Hey ya’ll i’m helping my friend with putting in a new intake on his sbo and he went with a holley street dominator but we aren’t sure what bolts to use for the thermostat housing. We’ve tried 5/16 coarse thread and fine and he said he tried M8-1.25 And all seem to be small and 3/8 is to big so we’re stuck
Anything helps really

 

Are both holes the same size ?

You can check hole sizes with the shank end of drill bits. Start with the tap drill size for 5/16"-18. Then 5/16"-24, 3/6"-16, 3/8'-24. Its possible theone or both holes have been drilled and tapped for a "Heli-coil" which would use different tap drills.

 

The bolt holes in any oldsmobile intake for the thermostat should be 5/16 coarse.

 

I don’t see any threads in those bolt holes. And this looks like a crack between the bolt hole and the thermostat opening. Probably not as much of an issue as no threads in the holes.

 

I see indications of a crack from both bolt holes. I wouldn't start installing this manifold before you figure things out and repair them.

 

Once you get the cracks and thread sorted out, why don't you try using the bolts from the original manifold?

 

Hopefully your friend got a deal on that Holley intake manifold used. If there are no threads you can always fix the holes with a heli coil.

 

Looks to me like a repair of the cracks has already been attempted, with the repair intruding into the round pocket that the T-stat fits into.

 

You need to drill and tap that for the next size up…3/8.

I just had a stripped hole on a factory manifold last month.

 

Agree, with the weld up on the holes. I like to use threaded inserts to fix things like this. Not Heli-Coils. They seal better, as this goes into the coolant passage.

 

Yup, that manifold has already had a hard life.
Note the weld blob goes into the recess for the thermostat. The thermostat is supposed to sit in that ring, which leaves a flat surface from the intake to the thermostat for the gasket to seal. As-is it would push the thermostat up and it'll be quite difficult to get a good seal. Gotta grind down that area so the thermostat sits flush. Doesn't need to be precise - just low enough that the thermostat is below the intake so the gasket is sealing the intake to the outlet housing and not trying to seal the thermostat itself to the housing.

If the welds are really good then opening up the holes and using a coil or an insert to get back to 5/16 will work. If the welds aren't great then it'll just crack again.

One thing I keep in mind: If a helicoil fails then you can drill out to a bigger side for an insert - inserts need bigger holes than coils. If you go straight to an insert and something goes wrong then you're in deeper trouble. In general I wholly agree - inserts are better than coils. Here the problem is coolant can crawl up the threads and out the top of the bolt. That's easier with a coil because, by necessity, there's more room around the coil.

 

I appreciate the input from everybody i’m going over to his place now with some heli-coils and a dream i hope the welds hold up if not then that’s a different days work. Also he got it for 150 I still think it’s a decent deal