forgot to mention,hp500 does not use a high volume pump.sc800 does and i don,t know about the sc600 but i think it uses a standard volume high pressure pump.

I don't typically use Merc's "production numbers". I use their "service numbers" and seem to be more of what we are all accustomed to; The other thing, if using Service Manual #3, the original HP500's were really no different than GM HO short block with a Melling oil pump and the Crane 731 cam/springs.

Service #'s from Manual #3:
525SC: Main 1-4: .002-.003, #5 Main: .003-.004; Rods: .002-.0035
600SC/800SC: Main 1-4: .0025-.0035, #5 Main: .0035 - .0045, Rods: .0025-.0035

if I remember correctly even when using 5-30 ...there was still ticking at cold. Like I said using a lighter oil doesn't drop you low enough on the viscosity curve. The only alternative to different lifters is running oil heaters which sounds crazy to me on a stock blue motor.

Along the same lines; on thes engines that have the lifter tick, were your lifter bore tolerances checked? Curious if there's any correlation between the ticking and loose lifter bore clearances on older engines.

Not sure but wouldn't that be a constant tick regardless of temp?

Always a good question and something that should be checked.

Here comes another 'but', the Morels will check a little larger OD than most of the other aftermarkets that aren't made by Morel. Thus the reason why some aftmkt blocks (like Dart) that lifter bores are a little smaller than GM, will have lifters sticking with Morels and not usually the others.

One thing to watch also is going from a .700 wheel to a .750 is that your oil hole in the lifter does not extend out of the lifter boss. On merlin blocks this can be a problem.

I'm not sure. I think it would depend on the different rate of expansion between the cast iron block and lifter body.

Last summer I had a friend run his 500's EFI WFO for about 30 minutes and the lifers were ticking all over the place when he came off plane. A few minutes later once the oil cooled down, the tick went away.

Don't quote me on this but I think my machinist told me that the high rev (polished) morels have a slightly smaller body diameter. My Dart blocks didn't need to be machined to fit the new lifters, they were good as is.

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What type/weight of oil and what where the oil temps ?