Question on tire sizing

I am in the final stages of building a 1327cc Sportster cafe racer. Since discovering that my motor builder put a long stroke kit in my motor and it is 1327cc (81ci)and not the 1032(63ci) I thought it was one of my engineers has raisied a concern of the weak link in these old motors of the weakness in the cases at the transmission. He recommended I get a cush drive wheel from a metric bike that relieve the stress to that area. It will also give me an easier selection of taller gearing to choose from. So I have purchased a Kawasaki Ninja EX250 16" wheel that will line up correctly on this bike. My question is I am running a 4.00x18 tire now that is 4.5" wide and want to find the widest metric equivalent to that? The 4.00x18 has only about 1/4-3/8" room between it and the chain so I am limited on how wide I can go. I checked and apparently a 130/70x17 is 5.9" wide, way too big. Can anyone direct me to a chart or link to help me determine tire sizes in width?

I need 4 more posts before I can post a pic of the bike.

I will probably be getting a suitable 17" wheel also.

And a suitable 16" tire

And a suitable 17" tire

Here is what I am working with:

This any help to you??

Tire Size Conversions|

I wouldn't go too crazy with tire width if you're going Cafe racing...the wider the tire,the harder the cornering....

What old Sporty engine ya running?? I've never heard of Sporty's being weak in the cases,early '90's Big Twin Evo cases,yes,but not Sportsters... IME Sporty's are damn near bullet proof

It's a 1962 XLCH with Sputhe dual carb heads. It has a 4.865" stroke and 3.25" bore for a total of 81ci. The early cases were notorious for breaking when putting excessive power to them.

The chart helps a little. Says I can run a 120 at most as an equivalent but doesn't tell me how wide a 120 is. I'll check some other charts to find that out.
Thank you.

120 mm = 4.72 " 1" = 25.4 mm

Didn't know that....thanks Good luck with the project...keep us posted with progress

But does that number relate to real world numbers. The tire on my 4" rim now is a 4.00x18 and it is 4.5" wide.
The original Sportster case was made for the old 45ci with only 45HP. they had steel rear motor mounts and when putting more horsepower to them the cases would break in the transmission area. Instead of fixing the cases by making them stronger they instead put in a weaker motor mount made from aluminum so it would break before the cases would. They should have made the cases stronger which they did finally in later years. My cases have already been welded for added strength or from previous damage. There is a process where an aluminum sleeve is welded around the trans shaft sleeve in the case to strengthen them.