Fastback and 850 engine

saint · Jun 14, 2009

I have what I think is a 1968 or 69 fastback without an engine. I recently aquired a 850 engine and I was wondering if this engine can be mated to the 750 gearbox and if, in general, it is a drop in operation.Thanks for any comments

swooshdave · Jun 14, 2009

Ron L · Jun 14, 2009

With the exception of a Mk3 850 it's pretty straightforward. However, if you have a '68 frame with the gusset (widow-maker) rather than one with the bracing tube, I wouldn't put a Briggs mower engine in it! There is a '68 on eBay (listed as a '69) with this frame and very low miles that is currently at $7250 that I would not ride around the block (without replacing the frame).

A Mk3 has a different crank and primary for the left side shift, so requires more engineering to fit.

I also prefer to use the gusseted 850 swingarm even with a 750 motor as it helps to reduce the warpage and helps keep the wheels in line.

Berticus · Jun 14, 2009

I put an early 850 engine into a 1972 750 roadster. Don't forget the carburetor belongs to the engine, not the frame. I did and was all excited about starting the new engine up only to realise I needed to send away for new jets etc. Bit of a disappointment!

A question that occurred to me later was the gearing: does the 850 rev less than the 750? Is the standard gearing any different? Mine runs perfectly well as it is but I don't exactly squeeze every last ounce of performance out of it.

Ron L · Jun 14, 2009

US supplied 750's normally came with 19 tooth countershaft while 850 Roadsters came with a 20 and many Interstates arrived with 21's. 19 will give the 850 a little more spirited acceleration, but the 20 or 21 gives nicer freeway driving.

saint · Jun 14, 2009

Ron
I checked the ebay commando, and my frame it identical.I'm a newbe to Commandos ,could you be more specific to your objections regarding this frame ,is it balance or strenght?is there a cure?Thanks

Ron L · Jun 14, 2009

The original '68 Fastback frames broke without warning at the steering head. In fact, there was an NHTS recall on them in late 1974. Norton replaced many frames under warranty with the frame with the small tube running to the lower end of the steering head. This frame was discontinued within the first few months of original production of the '68 Fastbacks.
The cure would be, assuming your frame is not already cracked, to weld a tube to the reinforce and strengthen the steering head as the factory did. It might be easier to find a replacement frame, preferrably one from a '69-'70 model. Those all have the replacement frame and the frame mounted centerstand.

79x100 · Jun 14, 2009

Has the eBay bike got a repair on the top tube ? It looks as if it's been cracked and welded :shock:

dave M · Jun 14, 2009

Saint, Don't panic yet, send a photo of your frame so that it can be ascertained whether you have indeed got one of the very early dodgy ones.

Ron L · Jun 14, 2009

79X100,
I don't think so, I believe that is the stock gusset you see in the photo showing the frame number. Since the there is supposedly under 1000 miles on the bike, it is a good chance it hasn't cracked....yet. I e-mailed the seller about the frame, but he was adamant that the frame was strong. It was a nice original early fastback. A perfect garage queen. I just wouldn't ride it!

saint · Jun 14, 2009

Thanks for your answers.I dashed to the storaged place where I have this potential commando, and I noticed that the steering head is held by the large backbone tube and just under it there is a smaller diameter tube welded to the head and the underbelly of the large tube and at the bottom of the steering head one can see two tubes which go down and form part of the engine cradle.I hope my description makes sense but I think that this frame is not the gusseted type.But I may be wrong.

dave M · Jun 14, 2009

It sounds like you have one of the good frames, which should pose no problems at all.

Fastback and 850 engine

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