A65 rear tyre sizes

[ferretjuggler]

Apologies if this question has been asked a million times before.
Can I fit a modern 120/90/18 rear tyre to the rear wheel of my 1969 Lightning ?
A foray into my loft has dug out a brand new Cheng Shin.
Not exactly top notch I admit but it's new and it's here
I'm just wondering if it's too wide and may foul the swing arm or chain guard.
I'd rather ask now then spend effort fitting it only to have to take it off again

[ChrisG]

Your lucky day!
120/90 is 4.00, 4.25, 4.50 in old money. The rear tyre size for the A65 as BSA intended is 4.00 X18 so Bingo! It will fit!

[ferretjuggler]

Many thanks.
I've got another problem now.
Only a simple thing
Probably better if I start another thread