Maximum rear tyre width ?

[Grewth]

A couple of weeks ago I bought the mortal remains of a CB750F2 cafe racer.
I've used some parts for my F1 restoration,  and now I'm wondering what I can do with the sorry looking pile of bits that remain.
Looking at a few cafe racer builds on YouTube,  I'm just wondering what is the maximum width of rear tyre that I can use with the CB750 engine and frame  ?
I'd consider a substitute (wider) swing arm, but I'd want to keep the main frame original(ish)

[Martin6]

It might be governed by the chain run as well as the swingarm, especiallyif you retain the chain guard.

My Norton Commando came to me with the wrong tyres 3.60 width, instead of the spec 4.10. When I put the wider 4.10 on it, I was surprised just how much slower the handling became. These CB750s aren't the most nimble in the first place, so worth considering if you can get the right profile in a wider tyre, to avoid trashing the handling.

[Grewth]

I don't want to seem negative,  but I'm not really worried about trashing the handling.
I've got a CB750F1 that I'm rebuilding to close to standard spec.
I had one thirty plus years ago, so I know what it will handle like, and I can live with it.
The F2 has come with a set of really manky comstars, even the stainless bits have gone rusty,  along with rotten tyres.
I'm thinking about a set of upside down forks off a modern sports bike, with maybe three spoke alloy wheels,  and modern brakes to match.
Chain/tyre clearance at the rear would seem to be the limiting factor.
Obviously the purist restorers won't like it, but what the Hell ?
It's not as if I'm hacking up some mint collectors bike.

[flatfour]

I have always used 4:10/18 TT100 tyres on the rear of the various CB750's that I've owned, and indeed on the 500/4's (three of them) too. My Benelli Sei also worked well on the same.

For me, the handling seems ruined by modern low - profile tyres, and they change the angle of lean when the bike is on the side stand too. As a matter of interest, when I was buying these bikes new in the 1970's, our Honda main dealer always recommended a change to TT100's in 4:10 sizes front (19") and rear (18") for both the CB750 & 500 Fours.

[SumpMagnet]

The limit here will be down to the chain run, unless you plan on some modification of the engine mounts to offset the motor. You may be able to get some extra clearance with a modified gearbox sprocket. You still need to keep it inside the frame rails though.

[Grewth]

Agreed.
Swing arm can be modified or replaced,  ditto for torque arm.
I'm not keen on the idea of shifting the sprockets outboard,  there's a hell of a load on that front output shaft.
I'd not considered moving the engine over.
Looking at the mounts, I think maybe 10mm wouldn't be too difficult, or look too obvious

[Spitfire]

I always used to use TT 100's, 410-19 and 425 - 18 on my 750's.

Cheers

Denns