Or I could ask the owner again about the possibility of drilling a small inspection hole..
Roger, I came to the conclusion that Egli really did not stray all that far from the Series C Vincent ufm when designing the Egli top tube portion.
He did jettison a lot of weight and at the same time remove a great deal of flex with the addition of those little side struts that run down to the lower engine mount.
When cornering one is in compression the other in tension to keep the rear wheel in the same plane as the front wheel.
As far as the Egli top tube, I used the general layout of the C Vincent UFM for lug position fore and aft. For steering head angle I used 27degrees, same as Norton Manx.
My headlug offsets were a one off for that bike due to the wide tire and mono shock.
Glen