Finding 30 BTDC on a V12 without access to the flywheel?

Apologies for the non-F car content.

I am chasing a rough idle in my 92 Diablo 2wd. Fuel and spark issues have been tracked down and corrected, but the issue persists.

We want to check timing next.

A Diablo has three references which need to correspond to get the timing right. At 30 degrees BTDC on cylinder 1, the bottom phase sensor and the tab on the 7-12 exhaust cam need to be lined up. At that moment, an arrow on the ignition rotor needs to line up with a mark on the distributor rotor base.

All this assumes the cams are lined up correctly, but fortunately that can be checked with the engine in place.

Both the phase sensor tab and the ignition rotor are infinitely adjustable, so I need to check both are set correctly.

This is easy with a degree wheel and the engine out of the car, but not so easy with the engine in the car. There are (as far as I can see) no marks on the flywheel when peering through a sensor hole (unlike a Countach, a Diablo has no timing inspection hole).

Obviously it is easy to find TDC on every cylinder, but given the firing order, is it possible to calculate which cylinder is at TDC 30 degrees before cylinder 1 reaches TDC?

By my guess a cylinder fires every 720 degrees of crank rotation. If the firing of the cylinders are equally spaced, then one cylinder fires every 60 degrees of crank rotation. Which means one of the cylinders should be at TDC on the exhaust stroke 30 degrees before cylinder 1 reaches TDC. Is my logic sound? In another way of thinking about it, how can I calculate which piston is 360 degrees away from TDC on the ignition stroke when cylinder 1 is 30 degrees BTDC?

If I am correct, this would be a "cheat" not printed in the service manual, very Lamborghini.

 

Alas, no vibration damper at the end of the crank on a Diablo.

I did get a call today from a gentleman in the UK who knows a lot about early Diablos. There IS a flywheel inspection window, but remarkably well hidden. I did not find it after quite some time looking.

There is a dot drilled into the flywheel 30 degrees BTDC apparently, so my mental gymnastics will remain theoretical for now.

 

A Diablo is supposed to have an inspection window drilled on the bottom of the sump where it meets the bellhousing.

But mine was built without one. Remarkable. Must have been built a Friday afternoon.

 

The Diablo had more design changes than most! The VT version (1994) necessitated an entirely new and larger flywheel and clutch. Oh, and they reversed the direction the crank spins...