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  • Engine break-in RPM

    I'm putting a new engine in my MG Midget and I'm wondering about the break-in period. The formula I always used on my motorcycles was "50 mph for 500 miles" with occasional engine braking to draw oil up around the pistons.

    It occurred to me, though that the recent change I made from a 4.2 differential to a 3.7 would change the rpm drastically at that speed. Is there a better rule of thumb for this process?

  • #2
    Drive it normally and after about 10 heat cycles dump the oil.
    After a hundred miles kick it's butt a few times.

    The thing about breaking in an engine all the parts like a different approach.

    Rings and pistons want varied speeds and loads.
    Bearings want slow and steady.
    Valves want high rpm to seat them.

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    • #3
      when building race engines. on start up take it up to 2800-3000 rpm for 20 minutes . change oil check valve lash and timing and co racing

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      • #4
        The break-in on my GS1100E years ago was recommended to use no more than 35% to 45% throttle opening for the first 125 miles. The idea was to limit the pressure on the rod bearings until they wore in. My anal side required that I dump the oil and change the filter after the trip home... that was about 20 miles, then I went on 500 mile intervals for the first few changes. Oil is cheap insurance.
        Have fun
        paul
        ARS W9PCS

        Esto Vigilans

        Remember, just because you can doesn't mean you should...
        but you may have to

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        • #5
          not to mention all the tech advances in engine oil from when that engine was designed, up to modern oil today.
          I don't think there is any way to co-relate what the designers wanted back then, to where we are today with modern oil.

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          • #6
            What are you putting into your Midget, a 1275? If so, I suggest you seek out forums for the many people who race these. They'll have many suggestions for breakin. All conflicting, of course.

            OTOH, if you're shoehorning a 327 in there, who cares? You'll never be able to stress a V8 in the slightest with a Midget.

            -js


            EDIT: If it's a British engine bear in mind that they all *needed* to have their internal rough edges worn down to semi-smoothness. When I built my 500cc BSA engine I was told that if I used synthetic oil the rings would never seat. Made sense to me, and I used 30wt non-detergent for the first 500 miles. Broke in nicely, no smoke at all. Then I switched to synthetic.
            Last edited by Jim Stewart; 10-18-2020, 08:34 PM.
            There are no stupid questions. But there are lots of stupid answers. This is the internet.

            Location: SF Bay Area

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            • #7
              Most of the newer engines really don't need a lot of break-in, because the parts are machined far more accurately than they were 50 years ago. Break-in was really intended to wear down the machining marks and so forth on the rubbing parts, primarily the rings and bores, but also bearings, and cams, etc.

              Changing the oil was not due to oil issues, but intended to remove all the particulates that were worn off the parts during break-in.

              Assuming the engine you are putting in was rebuilt "the old way", I'd go with the old standard plan of varying speeds and no sustained high RPM for 500 miles. Changing oil after 50 or 100 miles is not a terrible idea, most of the metal bits that will appear during break-in will probably be in that load of oil.
              1601

              Keep eye on ball.
              Hashim Khan

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              • #8
                Hah! I beat Jerry by one minute posting! He speaks wisdom...

                -js
                There are no stupid questions. But there are lots of stupid answers. This is the internet.

                Location: SF Bay Area

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                • #9
                  In my petrol-head days half a century ago, my car-club mate who rebuilt engines for a living advised me to alternate foot-flat acceleration to high revs under load with longer periods of gentle running. The idea was that the former forced the running-in bits of the engine into close contact, while the latter allowed the rubbed-off debris from those close encounters to be washed into the oil filter out of harm's way. That made sense to me.
                  Simply limiting the top speed doesn't achieve the desired result nearly as quickly, and it's also very frustrating for both the driver of the running-in car and those of the cars stuck behind him.

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                  • #10
                    Dad bought one of the first Morris Minors with a 1098 engine. He put a piece of thick rubber under the accelerator pedal that gave about half throttle and drove it with that for about 500 miles, sometimes reaching about 65 or so mph on flat or downhill stretches, and making her rev freely in the gears.
                    That engine ran for over 10 years with the only attention being oil changes and a new set of big end shells at about 120,000 miles and when sold was still running strongly but burning a little oil at about 180,000 Miles. The next owner reported that it was still running well at 200 000 miles when he scrapped it due to rust in the body.
                    regards David Powell,

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                    • #11
                      Some of your break-in procedure depends on the ring material used. Are you using the old cast rings or something more modern? Was the bore finish matched to the ring material? Anything done in the valve train that would affect the procedure? Old style or something more modern? I do agree with the above poster who says no synthetic oil for break-in. A high zinc content oil should probably be used.

