[Spridgets] Help!!! Dumb question of the day...

[Dave G.]

So I'm looking at the thermostat bolts that go into the 1275cc head...
I bought the S?S 5/16 x 18 bolts, washers and one nut (just to check).
The nut will not go onto the studs that are in the head... a few turns max 
and then the nut jams...
It feels like the threads are wrong...
Here's the dumb question...

Are the studs different size threads on each end??
Or do I need a 5/16 x 20??

TIA...

--
Dave G.   KK7SS
'65 MK III Sprite in Richland, WA
"Real stupidity beats artificial intelligence every time."
    Terry Pratchett, 'Hogfather'