...Lapping does Not widen the Seats. It Only deals with the 'One' Angle where the Valve MEETS the Head (Seat). 'That' Single angle is cut on the Valves at 44 Degrees, on the SEAT at 45 Degrees. You can see the two angles meet at a Taper. Lapping Insures the 'Contact Area' is a 100% Custom FIT!
The Bores are Perfectly Round, there should be Hash Marks at 30 Degrees in Both Directions. These Hold 'Oil' from the Oiling Ring to Lubricate the Cylinder Walls, Piston and Compression Rings. Ofcourse the Oiling Ring.
The Pistons are Cut as a OVAL, this is so there is 'Room' for the Piston the Expand when HOT. When Cold these Leak in a 'Leak Down Test'. The Ring Gaps. although 'Staggered' will Leak when COLD. These Gaps are Calculated to come nearly Closed when Hot.
When COLD any Engine will show leaks in a L.D. Test, even though the Valve seats are PERFECT! It would be just about Impossible to Do a L.D. Test on a HOT Engine. But more than likely...Very Successful.
I agree with Doug and All Others who Lap their own Valves. Shops are in a Hurry to be done and get paid. And some just don't Have the Disciplines!
...Some racers will Have Just One ROUNDED Seat Cut, in the Head, with the Valve cut 'Normally'. With an even THINNER Contact Area! and No other angles in the way of the Fuel/Air Flow, within the Head. These Must be 'Drag-Racers'.
Doug Makes Sure of His Valve Seats!! You Dig!! I Applaud You!
All the Best,