                      If this is a restoration of a stock engine then go with the procedure from when it was new.

                      lg
                      no neat sig line
                      near Salem OR

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                      • #12
                        MG engine? Doesn't matter how you break it in, it'll need another rebuild in 2,000 miles anyway

                        Ian
                        All of the gear, no idea...

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                        • #13
                          Originally posted by Commander_Chaos View Post
                          I'm putting a new engine in my MG Midget and I'm wondering about the break-in period. The formula I always used on my motorcycles was "50 mph for 500 miles" with occasional engine braking to draw oil up around the pistons.

                          It occurred to me, though that the recent change I made from a 4.2 differential to a 3.7 would change the rpm drastically at that speed. Is there a better rule of thumb for this process?
                          Thats really cool, I like the MG.

                          Cant help you for "break-in, chit".

                          Every engine is different.

                          I have rebuilt everything from our lawnmower, Honda CR, stolen Honda 460s (not my theft). With that package he said there were more. No,

                          My friends were my friends. Not yours. So yes. He yanked seven brand new bikes, including the gouldwing, whch he drove like he owned it. LoL Circ{79-80}

                          Of those vehcle that I fla out drove the wheels off of LOL The CR460 I had had crs before. mostly clapped out and cobbled together.. My 125 was a beast/

                          I built my CR125 back from crap.

                          Engine building.

                          Seems like I have built all the configurations. From Lawn mower to a really nasty GM 350 (once over) old fashion block.

                          Here is where the weird comes in. I build engines to survive anything as long as the oil sump nd pump are working.

                          I give a flat tappet, hydraulic lifter dino old bones. It is in my car so I can set the "red line". When I drive it, my car? I set the fuel and spark map for me. No rev limits, there shoud be. Wont make any power above 5500rpm..

                          And yet my dumm azz cranks her you to 7800-8000rpm.

                          Give the car to freinds. Say test it. I want to find the weak point in the way I service engines is all. JR

                          P.S. When you have this engine I buil
                          My old yahoo group. Bridgeport Mill Group

                          https://groups.yahoo.com/neo/groups/...port_mill/info

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                          • #14
                            You say a new engine. Do you mean like a "crate" engine or just an old block that you or someone rebuilt with new parts? The reason I ask is because my dad worked on the assembly line at Ford 50 years ago and I remember him telling that when the new engines come off the line they would go to "hot test." There they hooked them up, fired them up and revved them up to get them hot to pump up the lifters and check for leaks. I would assume that all engine manufacturers do this.
                            I really don't think a break in period is required with a new OEM engine. I would be surprised if car manufacturers expect more than 2% of customers to follow a break in procedure and thus build accordingly. This might be a holdover from the old days of poured, hand scraped Babbitt bearings and just car manufacturers hedging their bet.
                            I have only personally rebuilt a few engines in my time and the only break in I followed was for the new camshaft and lifters. The instructions specified something like 2000 rpm for 5 minutes on initial startup. To me that sounds like what Ford was doing in hot test.
                            I can see this devolving into a discussion like whether a clean car gets better gas mileage. You can probably prove it does but the cost of the car wash far exceeds what you are saving in gas.
                            Last edited by flathead4; 10-19-2020, 07:46 AM.
                            Tom - Spotsylvania, VA

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                            • #15
                              When I started driving in the 1960s, it was common for new cars or ones with a replacement engine, to have a sticker in the rear window 'Running in. Please pass', and stick to 40mph for a few hundred miles. By the time I got a Morris Marina company car with the 1275 engine, around 1980, it had gone up a recommendation of 50mph for 500 miles. Soon after that, manufacturers jut advised taking things reasonably easy for a few hundred miles.
                              'It may not always be the best policy to do what is best technically, but those responsible for policy can never form a right judgement without knowledge of what is right technically' - 'Dutch' Kindelberger

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