IMO there's no reason to do break-in on any fuel other than what you plan to use. If that's propane, great. If it's gasoline, don't bother doing a propane conversion just to break in the engine.

I've never done anything fancy with the deglazing process.

Initial engine oil fill should be on 5w30 regular oil, not synthetic. Do one initial start-up to make sure everything works properly and check for leaks. On the next startup, be prepared to drive it. Get to somewhere that you can do the following. At about half rated max RPM (probably 2500-ish in your case), accelerate hard for a couple of seconds, then let it coast back to the initial speed, and repeat that about 10 times. Then come back to the shop and drain the oil, and refill it once again with non-synthetic. Drive normally but avoid lugging it down and avoid long periods of full throttle for the first few hours of operation.

Yes, I should have clarified the reason for breaking in on propane is that it will be run on propane only afterwards.

Oil viscosity? I have never used 5W-30 in anything. 10W-30 most of the time. I agree with no synthetic for a period of time.

Makes little difference for what's being done with it - although, I have a strange feeling that enginesrus would second-guess the 5w20 factory oil recommendation for many newer engines (including the Chrysler Pentastar that I have in the van).

Evidently, factory oil recommendation for the 454 was 10w30.

running in means to have controlled wear for a certain period to improve parts conformity. later on that should result in easier lubrication of the parts involved and less wear. running in on propane may take somewhat longer because you are fuelling the engine with a dry gas and not with a incompletely vaporised fuel. however, i fully agree with BrianPetersen that using a fuel differing from what is to be used later on would be a unneeded complication.

modern engines usually come from the factory with no specific type of running-in oil - but of course the virgin mating surfaces have been treated in such ways that easy run-in will take place on the prescribed oil under normal driving conditions.

in your situation it might help to temporarily use a rather thin oil with a somewhat lower specification as wear reduction is concerned. a lower viscosity oil will assist in cooling not yet fully run-in parts and also might "allow" somewhat more wear.

the complete running-in process has two sides to it: you want temporarily accelerated wear with some extra cooling capability of the oil used (which is accomplished by the use of a lower quality thin oil) and you want the load to actually provoke the extra wear you want. thus, once you have checked that there are no leaks etc and oil is getting to all the parts that need lubrication, you will need to apply a load to actually get that wear. applying a more or less "excessive" load within the mid to high rev range will generate that wear and is helpful in getting the result needed. you should initially be careful not to apply that load too long, because local overheating of not yet fully mating parts might occur. hence the advises to vary the load for a while.

a relatively short running in period should suffice. after say 2000 km the engine should be fully fit for
normal use and the prescribed lubricant with the standard oil change interval can be used.

new engines usually come with a cross hatch pattern. the pattern plays a role in oil retention on the cylinder wall to aid in lubrication of the rings and to prevent excessive oil consumption, but the pattern has no specific role during running in. if you roughen up the bores with a ball hone, you might get the same effect, just do not overdo it.

Why would you use plain cast iron rings instead of something with a face coating that offers a bit more wear and scuff resistance?

5W-20 oil in new cars is for faster cranking (starts) on the EPA emissions cycle and a bit less viscous drag for fuel economy. Since film thickness is proportional to viscosity, thin oil in a new engine might keep the parts closer together for faster break-in. Is that a good thing?

Better cooling with thinner oil is owing to increased flow through the bearings?

Thanks for the replies everyone!
Cast rings are recommended in old propane training manuals. I used to work in the industry during the 80's and 90's.
I know moly is a bit better, but I thought plain cast might seat in quicker. I might be wrong, but I doubt it's a deal breaker for this build.

" Cast rings are recommended in old propane training manuals. "

Before you posted that, I was going to question if non-faced rings were appropriate for propane, having listened to stories about how gaseous fuels can be kind of tough on some components.

Now I would only ask, was the wording "recommended for use in" or "acceptable for use in", along with some qualifiers in the context, like, in used bores with measurable taper and out-of-round?

I figure unfaced rings simply can not provide nearly as long service life when subjected to the flash bore corrosion and condensation at start up, until the cylinder walls get up to temperature.
Of course other factors might make iron the only choice.

Oil selection will be important. Something that is both suitable for propane and for the 454.
This might take some analysis.
And some special attention to oil delivery, since your fuel does not have any lubricity.

New engine clearances are ridiculous, my wife's new Toyota V-6 uses 0-W20.

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P.E. Metallurgy, Plymouth Tube

Skip the abrasives, snake oils, etc, and do clean the block and all parts thoroughly before final assembly. Deglazing with the ol dingleberry will spread all manner of crap throughout the block so don't skimp on cleaning. Depending on the condition of the bores, the only thing I would do beyond a deglaze would be to check for and remove any cylinder top ridges with the appropriate reamer.

JMO but the best break-in method by far is good old fashioned motoring either on a motoring dyno or simply using a large electric motor. Spin it up to 3k RPM, make sure there's good oil pressure, and let it go for 20-40 hours. If you'd prefer a running break-in, I've never been a fan of stressing new engines with various heat and speed cycles and usually recommend a few roadtrips with hours of highway driving. Keep the RPM relatively low (<3k) and be slow on throttle acceleration. I'd use standard oil viscosity for that engine (10W-30?) but see if you can find it formulated for gaseous fuels, worst case ask either a Cummins or Cat dealer.

You are reconditioning a used engine using low tech parts and procedures; don't overthink it. The thorough cleaning after dingleballing is the best advice thus far. Cast rings seat easily and quickly. Bon Ami? The horror! The horror!

jack vines

Thank-you for the replies!