390 or 420 top end. How to tell?

Guys,
A couple weeks ago you helped me buy a 1980 390 with a 420 top end. I,m happy to say I now own it. Comparing it to a 390 motor I had already, I see no difference. Is there a way to spot the difference from the outside without pulling the head?
Pete

Nope, you have to pull the head and measure the piston/bore diameter.

count the fins...

I was just comparing 420 to 430 last week. The 430 casting seems to be the same as the 420 but shortened on top. I couldn't figure out about the bottom one of the 420 ones was shortened and the other maybe was maybe wasn't. The cylinder heads don't interchange 420 430. I have two 420 top ends loose and one on the bike, two have the spark plug pointing straight up and the other one is angled. One has a small hole to make starting easier going from inside the exhaust part of the head to a hole above the exhaust port opening.

Does the 390 have a divided exhaust port like the 360? If not it kind of seems not a great idea the 420 bore/stroke ratio as about as oversquare as any model they ever made isn't it? The 420/430 piston must not be in balance for the crank weights of a 390.

The 420 would have an 86 mm bore I think I could make a small wood stick that size and figure a way to see if that was in fact your bore diameter working through the exhaust port. I think 9 fins for the ones I am typing about.

I think the 390 is a divided exhaust port. I,ll look for sure later today. Both cylinders do have 9 fins and the heads look the same. Pipe looks to be untouched stock. Someone here mentioned was it modified to fit the 420.

George at Uptite told me the 1980 390 was the only cylinder that has enough cylinder wall thickness to accept the 420 piston.
He also said the stud spacing is wider in the 1980 390, as Husky was getting ready for the new cases and cylinder for the 420 in 1981.
My 79 390 has a single exhaust port (not bridged) and the studs measure 2.75" front to back and 3" across. Difficult to measure with the cylinder on the bike though.
Ron

Thanks T I'll give a look and Ron I'll see what I can.
Oh and heres a pic.