Are 365hp and 420hp valve covers the same?
 

It's an older merc 330 hp 454, taking off the peanut heads and putting on square port heads on a buddy's boat. It also has 1.7 ratio rocker arms. From the top of the head to the rocker arms is right under 3" and I just told him to get factory valve covers because He got some from jegs that weren't tall enough.

They may be different, but the stock 454Mag will clear roller rockers. You have to grind off the oil drip tabs on the inside of the valve cover. Also depending on the baffling and which roller rockers you have, you may need to grind the corners off the #1 and #8 intake rocker arms.

If there is still any issue with clearance, he can add a thick valve cover gasket.

The old Merc 365hp valve covers are a kind of a sheet metal material whereas the Merc 420hp engine has cast aluminum valve covers. Don't know about how they fit compared to each other, but I can tell you that the cast aluminum one is a much nicer looking valve cover.

The 420 covers you are looking at are made by Weiand. They are an oooooollllllllldddddd style cover. I love them!
Merc used that cover on all the HP engines up to the early/mid 90's.

In 1987, the 330 HP were sheet metal. The 370 and 420 HP were the cast alum - same type...except for the Merc sticker. The 365 HP was a later model year 1988-89? Not sure of the valve cover type.

These are in the swap shop $100 for both sets, valve covers that is....

I'm pretty sure the 420 valve covers have 55 more horsepower the the other ones. I put a set on my boat and pick up 3 mph.....

My 1993 454mag/365hp valve covers were identical in dimensions and design to my 1996 525SC valve covers. When I added roller rockers, I had to grind off the drip tabs. On my 525SC, you could see where they were ground off at the factory.

I have two sets of '89 454 Mag (365 H.P.) valve covers, stamped steel type. If you can use them, just cover shipping. Also have the original factory intakes and carbs for same in VGC.

Let me know if you are interested.