamp bearings?
Havnt decided if I am going to swap the fork or not.

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post #10 of 11 Old 12-08-2015, 10:54 AM
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They used standard ball bearings and races. Honda only used taper rollers on a select few bikes, starting with the GL1500.

However, you can retrofit taper roller sets and I for one strongly recommend them. K&L has a nice kit for Hondas that includes Japanese (not Chinese) rollers. See my comments here on taper rollers in ball bearing Hondas and the corrected torque settings, installation procedure etc.: https://www.wristtwisters.com/forums/...ml#post1156634

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post #11 of 11 Old 01-04-2016, 10:09 AM Thread Starter
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And here it is, my RC42 from 2002, got it really cheap.


free image hosting

Soon to be fused with this stuff :


image free hosting

Started tearing the Honda down, found a tank, but I am a tad worried that it will look too small?, just placed it on top of the frame to see how it looks, need to cut and move the front pegs to get it down there properly
What do you guys think?, I am also bidding on a CB750 Supersport tank, If all goes well that might fit better?

imageupload

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