Cylinder head centralising studs.

[philthehill]

If you need to fit a 10th & 11th cylinder head fixing (stud or bolt) you do need to make sure that the head is fitted square on the block before drilling.
Only the o/s centre stud on a 'A' Series cylinder head is fitted, the other 8 are clearance and they are quite a loose fit in the cylinder head stud holes.
To ensure centricity I have adapted the following Minor parts. Two rocker cover retaining nuts and two short cylinder head studs.
The rocker cover retaining nut in standard form is a nice close fit in the cylinder head stud hole. The rocker cover nut is drilled and tapped right through.
Note:- Not all rocker cover retaining nuts have the same shank diameter and therefore may require a slight skim to fit in the cylinder head stud hole.
The two short cylinder head studs are reduced in diameter at the UNF end and rethreaded to 5/16" UNF.
To centralise the head on the block the two modified studs are screwed into n/s front and rear stud holies in the block. A standard stud is screwed into the centre o/s stud hole. The head is then fitted and the modified rocker cover nuts are fitted to the two modified studs and tightened down.
This action now provides three fitted studs in triangulation so keeping the head centric to the block and unable to move.
The head and block can now be drilled.
The 10th stud at the rear of the head is 3/8" UNC into the block. The 11th fixing at the front of the head can be either a 3/8" UNC into the block or as per ex factory a 5/16" UNC bolt into the block.
Whatever you use for the 11th fixing it should not be torqued more than 25lbf ft as the metal above the water pump aperture is quite thin.
A tip is to paint the 11th fixing RED to identify the difference in torque setting.
Below are the modified centralising studs:-

Head aligning studs.JPG (1.55 MiB) Viewed 1011 times

[philthehill]

Further to the above and some random lock down thoughts
Is it a coincidence that the OD of the rocker cover nut shank is the same as the ID of the stud hole in the head or was it planned to be?
Were the rocker cover nuts the same size so as to assist in keeping the head central over the head gasket? The head is able to pivot quite a bit around the OS centre stud without the centralising studs being used.
The head gasket is usually quite a good fit around the cylinder head studs so would not need to be centralised but the head can move around on the gasket. The contact between the head gasket fire ring and the edge of the combustion chamber especially Nos 1 & 4 cylinders of the head could be reduced if the head was on max pivot leading to reduced life of the head gasket.
The centralising studs can be used to ensure that the fire ring has maximum contact between it and the head and also ensure that the head is is line with the bores. Once the other studs are lightly nipped up but not torqued the two centralising studs can be replaced with normal head studs and the head torqued as normal.
Phil

[kingsley]

Excellent info Phil.
Thanks for sharing

[Sleeper]

Ditto

John ;-)