Flathead merc valve noise?

Had to install new radiator for a 50 merc flathead and pulled the heads to be sure there were no blockages... installed new studs, gaskets, when it got back together now it is like a continuous rapping like the valve train is noisy or the pistons are constantly slapping.... any thoughts? I have only run it for a couple minutes as I didn't want anything to get to messed up. Thanks for any help.

 

If that's all you touched...
Perhaps a gasket glitch with a gasket overhanging and a piston slapping that??
If valve (or piston) clearance was really marginal, a new gasket slightly thinner than the old one could create head interference. If heads and cam are stock this is unlikely.
Crud or washer dropped in there??
Nothing here suggests an obvious immediate conclusion. Pull plugs and see if you notice anything funny...if not, spinnit with the starter (will turn faster with plugs out) and see if noise is there that way. If nothing turns up at that level I'd worry enough to pull heads.
Oh...on a late FH you probably pulled distributor too...if so look in there for rotor interference.

 

If the engine has been bored at some time in the past, and you installed stock gaskets, it's possible the pistons are contacting the edge of the gaskets, as Bruce Lancaster said. A bit unusual, but it has been known to happen.

4TTRUK

 

K thanks for the replies, I pulled plugs, couldn't find anything, I pulled the heads and could see where the pistons infact were slapping the heads, just barely but enough to tell. I think the gasket thickness like BRUCE suggested is the culprit. I also now noticed a howling like a bad bearing so now I think I am just going to pull the motor and go through the old gal. Thanks for the replies.