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For you Rotaxians that need to know what’s recommended for a top end overhaul I’ll tell you this so listen and listen very carefully:
The con rod’s connected to the, WRIST PIN.
The wrist pin’s connected to the, NEEDLE BEARING.
The needle bearing’s connected to the, PISTON.
The piston’s connected to the, PISTON RING.(Mikaela put me up to that one, she’s been teaching preschool too much)
The needle bearing on the crank is (I’d guess) the weakest point on the bottom end. Knowing what I know now would i do it again and only do a top end? Probably not. The extra money is worth a peace of mind. Should my needle bearing have failed? Probably not since it didn’t have enough hours on it but hey, that’s racing right? I did hear something from an engine builder however who said since we’re at such a high altitude our engines do get stressed a lot more than those closer to sea level since we run so lean vs sea level and lower lubrication (I’d have to say this must have been concluded after many beers but hey, what do I know. Perhaps Brad could chime in here since he’s the engine expert in my opinion, or Jeb Clampett for that matter) 8)
Piston, ring and wrist pin bearing Rich.
At our altitude the engine produces 23% less power than at sea level so there is 23% less stress. It is true you use less fuel but the engine does not run leaner as the optimum fuel to air ratio is the same at any altitude. The ratio of oil to gas is the same so the engine does not get less lubrication. On the other hand if you go off the track regularly and suck dirt into the engine through the lousy air filter the Rotax has this will cause accelerated bearing and piston wear. All it takes is one small piece of dirt to stop the lower end bearing from turning. In just one second the crank pin will turn over 200 times against a stopped bearing which will flat spot the rollers and fry the bottom end. All of the power (load) goes through that bearing. If you look at the design life chart for a roller bearing of that size using the rpm the engine runs it is not even on the scale to determine hours of expected life. Race engines exceed the normal design rpm operating limit of bearings so the life of the bearing is limited.
You are right on blink! My four decades of working with race engines has proven to me that these engines put a lot of stress on parts that lead to regular maintance. This is no more true at our altitude than it is at sea level. More often than not, however, it is outside factors (dirt, water, improper fuel and/or oil mixture, etc) that lead to premature failures. Thanks for the insight on our altitude.
the swami’s have spoken…
I guess in my first two years of working the “John Deere” sponsorship route I should have expected this…
Brad is correct about dirt and Rotax motors. I learned the hard way a couple of years ago by ruining a motor by getting off track and sucking a bunch of dirt through the filter. If I get seriously off track now, I immediately shut the motor off and don’t even drive it back to the pits. I keep a spare clean filter foam so I can swap it quickly and be ready to go again.
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