how much do valves 'seat in' after fresh rebuild?

[lvporschepilot]

My general rule for a fresh engine when performing a leakdown test is to yield 5% at the most prior to the engine being run at all. No leakage through the valves, and any leakage should be via the unbroken-in rings at this point.

I was talking with a local machinist today who cuts his valve seats, laps the valves, puts it together and does not (!) run a compression test prior to firing the engine up and running the engine at least 400-500 miles. His reasoning is the valves are going to "bed in" after a few hundred miles of running and saw no real point to leaking down freshly cut and lapped valves. And of course after the 'bed in' peroid, you pull the covers and check your clearances and some of the valves will be a little tight due to bedding in.

While this reasoning makes sense, it's not my personal preference. Anyone else do this?

John

[cgarr]

Valves should not leak on a fresh valve job, that's why they are pressure or vacuum tested when done but there are a lot of parts that do have to get happy with each other (rings etc) thus the break in period prior to a leak down test. Lapping valves will not help the seal it will only show you what the contact surface looks like. There may be some debate about seat angles but I like to cut a 46 degree seat and leave the valve at 45 which gives you a slight interference fit which does nothing with the initial seal but over time maintain a good seal. I have really had no problems with valves sealing.

[Two Rivers]

Valve lapping is old school. With the modern tooling the seats do not need lapping. I use machinist blueing for the tell tell. I also cut the seat angle to 45 1/2 degrees and after 1000 miles don`t see much change in the valve clearance.