VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1912-1928 1925 Head Bolts - British Standard Thread?

There is an listing on ebay for 1925 Chevy head bolts. The listing states that the original head bolts for the vehicle were 1/2"-12 British standard thread. Can anyone confirm it is British standard and not American National Standard Thread?

Thanks........

Yes they are British. You can buy new bolts at The Filling Station.

According to my'28 Master Parts Book,they're described as-"1/2" X 12 U.S.S.,and they have a 60 degree thread form,unlike 1'2" Whitworth,which has a 55 degree thread form.

I would not buy used head bolts because they might be stretched.

When I took the head off my '28, three of the bolts were stretched. The way I checked is that if they screwed all the way in easily using only my fingers, then the threads were good. The three that I deemed bad all stopped going in easily at the same place, so I figured the bolts were stretched at that spot.

Cheers, Dean

The bolts are reproductions. What was strange was that they are British Standard. The last time I encountered British Standard in an American built engine was when I was doing volunteer work at the National WWII Aviation Museum in Colorado Springs. I was tasked with tearing down a Merlin engine (P-51)and none of my standard or metric sockets fit any of the bolts or nuts. The Merlin was designed in Britain and the majority of them were built there but the one I was working on had a data plate that said built in the USA by Packard. I was contemplating about writing a nasty letter to sears/craftsman about how poor their sockets were designed when an old aviator that had been flying since the dinosaur age came by and laughed at me said it's British Standard and your going to have to buy a whole new set of tools.

I have made new 1/2"x 12 screws today for a -27 engine.
On two of the pictures you can see how the screws are stretch.
The end of the screws the thread are fine. Half up the are a problem.

Captain D

CJP'29's post above is correct. They are not BSW (i.e. Whitworth) bolts. Up to 1907 1/2x12 was the standard American coarse thread. For some time after this the two 1/2 inch coarse threads were both accepted for normal use, and some manufacturers, e.g. Chevrolet, persisted with the 1/2x12 thread for special purposes for long after they had fallen out of common use during WW1.

The Chevrolet head bolts have a 60 degree thread angle, while the BSW 1/2 inch bolts use a 55 degree thread angle.
The 1/2 inch Whitworth bolts will usually screw into the 1/2 inch (now known as USS) tapped holes in the cylinder block but will not have the same holding power as the correct bolts.

Frank.

So is The Filling Station selling the wrong head bolt or are they just mislabeling them? I bought a set from them along with the tap when I had my head rebuilt about 8 years ago. I never had a problem.

I bought new head bolts and a tap from FStn. The Chinese tap wasn't a good enough match with the new bolts thread pitch or angle in my opinion, so I didn't use it for fear of spoiling the threads in the block.

The bolts fit the block OK.

I've just checked the tap angle with a 55 deg Whitworth fish gauge and the angles match, so something's not quite right there.

I am going to sum up the information contained in the above posts.

1. The original head bolt thread for a 1925 is 1/2"x12 U.S.S. 60 degree thread form which went out of use around 1907 with the exception of a few companies like Chevrolet.

2. The Filling Station sells reproduction bolts that are 1/2"x12 British Standard Whitworth that have a 55 degree thread angle (not the correct 60 degree angle). The bolts will work but the holding power will be less due to the incorrect angle.

3. The Filling Station sells a tap that is not correct for their bolts and does not match the original threads in the block. The tap has a 55 degree Whitworth thread angle.

If I made a mistake in my summation please correct me.

My thoughts on all this.

If a 1920s engine has been de-coked a few times and had the ocasional valve re-grind, with the stretched bolts re-used a few times and over tightened, the thread wear in the block would most likely be more than a 1/2 deg each side of the V.

So in reality, unless you're building a high compression custom engine I doubt it makes a lot of difference to us if BSW or USS are used, the repro bolts are high tensile so the softer cast iron threads should wear enough to fit as you tighten the bolts to a higher torque than standard.

This must have happened to hundreds of engines with FStn bolts fitted, the only problem I've heard about is where a guy tapped his re-built block UNC to clean up the threads wished he hadn't as his new bolts didn't fit well at all.

If in doubt a much better way is to have some studs made and use UNF nuts on top.

From my "1928 Chevrolet Master Parts Price List" book,here are the following head bolts and their part numbers:
Cylinder head Bolt,Short,1/2"-12 X 1 15/16" U.S.S.(Except D)
P/No. 326535 price 6 cents ea.
Cylinder Head Bolt,Long,1/2"-12 X 5 3/16",U.S.S.(Except D)
P/No. 326534 price 20 cents ea.

If the Filling Station head bolts are made as per original,then they should have the 60 degree thread form(check with a thread gauge),if not,then they'll be 1/2" X 12 TPI Whitworth bolts,which won't have the same holding power as CaptainD pointed out.
Todays standard off the shelf 1/2" coarse threaded bolts are 13tpi with a 60 degree threadform(UNC)or 1/2" coarse threaded bolts with the 55 degree thread form,12tpi(Whitworth)
Much easier to make studs out of hi-tensile steel rod,1/2" X 12 tpi 60 degree form for the block end,and 1/2" X 20 tpi(UNF)and use tall series nuts to hold the head down as per franco's idea.Studded my engine years ago,no problems since.

Steve from TFS here. We had these bolts made in 2006. Since that time we have sold 100+ sets of these bolts. What I have not heard in this thread is anyone that has had a failure using our bolts. Yes, in 2006 we had the threads cut to the British Standard thread. And, yes, it is slightly different from the USS thread used in the 4-cylinder era. However, this does not seem to have caused a problem. In the 10 years we have been selling these bolts, the only two problems I have heard about was one fellow that used a 1/2-13 tap instead of a 1/2-12 and one guy that torqued his head to 110 pounds!

Before we went to market with these bolts, I had a tech advisor from VCCA check them out and he was very supportive of the product. Today I checked with a fellow VCCA member in our local region who is a retired machinist. He did not feel that the difference between British Standard and USS. in this case, would cause any problems, as long as they were both 1/2-12 threads.

I just took a photo of the threads of our bolt and the tap we sell. While you can see a very slight variation, the difference is almost nil.


The real question that must be answered is will these bolts work and function properly? I must say - absolutely! We strive for perfection in the parts we have manufactured. While I would prefer to have the exact correct thread, these bolts are certainly usable and will function properly.

As CJP's 29 said

"Much easier to make studs out of hi-tensile steel rod,1/2" X 12 tpi 60 degree form for the block end,and 1/2" X 20 tpi(UNF)and use tall series nuts to hold the head down as per franco's idea.Studded my engine years ago,no problems since"

I studded my 28 with the same studs in 2014, no problems here.

Cheers

Ray