All great advice there until that last statement. I would much rather have a talented/experienced/concerned amateur do the examination than the average 'pro'. All one needs is some general engine experience, the more the better, good measuring tools and the book to evaluate the condition of parts. That , a clean workbench and a methodical approach to cleaning and reassembly has produced a LOT of reliable rebuilds in the garages of the world, mine being one of them.
Many pros take the approach that installing all new parts both increases their take and insures against any failures being blamed on them. A fair way to do business from their perspective, but has no concern for the customers pocket book or real needs.
It seems that Ken and I are of different opinions here.
As I see it, Onan designed these engines to last at least 3000 hours before needing to be rebuilt but that lifespan was predicated on the engine being serviced properly.
So.....let's say that an engine didn't get serviced properly and finally failed at 1500 hours because the oil wasn't changed at every 50 hours. Instead, the oil got changed at 250 to 300 hours of use and decarbonizing didn't take place at proper intervals either.
The engine gets dismantled for rebuilding. The bores are measured and a 20 thou cut cleans both of them up nicely. The valves get ground and new guides are installed. The crank throws must be turned 10 thou to clean them up so that means that new rods are required along with pistons and rings. All of that is pretty straightforward but now it starts to get a little trickier thanks to the neglect this engine was subjected to.
This is a block that had no oil filter and that filthy oil was being pumped around for 5 to 6 times longer than it should have been. The crank end-bearings, the camshaft, the camshaft end-gears and the oil pump itself were constantly being bathed in this dirty oil filled with particulate that was grinding against all those parts including the crank and camshaft timing gears.
The whole purpose behind rebuilding an engine is to reset the "wear clock" back to zero so that there is once again a reasonable expectation of 3000 more hours of trouble-free service. On a perfectly maintained engine that inexplicably failed at 1500 hours, all those "other" parts I mentioned will have 1500 hours on them if they are used in the rebuild. That might be OK but what about the same parts from the abused engine? For sure, they have 1500 hours of run-time on them but in truth, they actually have a lot less life in them due to the bad lubrication they were forced to endure for that 1500 hours.
They could have the equivalent of 3000 or even 4000 hours of wear on them and be very close to the end of their life. Pro engine builders know this and years of experience have taught them that there are some parts that you just don't gamble on as "looks OK" and "feels OK". There are some items that can't be measured for wear. Instead, you just replace them because it's the smart thing to do.
Rebuilding an Onan is not an inexpensive process. If the job is not done with extreme care and thoroughness, then the cost of the machine work, the rods, pistons, rings and gaskets is thrown into jeopardy over the cost of some bearings or an oil pump. The camshaft should be profiled to make sure it is delivering the lift and duration the manufacturer designed it to provide. Very few backyard mechanics have the tools and talent in their garage to carry out all the measuring required to make sure all the parts meet factory specs. If you know someone that can do all of this, then more power to you.
In a recent thread on this site, someone entrusted there Onan to be rebuilt by a party who said they could do the work for a mere $500.00. As it turned out, the engine came back to them and huffed oil out the breather badly and they finally concluded what I told them in my initial reply. YOU CANNOT DO A PROPER REBUILD FOR $500.00. Fortunately for them, all of their money was refunded. Rarely does that ever happen. But even so, look at the aggravation they went through having to pull the engine, take it to these guys, pick it up, put it back in the tractor only to find it pushing out oil relentlessly.
Hours upon hours were spent trying to figure out what was wrong and if it could be cured. I don't know what they eventually did but my guess is that they pulled the engine once again and took it to someone who knows how to conduct a proper rebuild that will give them an engine they can use for many, many years. I'm not saying that Ken is wrong but he's coming at this from his own perspective of carrying out his own work. Apparently he's spent a considerable amount of money to purchase the specialized calipers, dial indicators and micrometers etc needed for checking all the parts.
Few of us are willing to make such an investment in these tools or in the time it takes to become self-taught on how to use them. Pros already have the tools and they make those measurements many times each day, every day. They know what to check and how to check it without having to consult some do-it-yourself manual because that's there trade. There's a lot of jobs that I'm more than happy to take on because I know that I can do them just as well as the Pros but there are some jobs I prefer to relegate to them because I'd rather pay for their expertise than run the risk of me making a stupid little mistake that could come back and bite me in the butt big-time.
I've been down that road in the past where my arrogance over-shadowed the common sense I should have exhibited and it ended up costing me far more money than it would have had I just paid the Pro his fee.
