A Series manifold stud spacing

[Maddog]

Can anyone please confirm that the nominal spacing for the manifold studs on a conventional 5 port A Series head is 2.5 inches between centres for the end exhaust ports, and 3 inches between the inner four studs? I have measured up a 12G940 head and the spacing seems very close to this, but the four end studs are all out by about 30 thou. Probably just manufacturing tolerances but would like to know the drawing dimension to satisfy my OCD.
Maddog.

[Spider]

The old drawings I have are not the easiest to read.

Between the Centre 2 Studs is 3" even.

From these to the next 2 out each way is 3-1/32".

Now,,,,, the end ones are 2-19/32" from the Centre of the End Head Studs / Rocker Studs (On the row of 4). Don't ask me, it's how it's drawn.

[Maddog]

Thanks Spider, that is fascinating. So, based on my measurements the 30 thou that I thought my studs were out due to shoddy manufacturing tolerances is in fact actually pretty spot on to the 1/32 that you have quoted. I will adjust my dimensions!
Cheers
Maddog

[mk1]

Copy of the salient bit of the drawing for a 295, dims are the same across all A series heads.

Click on image for larger version.

12 G 295.jpg

[Spider]

Thanks Spider, that is fascinating. So, based on my measurements the 30 thou that I thought my studs were out due to shoddy manufacturing tolerances is in fact actually pretty spot on to the 1/32 that you have quoted. I will adjust my dimensions!
Cheers
Maddog

Ha, so you can measure !

I think the drawing Mark has kindly posted up shows the weird way they dimensioned the very outer studs.

[GraemeC]

I think they're referenced from the centre line of cylinders 1 & 4 rather than the rocker stud (although appreciate this is effectively the same). I agree it is a very odd way to dim them up though - maybe a gasket or manifold drawing would show something more sensible (if they were available).

I'd have got a right ticking off if I'd dim'd a drawing like that - lots of scope for tolerance stack up creating all sorts of errors!

[Spider]

I think they're referenced from the centre line of cylinders 1 & 4 rather than the rocker stud (although appreciate this is effectively the same). I agree it is a very odd way to dim them up though - maybe a gasket or manifold drawing would show something more sensible (if they were available).

I'd have got a right ticking off if I'd dim'd a drawing like that - lots of scope for tolerance stack up creating all sorts of errors!

I did double check on the drawing I have off just what they were measured from and yes, it was the drilling for the head & rocker studs.

Totally agree on the tolerance stack !

[Spider]

While looking for Camshaft Drawing, I stumbled over a stock Cast Iron Manifold Drawing this afternoon (don't ask,,,,,)

It shows the 2 centre studs as being 1-1/2" from the Centreline.
Next 2 studs out, 4-17/32 from C/L,
Next 2 stud 5-31/32"from C/L, and
the outer 2 studs on a dead 7" either way of Centreline.

[GraemeC]

8 studs on a manifold?

[Spider]

8 studs on a manifold?

More is better !

Scratch those at 5-31/32" - that's the port centres.

[Maddog]

Thanks to Spider for the original BMC drawing dimensions. To close the loop on this, for anyone else looking for the information, here is a table I compiled to give cumulative measurements from left to right indicating the stud spacings (space between stud centres).
Maddog

[Spider]

Cheers Maddog. That table should be handy. I take that it's referenced from one of the studs at one end ?

[Maddog]

Spider, yes that's correct. Apologies if I didn't point that out.