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Looking at replacing my 4cyl 171 head bolts,have some 1/2 inch bsw g2 bolts,dose anyone know if g2 bolts are the same as the original head bolts. Thanks

I would not use them. Look for some regular head bolts. If anything, use grade 8.

Be careful about getting the correct thread on the new bolts. They are threaded 1/2-12. Yup, 12 threads per inch, NOT 13.

The Filling Station sells them.

Cheers, Dean

Whatever bolt strength you select make sure you only tighten to the original torque value (if there was one back then).

Even though the bolt might be stronger the internal threads and the assembled parts might not be able to handle the higher clamp force created by the higher torque value.

The original bolts were soft steel. They were tightened to "tight". Most of us use 60-65 ft-lbs.

Seems you can only get grade 2 bsw hex head bolts,any higher grade only come with allen key heads,not a good look on a old chevy,dont want to increase head bolt tension,just want a replacement for the original ones,it looks like the filling station will be the answer.

Johnhill
Chevrolet did not use BSW thread (British thread), most American vehicles used UNC or UNF thread pattern which is slightly different shape and thread count.
Tony

Tonyw i think unc is 13 threads to the inch and bsw is 12 threads to the inch??

Without checking you could well be right, SOME bsw bolts will work with unc nuts and the other way around but not all.
Tony

Im looking at useing them for head bolts ,threads are in the block bsw not unc

Im looking at useing them for head bolts ,threads are in the block bsw not unc

So for my understanding, what did the US blocks use for the thread in these blocks?

Filling station lists one head bolt size up to 1928

Filling station lists one head bolt size up to 1928

Old216 looks like all 171 4 cyl head bolts are 1/2 x bsw 12 threads per inch??

And the 6 main bearing cap bolts are bsw as well

Considering most of the US adopted UNF and UNC as standard prior to 1900 I am surprised that Chev used BSW at all.
Tony

There is an old saying in the computer industry: "The nice thing about standards is there are so many to choose from".

USA brass era cars didnt even have interchangeable parts prior to 1908 or so, let alone standardized bolts. My dad was a brass car enthusiast, and kept a bucket of really old bolts. There was a lot of strange stuff in there. Weird thread pitches, radiused threads, similar to Whitworth, and so on. In those days they just made whatever. There were a bunch of standards that predated UNC/UNF to pick from, if they followed a standard at all!

I think the radiused threads went away, and things became somewhat standardized in the US during WW1, in the interest of making more bolts faster. Some screwball threads hung on for a lot longer.

Getting back to Chevy Four head bolts, there were a few old threads about it. It isn't exactly Whitworth, but it is close. A little over a year ago franco posted this:

Real standardization finally came about during WWII, and in the 30s, all sorts of goofy stuff is still possible. There is a little access panel on my 36 Pontiac in the RF fender to access the valves (its a flathead). The bolts look like 1/4" UNC or UNF (I forget which), but a 1/4" bolt wont go in. IIRC it turned out they were inch threads turned on a 6mm diameter.

There ya go you learn something new every day,wikipedia has a explaination of bsw v u.s. special thread ,so looks like i wont be useing bsw bolts from the local bolt shop.i have ordered a head bolt set from the u.s. off ebay, lucky i managed to find 6 main bearing bolts out of 2 motors that weren't streached, anyone know how easy bolts with this old u.s. special thread are to get in case others in the chevy club here are looking for them thanks

Just looked at add for chev head bolts ,says thay are British threads not american ???????????????AMG ebay,chev 4 head bolt set

Hopefully some of our Chevy Four gurus can comment on that.

My understanding is that the Filling Station's bolts are the most correct thing available, and the only detail that is wrong is that they have 55 degree threads instead of 60 degree.

I believe the owner jumped into a thread and commented that 60 degrees would have driven the cost through the roof, and that he had been supplying 55 degrees for over a decade with no comebacks, the implication being that it is close enough.

Take my post with a grain of salt. I don't think I imagined it, but I cant find the thread.

Ok so by running a 55 degree (witworth )tap through the 60 degree uss origonal threads ,allows the replacement witworth head bolts to be used ,thay are both 20 threads to the inch,just thread angle is different ?

Talk about confusion! Mercy!

I hope this works out.

Best.

Charlie

The head bolts are not fine thread. Typically it is more difficult to cut fine threads into cast iron. Therefore most drilled and tapped holes in cast iron are coarse thread, whatever the thread type. Even the British with their obsession to use fine threads everywhere used coarse threads in cast iron. For example the studs to attach the head for a Triumph TR4A block are coarse on the end that threads into the block but fine on the upper end where you use a fine thread nut.

As you read through this post you will see that there is a difference in pitch. The 1/2" BSW is 12 threads per inch while the UNC is 13.

I have no direct experience with how it would work to mix those. My engineering assessment is that with enough torque you could force them together. But you would never be able to tell when you were actually clamping the head to the block. The torque to rotate the mismatched threads would be so great you would never be able to measure the torque created due to the clamp load. And good luck getting things apart.

We are not talking unc we are talking uss, since,uss is 60 degrees by running a 55 degree witworth tap through it,im gessing this would turn the uss thread into a 55 degree thread,and this wouldnt work the other way around.? Ps . uss or uns im confused ???