Thermostat housing bolts
 

What sizes are they in length and thread? The lower bolt broke off in the head.

Top one is 1 3/4 long, bottom is 3/4 long.
5/16-18 thread

Can these be replaced with regular bolts and a washer?

what do you mean by regular bolts?

So if I have at least half of an inch of the bottom bolt in my hand, not much is in there, right?

He means as opposed to irregular bolts, found next to the blinker fluid.

You can get the right length Grade 5 or grade 8 bolts at Lowes,

Is grade 8 that much more brittle? I don't want to get good at doing this...

Holy crap, I never knew the marks had meanings. Cool bit of info. And I'll probably get grade 8. Lol

Someone once said that knowledge does not ends with a diploma, only with a death certificate.

I disagree. Only the dead guy knows if there is life after death or not, so truly you never stop learning.....

So safe to use non-flanged standard bolts with a regular washer or should you use a flanged bolt? Also, is grade 8 the best or should one go stainless?

I would just be careful using grade 8 as the housing is said to be farily easy to snap, and reviewing the construction of mine this weeekend, I would agree. Also, make sure you use some thread sealer on the top bolt unless you want to enter yourself in a wet t-shirt contest.

Regular screw? Yes. Washer? Don't know if there's room.

I usually replace these with brass or bronze, CRES if I can't find either of those. I don't like using steel in water, if I have a choice.

(You can probably find brass, bronze, and CRES screws at the local marine supply house, definitely Fastenal, and probably any local MRO house.)

CRES = Corrosion RESistant alloy steel. "Stainless" is a trademark.