Chevy Small Block Bolts Question

[Keep it stiff]

I need to change a couple of water pump bolts to be longer and with allen key head to be able to accommodate an alternator bracket.

Can someone tell me please what the thread is? These imperial sizes are a whole new world to me! I think they might be 3/8th and if so UNF or UNC?

Thanks.

[Panamax]

If you google for "imperial bolt sizes" you'll find some excellent explanations of the difference between UK and American threads etc. The assortment is baffling at first with various combinations of bolt diameter and thread pitch, but you'll get there.

Example from a quick click,
" AF was introduced much later and refers to Across Flats and was related to UNC(universal national coarse) and UNF(universal national fine) threads in the USA . A 1/2 inch Whitworth socket is for a nut that fits on a 1/2 inch diameter thread, and is usually 1.5 times the diameter of the screwed rod across the flats of the...."

[Oneball]

3/8 UNC

If we’re talking about the alternator stay You’re better off getting the correct stud. They go into the water jacket and can leak so a bit of thread seal is a good idea and you don’t want to be loosening a bolt to change belts.

[Error_404_Username_not_found]

Given that you seem confident that it's an SAE thread, which includes UNF and UNC, first you need to establish the nominal diameter. If you don't have a vernier, one trick is to use an open ended spanner. A 3/8" inch spanner will comfortably sit on a 3/8" thread but the next common size down (7/16") won't.
Next count the threads.
3/8" UNC will have 16 threads to an inch but 3/8" UNF will have 24 TPI.
Look here for other sizes.
https://www.engineeringtoolbox.com/unified-screw-t...
Hope that helps.

ETA: M' learned friend above has more specific knowledge about this engine. Good post.

[Keep it stiff]

Thank you.

[roscobbc]

Generally bolts or studs secured in to cast iron blocks will be UNC - UNF will be too fine with the potenial risk of stripping thread in block......hence why you'll see studs with different threads each end. My experience is with big block chevys where the water pump bolts (not studs) are not all the same length.....and it is all to easy for the uninitiated to use a longer bolt where a short one should be used......an over enthusiastic wrenching can result in the bolt 'bottoming-out' in the block and stripping its thread.