As an introduction though, why not go to the Blog overview first.
These are magnificent chrome on brass hand made vintage style bonnet catches made by a small workshop in Lancashire England. Only this type will pass the UK MSVA test due to the fully radiused nature of them.
Now, the first thing to do prior to locating these is to ensure the fit of the bonnet halves which will have been hinged by me. Correctly locating them is to bolt in the hinge pintles and ensure that the nose cone is correctly spaced. The nose cone has slots enabling it to move fore and aft, however, you might wish/need to enlarge these accordingly.
The next thing to do is to mark a line from the front edge of the bonnet as can be seen above, from the edge of the nose cone, back to the edge of the scuttle where the bonnet touches. Then mark a line in front of the first one, 15mm from the first line.
Then mark the bonnet halves as below:
For the front catch, mark the front of the bonnet halves with a co-ordinate 50mm in from the front edge and 85mm from the lower edge of the bonnet, draw a horizontal line. Disregard the upper line shown above. Centre the catch knob, then drill holes accordingly through the bonnet. Note, I have used M5 socket button head screws with nylock nuts and washers on the rear. You might want to rivet these catches. SBH screws allow you to move the catch around a little to centre it.
Having fitted the top catch/latch, draw down the knob centre line and extend it at right angles to the upper deck aluminium plate as shown above, then centre up the main catch mechanism and drill holes accordingly. The holes in the catch plate and mechanism are 4mm so you will need to drill these out to say 5mm or 5.5mm (allowing for movement) if using M5 SBH screws.
This shot clearly shows the upper catch and the knob centred up on the vertical line.
Here we are at the rear catch point. We now need to use the following measurements/co-ordinates:
A vertical line drawn 50mm in from the rear edge of the bonnet, and a horizontal line 65mm up from the bonnet lower edge.
Note that the main mechanism is then located below the bottom edge of the aluminium top deck trim strip. I haven't shown the side panel fitted at this stage, but of course, you will be fixing through the side panel and into the horizontal top chassis member. I have used riv-nuts and therefore M5 SBH screws, but you might well wish to use rivets. I would advise obtaining a riv nut tool...they are invaluable for all kinds of jobs.