Ferg has suggested an alternative method for locating and drilling the holes in the landing gear frame, through which it is bolted to the cockpit module.
Having just completed this phase of the build, I penned the following in the hope that it might save some other poor fish a few moments' head-scratching - to say little of the cost of a tight 90deg drill chuck.
Please note I have not cleared with any authority
regarding the 1/16inch datum holes regarding structural integrity,
so in your bailliewick that may be a wise precaution. Filled with
420, I can't think they amount to much.
"DRILLING HOLES FOR GEAR/ENGINE MOUNTING FRAME Mark 1 manual, page 22-2
Assuming you have correctly inserted the mounting frame to the firewall, the method of positioning, drilling and mounting the -3 bolts becomes paramount.
The manual says, "Mark out, centre-punch and drill the attachment tubes according to figure 3", and ".... mark the tubes' centrelines onto the top of the wheelwell as accurately as you can."
Because the 'attachment tubes' (I call them longerons) are at discrete angles, their centrelines are quite difficult to divine - as has been found by some builders. Piercing them with a drill at the accurate centre is a soul-searching experience - as the bottom pair are easily drilled from outside (in the wheelwell) but are inside, and the top pair are similarly outside but easily drilled from the cockpit side. This begs the question. 'How to position?'
It eventually occurred to me to use a 'temporary datum' hole as I had done on an earlier job.The lower longerons extend inwards from the firewall FORWARD face by a bit more than 5 inches. So I drilled a 1/16 inch hole about 5-3/8 inches from the firewall for'd face along an estimated centreline. I then inserted a tight-fitting nail and peeked around the corner. Not bad. I missed the centreline by 1/16 inch and the end of the longeron by 5/32.
The hole became the datum by which I could correct and site the two horizontal holes in the lower longeron. I drilled with a new 3/32 bit, inserted another tight-fitting nail and peered down the longeron to happily find the nail(s) centred, and then re-drilled with a 4.8 mm bit.
I won't go on but the same principle can be used for the upper longerons. This saves the purchase of a special drillbit chuck in tight corners as all 6 horizontal and 2 vertical boltholes may easily be accomplished with a normal handheld electric drill - assuming proper aiming and so forth.
The only reservation to this scheme is that the 1/16 inch datum holes should be vetted through PFA (for UK builders) for structural integrity and filled with 420 flox I suppose. However I can't think this poses any more stress than the myriad cleco holes along the waistband. [I imagine the PFA might like the foam cut back from the holes by a few mm (if you missed the embedded metal plates) so that the flox can lock itself to the inner skin surfaces, in the usual way - JFC.]
Any questions answered, flamers denied.
Ferg is at VE3LVO@rac.ca