I have a 1952 mercury with a newly rebuilt 255 flathead engine. I am wondering if it is necessary to use the break in procudure described in the manual from 1952. I’ve had other people tell me that since oil is much better now I don’t need to be so “strict” in following this break in time. Do I need to be worried about things not getting “set” properly? If I just take it easy on the car for the 1st 1000 miles or so should I be alright. I was also told not to use the overdrive for the first 1000 miles. What should I do?
Most of the break-in will occur in the first 20 miles (true even on the older oils) but it is a good idea to just drive it normally for the first 1000 miles. No wide open throttle, extended high speed cruising without letup, etc. The overdrive is not an issue. You’ll be fine.
My personal opinion is that an engine oil/filter change should be performed at the 1000 mile mark. This rids the oil of minute particles that will be present from the break-in. An oil filter is not going to catch all of those particles so the sooner they’re out the better is my position. JMHO and hope it helps. 
When I rebuilt an engine, the maker of the piston rings that I used called for about 8 to 10 accelerations from 40 to 60 mph at full throttle to get the rings seated to the cylinder walls. Yes, they (Perfect Circle) said full throttle. Part throttle does not force the piston rings against the cylinder walls and the rings do not break in.
Please note, I did not say to rev it to the redline.
Here’s a link to a second opinion from a major aero engine maker. They caution strongly against running the engine at less than 65% full power for the first 50 hrs.
http://www.factoryengines.com/docs/Hard%20Facts%20About%20Engine%20Break-In.pdf
I agree with both of those points. In addition to the minute breaking particulates, I’d guess that newly rebuilt motors have more bypass than a broken in engine, at least until the rings seat in, and that manifests itself as dilution and bypass-gasses contamination.
I suspect the “floor it” idea comes from the fear that an engine not “pushed” will not travel its pistons to the top of the cylinders and will wear a “shelf” into the cylinders. OK4450, you’ve rebuilt tons of engines. Perhaps I could get your feedback on this?
If I had a prize like that I think I would treat the new engine normaly for about 100 miles. Avoid interatate speeds of 75 or so and keep an eye on temperature, Oil pressure and such. Aftet that simply enjoy your great car.
LEE
Small airplane engines have nitrided steel or hard chrome plated cylinders. I would not be inclined to equate an airplane engine’s break-in requirements to that of the OP’s cast iron cylinder engine. The cylinder bores have the more critical requirements regarding break-in of all engine components. I’d go with the break-in requirements in the original owner’s manual with maybe a light extra push to allow for modern oil protection.
Note too that the latest oils with SL and SM designation do not have enough metallic antiwear compounds (Google ZDDP) to adequately protect the flat face cam followers in the OP’s engine. Modern engines can tolerate these newer oils as they have roller cam followers. The metallic antiwear compounds deteriorate catalytic converters which is at odds with the EPA’s most recent emission control durability requirements. It would be prudent to use a couple of quarts of diesel engine oil with the engine oil. If the OP has resistance to mixing engine oils, then use all diesel oil as the car has no catalytic converter to degrade.
I agree with you including the part about newly rebuilt motors having a bit more contamination due to gasses.
Needless to say, I don’t agree with the floor it idea. I’ve never heard any car maker recommend it and one can safely figure that the average owner is not going to do this to their brand new car unless told to do so by the car maker.
The piston travel is going to be the same no matter if the engine is at idle or 6k RPM. I also have to disagree with any aircraft analogy. Aircraft engines use different metallurgy, different oils, and operate under completely different conditions.
Airplane engines are usually idle, cruise, and full power with not much ever going on between idle and cruise. Even compression figures are far different than an auto engine (much lower).
There’s also another glaring difference between automobile and aircraft engines.
http://cgi.ebay.com/ebaymotors/Cessna-Aircraft-Tachometer-Tach-SL1010-001-1-1_W0QQitemZ230128540124QQcmdZViewItem?hash=item230128540124&_trksid=p3756.m20.l1116
Sixty-five percent of full throttle on a Cessna is not exactly an abundance of high RPMS (note that cruising RPM band/red limiter in the pic) and exhaust gas temperature is very critical during an aircraft engine break-in period.
JMHO anyway.
Thanks for the feedback. I’m an advocate of not driving a new engine hard, but I’ve heard the “shelf” theory occasionally over the years and thought I’d get your thoughts on the subject.
With respect to those who pointed out that full throttle forces the rings against the cylinder walls with more force, while it’s true that rings are canted, designed such that downward force pushes them against the cylinders, the spaces between the pistons and cylinder walls through which pass those forces are so minute that I doubt that the difference if radial forces of the rings against the wall is meaningful. I’m open to technical data to the contrary.
My own practice has always been to drive the engine the same way during the break in period that I plan to drive it long term. And it’s always worked for me.
I just realized that this is a 1952 engine, rebuilt (I assume) with replacement parts for a 1952 engine. I admittedly have no idea what the “break in procedure described in the manual from 1952” is. I’d be interested in hearing what the 1952 recommendation was. OP???
My guess is he should follow the 1952 recommendations.