Question for Joe Lucia and Our Engine Experts>Exact Length of Chevy BB Cyl. Bolts

[Larry E.]

Building a 1968 L36 for display in my "mancave". Trying to make it as original in looks as possible. Block is
made of HD plastic and is exact duplicate of the real block.(Went this route for weight) All add on parts
(eg.-heads-oil pan-timing cover-intake manifold) will be the real parts. Question pertains to the cyl.
head bolts. We know that the bolt size is 7/16 X 14 but need to know the exact length of these bolts
(Both the long bolts and short bolts) Also need to know the typical head markings. Do not want to buy
complete set of bolts(eg. Paragon and etc.) as I will not install them all. Will buy only the bolts I need
from AMK. Need your help on this. Below is picture of my block in the cave. Thanks in advance-Larry

[Gene M.]

Re: Question for Joe Lucia and Our Engine Experts>Exact Length of Chevy BB Cyl. Bolts

Easy. If just for viewing measure the head thickness at the bolt holes and add 1/2”.

[Larry E.]

Re: Question for Joe Lucia and Our Engine Experts>Exact Length of Chevy BB Cyl. Bolts

Thanks Gene for the response but don't have the heads handy to measure what you suggest and also need the
head markings of the bolts. Want to make the bolt order from AMK ASAP. Thanks--Larry

[Edward B.]

Re: Question for Joe Lucia and Our Engine Experts>Exact Length of Chevy BB Cyl. Bolts

Larry, see attached.

The short bolt is 2" long and the long bolt is 4" long. (Note that they appear about an 1/8" longer than what they are, but that's only because of the angle I took the pictures from.)

Markings are a plus sign with an M for the short bolts and a plus sign with an O for the long bolts, but I've seen other marking before.

Ed

[Mark E.]

Re: Question for Joe Lucia and Our Engine Experts>Exact Length of Chevy BB Cyl. Bolts

Larry,

What a cool project. If I was single, I would do something like this for the living room.

[Joe L.]

Re: Question for Joe Lucia and Our Engine Experts>Exact Length of Chevy BB Cyl. Bolts

Building a 1968 L36 for display in my "mancave". Trying to make it as original in looks as possible. Block is
made of HD plastic and is exact duplicate of the real block.(Went this route for weight) All add on parts
(eg.-heads-oil pan-timing cover-intake manifold) will be the real parts. Question pertains to the cyl.
head bolts. We know that the bolt size is 7/16 X 14 but need to know the exact length of these bolts
(Both the long bolts and short bolts) Also need to know the typical head markings. Do not want to buy
complete set of bolts(eg. Paragon and etc.) as I will not install them all. Will buy only the bolts I need
from AMK. Need your help on this. Below is picture of my block in the cave. Thanks in advance-Larry

Larry------


The long bolts were GM #3877668. GM says these bolts were 4-1/2" in overall length. I've never measured any so I can't say, for sure. This part number is GM-discontinued but I expect you can find some on eBay. Even used examples will work for your purposes.

The short bolts are GM #3877669. GM says these bolts are 2-1/16" in overall length. This bolt is still available from GM.

[Edward B.]

Re: Question for Joe Lucia and Our Engine Experts>Exact Length of Chevy BB Cyl. Bolts

Joe, I agree on the long bolt (4.5" Overall), but boy, all my short bolts are 2.5" overall! I can't find ANY that are 2.0625" overall. I don't have a ton of them, but they are originals, so I can't see why such a difference between what I have and what the books say.

Ed

[Joe L.]

Re: Question for Joe Lucia and Our Engine Experts>Exact Length of Chevy BB Cyl. Bolts

Joe, I agree on the long bolt (4.5" Overall), but boy, all my short bolts are 2.5" overall! I can't find ANY that are 2.0625" overall. I don't have a ton of them, but they are originals, so I can't see why such a difference between what I have and what the books say.

Ed

Ed------


GM spec for bolts often differ from reality. I don't know why this is. GM says that the bolts have the overall length I reported. However, I think that in the case of the 3877668, it is overall length that's specified at 4-1/2". In the case of the 3877669 I think that despite their use of the term overall length, what they mean is the shank length of the bolt (i.e. overall length minus height of the bolt head). This is actually the customary way to specify bolt length.

[Edward B.]

Re: Question for Joe Lucia and Our Engine Experts>Exact Length of Chevy BB Cyl. Bolts

Thanks Joe. Shank length is the way I was taught by my grandfather, a machinist for over 50 years, to measure bolt lengths. It seems strange that GM would specify "overall" length when it was an industry standard to use "shank" length. And in the case of the 3877668 and 3877669, one specified overall and the other shank!!! Very strange indeed.

Ed

[Joe L.]

Re: Question for Joe Lucia and Our Engine Experts>Exact Length of Chevy BB Cyl. Bolts

All-----


One other thing: on original big block head bolts the "cross" on the bolt head usually extends all the way to the ridge on the perimeter of the bolt head. Later bolts have the cross not extending all the way to the perimeter.

[Larry E.]

Re: Question for Joe Lucia and Our Engine Experts>Exact Length of Chevy BB Cyl. Bolts

Ed------


GM spec for bolts often differ from reality. I don't know why this is. GM says that the bolts have the overall length I reported. However, I think that in the case of the 3877668, it is overall length that's specified at 4-1/2". In the case of the 3877669 I think that despite their use of the term overall length, what they mean is the shank length of the bolt (i.e. overall length minus height of the bolt head). This is actually the customary way to specify bolt length.

FWIW: Made my order with AMK. To my surprise they had a bolt listed for the Chevy BB (Short Bolt) which they
claim is 2-1/16" in Shank Length. No bolt was listed for the (Long Bolt) so I had to find/look at other 7/16-14 bolts.
The one which was close was listed for the 260/289/302 Ford Heads. Shank length was 3.91". The head markings
short bolt did have the "+" marking but the long bolt has a "F" marking. Thanks for all the help>Larry