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Could anyone tell me the correct way the break-in a new big bore motor? How long should i let the motor warm up?How many tanks of gas do i need pre mix oil in? I have 400 main jets should they be changed? The reason I ask so many questions is I already lost one motor!
 

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Real world... the engine as built should be clearanced to run right from the start. Breaking it in should have nothing to do with the engine going down or seizing. The only reason you break an engine in.. particularly one that has been rebuilt or built as a performance piece... is to seat the rings and there are a lot of philosophies on this.. ie. varying engine speeds.. don't run a constant speed.. I think the varying speed is probably related to the rings seeing a lot of diffent conditions.. from vacuum to pressure loading to seat them. Or run it hard right from the start. No one here is an ultimate authority as to what really works or doesn't work.. don't think any one here has study it or done a thesis on it. But all will have an opinion based on their experiences. Real world, probably the best opinion would be to warm it up so you don't cold seize it.. 3 to 5 minutes (water temperature and piston temperatures need to get up to temperature) before you run it harder. The bores expand slower then the aluminum pistons..

Particularly with roller bearing cranks, needle bearings on the rod big and small ends.. there is little need for break in here. Most of the modern pistons have been moly coated to assist in initial wear in on two cycle engines...

And you would probably want to run it moderately easy (no one or two mile wide open throttle runs down a lake) for the first 3 to 5 hours or operation. My guess is a days good trail riding where you don't hammer the throttle wide open for any extented time.. will pretty much break the motor in. But then this is just my opinion based on my experiences with few facts.... and you are going to get 50 differing opinions here.

I can tell you that when we built NHRA stocker motors.. we just went out and ran them hard the first run.. and probably by the 4 th or 5 run the rings had seated.. and those engines ran as well as one that was broken in slowly. Albeit they were not two cycle engines.. but from the stand point of breaking them in... the principle is the same. You clearance the for the conditions you are going to run them. Only issue is the cold start problem with sleds.. and the rapid expansion of the pistons.. vs the rapid expansion of the bores...

If your first motor went down.. (and I don't know the conditions of the failure), it was from cold seizure, too lean, not enough oil, leaking air, snow dust, inadequate clearances or detonation. And proper break in is not going to help much for those casal factors.

One thing I don't understand ... is the heat cycling phenomena I hear on this site. I don't for the life of me understand what heat cycling is going to do for you.. does the grain structure in the piston alloy change with heat cycling 3 to 5 times? Does it change in the cylinder wall? Does it change the grain structure of the material in the needle bearings or the crank steel? I think not.. the temperature are not high enough.. Kind of sounds like an old wives tale to me.......... If you are not clearanced properly.. nothing is going to save the engine under operation conditions...

Just warm the engine up so things are at operating temperatures and don't hold it wide open for the first few hours of operation and let the rings wear in under lighter loads.. and you should be good to go...just my opinion and not endorsed by the forum or anyone else.. have a good one and happy sledding.
 

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djm - nice job!
I agree to almost everything you typed I just don't wait 3 to 5 hours of operation time!
If all seems well (do a few plug checks at different throttle positions) (usually to soon for pistion wash oh and REVs are a PITA to look for that lol) I'll break it in Exactly how I'm going to ride it! If it's a sno-x sled then that new engine will have seen 8000+ rpm with in 10 min of ever starting it. (run time)
lol there 2 answers both slightly different read them all and do what you feel comfortable with.
I too am NO expert!
05SDI
 

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nhrevx said:
Could anyone tell me the correct way the break-in a new big bore motor? How long should i let the motor warm up?How many tanks of gas do i need pre mix oil in? I have 400 main jets should they be changed? The reason I ask so many questions is I already lost one motor!

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It would be hard to make a call on jetting without knowing exactly what you are running for carbs and engine and pipes. What or how did the engine builder who made the big bore tell you. Even then.. this is something some one really familiar with your situation.. altitude and temperatures..engine, pipe and sled should make the call on.

I would say running a bit of premix oil.. is a good idea... but not a lot as it leans the fuel air mixture slightly.

One would need a lot more info to make any other suggestions.. ...
 
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