Cylinder Head Combustion Chamber Template

Hi,

I have been scratching my head in the garage and thought I'd pose the question on here.

I am modifying an A Series Cylinder Head and need to make a template of the modified cylinder head chamber profile so that I can easily scribe it to chambers ready for grinding.

Here is my modified chamber:

It's easy enough to make a template using thin card, but I need to be able to precisely re-locate the template on the other chambers.

In Vizards book "Theory and practice of cylinder head modification" he suggests that the only reliable method of locating the template is to use the valve guides. He shows a picture of a template and suggests making two holes that correspond to the centre distances of the valves.

What it doesn't show is how to achieve this and I am really struggling to some up with a method of doing this.

If I make the template first, how on earth do I ensure the holes for the valves are correctly located?

If I start by getting a sheet of template material and drilling two holes with centres corresponding to the valve centres, which I believe is 1.343"? How do I them transfer the template shape onto this accurately, ensuring that the valve centre holes are correctly located on the template.

Stumped......

I suggest a 3D template rather than just a 2D cutout template will be needed to get all cavities the same.

If you apply a mould release paste, or paste wax, then pour in a low shrinkage two part urethane casting resin, an accurate hard resin "plug" can be made. This can be used to duplicate the chamber exactly, with a duplicating mill, or digitized with a scanner or CMM to get data for CNC programming a CNC mill, Even plaster of Paris can work if a low shrinkage grade is used. If you pour the impression through a "deck plate" with holes lining up with machined holes in the head, and a centre hole for pouring in the impression material, the impression can be located properly to the head. If you do want to use the valve guides to locate it, you could plug the ports with plasticiene and use a couple of cut off scrap valve stems in the guides, cast into the impression, to locate it that way - but that is much more work for not much benefit or increased accuracy. Probably not needed unless you are tuning a formula one race engine.

.

By choosing your template material carefully, and then using a ball-pein hammer [very carefully] to repoussé the profile of your specimen into the card.

Then remove the card and cut round the outline.

… Similar to making paper gaskets from scratch.

MichaelG.

Edited By Michael Gilligan on 13/09/2021 21:32:38

What about body filler (P38) I’ve got a tub full?

I’m happy with how to make the template, what is the issue is how to ensure the holes for the valves are in the correct place to enable positioning it correctly?

.

Sorry … I thought that bit was obvious when you wrote:

If I start by getting a sheet of template material and drilling two holes with centres corresponding to the valve centres

It appears that you have already done one chamber: so I assumed that you would copy the outline from that.

… if I have missed some important point, please excuse me.

MichaelG.

^^^
MichaelG.

I needed to work out how to ensure the template could be lined up on each chamber using a couple of valves. Someone on another forum has suggested turning the head over and tapping the template material on the tops of the valve guides. Once the valve guide holes are marked on the template, place the template on the chamber and drop two valve me through the holes to hold it in place.
Then tap the edge of the profile through the template (as has been suggested).

This will produce a template with holes corresponding to the valve guides.

This template can then be placed on a chamber to be marked out.

Perhaps you need a third reference point, like that hole on the left hand side

3 points are better than 2

Edited By Ady1 on 13/09/2021 22:51:15

.

Only if each group is identically configured

… that third hole is amost certainly not as accurately sized/positioned as the valve guides

… If using two valve stems was good enough for DV, it would be good enough for me.

MichaelG.

Hi William,

How about a commercial 3d scan and have them output a file of the flat surface of the head, .this could then be output possibly to a sign-writers stencil cutter. This is food for thought for myself when I need to re-do a head gasket for my1926 Renault, as the previous gaskets re-cut from old gaskets and do not necessarily pass over the head studs.

Regards, Nimble Neil.

My Herbert 0V milling machine has a page in the manual for doing this very thing (actually for copying dies, but it's the same principle). It describes using a ball-ended cutter and an identical probe (just a metal rod with a round end the same diameter as the cutter) set over the pattern. It has a fast-travel x-axis screw too (2.5tpi) to facilitate the work. The cutter and probe are set up over their respective parts, the operator keeps weight on the quill handle and uses the traverse to run the proble over the die to be copied. The cutter follows the path of the probe and makes an identical pocket needing only minor finishing.

A bit like a lathe hydraulic copying attachment except fully manual and producing 3-dimensional copies rather than the lathe's 2-D.

Obviously the better ‘engineering’ approach, Pete

MichaelG.

“If I make the template first, how on earth do I ensure the holes for the valves are correctly located?”

Can’t you poke a (suitably sized) pointed rod through the valve guides to mark the rear of the template when it is held in the correct position? (Then create holes at the marked positions).

Posted by William Harvey 1 13 / 09 2021 20:43:15

What it doesn't show is how to achieve this and I am really struggling to some up with a
method of doing this.

If I make the template first, how on earth do I ensure the holes for the
valves are correctly located?

If I start by getting a sheet of template material and drilling two
holes with centres corresponding to the valve centres, which I
believe is 1.343"? How do I them transfer the template shape
onto this accurately, ensuring that the valve centre holes are
correctly located on the template.

Stumped.....

-------------------

I faced a similar problem in locating matching holes in a blind plate,
although not the same as your situation the solution is much the same.
Seen here in the photo the two halves of a belt housing ,the main
part of the fabrication has some soldered in bosses to hold the cover
plate ,as these were in fairly random locations the cover had to be drilled
to line up with these threaded holes.Set up on the milling machine a
clearance drill is dropped into a hole to set the position and then the
cover plate which is oversize is held in place and just spotted ,the hole
is then drilled ,with the first hole in place the same procedure is followed
until all of the holes are drilled.The oversize plate is scribed around the
profile and bandsawn and filed to match the body of the housing.
The second part is much the same in the the centre hole has to line up
with the location hole in the rear of the body ,the housing is bolted down
and the centre of the location hole is located,the large washer and bolt
fitted to secure ,the cover plate fitted and the opening milled away.

It is useful to have a milling machine to do this sort of thing ,as you have
a pillar drill much the same can be done ,i would think that it would be easier
if you had a sheet metal template and by locating the valve guide with
suitable size rod in the chuck and then using a small drill to spot the
location with the template just held in position ,fixing then the template
with a suitable fixture in the first hole use the same procedure
to locate the second hole.

Simples. Use a transfer punch of the correct diameter inserted down the valve guides.

If you dont have a range of punches. Grind a point on the end of an old valve, and use that.

This assumes that your template is a positive (a plate the shape of the chamber) and not a negative (a hole the shape of a chamber).

Cheers, Colin

The way I used to do it was by making a pattern from about 1mm aluminiun and filing it to fit in to the profile so that the top surface is level with the head surface. Drill the two holes through the valve guides. Drop the valves through the pattern and into the guides. Mark the head surface using the outside of the profile and machine/grind the chamber until the pattern is flush with the head surface. Using this method all the chambers can be very accurately ground to the final size. I found over the years that I didn't have to do any more fettling to achieve accurate matching.

I haven't flowed a head for more than 20 years but I have never got all the cast iron dust out of my hands!

Here's the page from the Herbert manual, if it's of any use.

Regards

Pete.

You clearly need to make the holes first then. It should be a simple task to find the valve guide separation.

Create identically spaced holes on your template material then form the outside profile. If you use thin perspex sheet you can see through to pick up the original profile.

regards Martin

Don't forget that you need to turn the pattern over for adjacent cylinders. Which to me means holes in the pattern to match the valve guides.