Advice Please - Spencer AMA bike

[SeeDoubleYou]

Hi I am new to the forum having just realised the virtues of the CB750s. In particular I am looking to build a replica Freddie Spencer AMA lookalike to race with the CRMC in the UK. Having done some initial research I have some questions I hope you can help with

1) I see that the CB750 DOHC produces about 70bhp and the CBX750 about 90bhp. So would a CBX engine drop into a CB frame. I can do mods if need be

2) The second perhaps easier route would be to start with a CBX. Question therefore is whether the main frame cradle on the CBXis similar to the CB and whether the CB tank and body work would follow the contours of the CBX frame. I appreciate the rear end is a monoshock but I could adapt with tubework and welding

Any view appreciated.  Thanks

[shinyribs]

Welcome to the forum.

As far as I know, we never got the CBX750 in the US, but it is very closely related to the Nighthawk engine. IIRC, the Nighthawk engine is loosely based on the CBX750 engine.

As far as bolting in? A nighthawk engine will not directly bolt in to a DOHC CB750 frame, so I am assuming a CBX750 won't either. That's not to say it can't be done (even easily done), it's just not a direct, plug-and-play type swap.

Very interested in your build. I hope you keep us in the loop. I would love to watch this take shape.

[shinyribs]

The frames seem quite different. Not sure how tank fitment would play out.

CBX frame.



CB frame.



The CB frame has the two upper cradle bars that are so much wider than the CBX's ''backbone''. Just guessing it seems a DOHC F tank should be able to go on easy enough.

[shinyribs]

SeeDoubleYou wrote

1) I see that the CB750 DOHC produces about 70bhp and the CBX750 about 90bhp. So would a CBX engine drop into a CB frame. I can do mods if need be



Any view appreciated.  Thanks

A DOHC CB900F is claimed to make ~90hp and a DOHC 1100F makes close to 110 hp. Both of those engines will bolt directly in to a 79-83 DOHC CB750 frame. Never tell them apart unless you look close at the clutch cover and a couple other tiny clues. But being in the UK I'm not sure about your MOT allowing those kinds of swaps?

[SeeDoubleYou]

Thanks for you replies chaps. I've decided to go the CBX donor bike and fit CB bodywork. I think it looks pretty straightforward, however looking at the underside of the tank I reckon it'll have to have a chunk cut out to fit over the rear bend on the CBx frame. I think for now I'll keep it as a monoshock but will go to twîn shock later.

The other problem with the CBX is the 16" front wheel. I've done some research on discs sizes and alternatives and reckon that the CBR 600/750 3 spoke front wheel may fit. If it does then I'll probably look to fit a similar wheel in the rear so that modern tyres can be used. Again I reckon a CBR750 Hurricane could be an option. Any views? I'll keep you posted

[shinyribs]

I've got a set of Hurricane 3 spokes here. Holler if you need some measurements. I had them between 1000F2 forks using F3 trees, but custom axle spacers of course. But, I do have a set of brackets to relocate the Hurricane calipers to 1000 F2 forks so they will align to the Hurricane discs.

[Beekeeper]

For a second there, I thought you were channeling Abbot and Costello.  Who's on first, I dunno, third base!

[Truck]

I'd think the easyest way to go would be a CB900f. Most DOHC parts wil be bolt on and you get the power you want without the headache of trying to get/make parts fit. is there a reason you want to got with the CBX frame?

[SeeDoubleYou]

Thanks guys. To answer your last question I opted for the CBX as its cheap power, with extra horses over the CB750. Also the race regs I'm trying to adhere to say that I must use the same frame and Swingarm as the engine of choice. Getting tank and seat to fit won't be too much of an issue although I do anticipate some cutting and shutting.

Hey skinyribs, if you could have a measure up on the Hurricane wheels that would be great. For the fron can you just measure the distance across the 2 discs at the front. I can get the wheel central easily enough and can make necessary spacers or bearing packers if the spindles are different diameters. Same for the rear across disc and sprocket. Also if you could make an approximate measurement from middle  of the hub to the inside of the sprocket I can also check the offset. Thanks all

[shinyribs]

Rear
-Sprocket offset from wheel centerline
-Outside to outside sprocket to disc
Front
-Outside to outside width of the front discs

I'm on it.

I do know that using the 'Cane rim on the back of a DOHC750 that the sprocket had to be spaced off the carrier about 6-8mm.

edit: I just had a thought. You reckon a US model Hurricane will be the same as what is available to you?