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How long will a big bore engine last?
This is a question that many have and I too really wondered when I decided to build a 120” stroker. My bike is a 2007 UltraClassic that has 45,000 miles on the clock. The first 10,000 miles the engine was a 103” build. I decided to go bigger and settled on 120”. After putting 35,000 miles on the engine, I decided to tear the engine back down for inspection and to see if there was anything that needed attention. Those miles include cross country trips, local riding, temps well into the 100’s, mountain riding and very spirited acceleration, although I do not routinely pull wheelies or do burnouts.
A little background first. I reviewed all of the available parts that are supplied by many for HD engines and settled on HeadQuarters for the top end. HeadQuarters worked the heads, supplied the pistons and cylinders that had been trued to a .005 overbore from the stock HD bigger bore, and the 575 cams and Black Ops lifters. Darkhorse Crank Works supplied the Jims wheels, their rods, and balanced, trued and pinned the rotating assembly. The initial build included a HD 50mm throttle body with 4.89 gm/sec injectors and a D&D Borzilla exhaust. I have since switched to an HPI 55mm throttle body, gaining a few hp on the top end but losing a few pounds of torque on the lower end.
During assembly, cleanliness is extremely important. Cleaning everything prior to assembly and then keeping it clean until installed will make for a long lasting engine. It is amazing what damage a small bit of dirt or one small metal shaving in the wrong spot can do. Everything was checked, and double checked. Piston ring gap, quench, oil pump alignment, cam spacer, and torque values were all meticulously measured. During assembly, a quality assembly lube was used and everything other than the pistons and cyls got a liberal coating prior to installation. The pistons and cyls were coated with 30wt. Oil.
During the assembly which occurred over the course of many days (I was taking my time as there was snow on the ground) every time I stopped work on the build, I covered the engine with a clean sheet to prevent anything from getting into the engine.
Prior to starting the engine, it was filled with HD 20/50 oil and a new oil filter. The lifters were primed by cranking the engine over without the plugs installed for short bursts until oil pressure was indicated on the gauge. Once started, it was run for about 3 minutes, shut down and inspected for any leaks (none) or problems (none). I broke the engine in making 10 runs from 30mph - 60mph at about ¾ throttle and then rode it as I normally would for about 50 miles and then changed the oil and filter. I rode it for about another 100 miles then again changed the oil and filter, using Redline 20w50, which is what I am still running in the engine. Oil change intervals are based on oil analysis and have been 5000 miles since the engine was built. One item that is often overlooked at oil change intervals is the air cleaner element. Oil analysis showed that the air filter needs to be cleaned at about a two to one ratio to oil changes.
Initially, the engine was tuned by me with a DTT TCFI IID. I was able to obtain a consistent 45mpg and engine oil temps in the 200-210 degree range and through seat of the pants tuning was able to eliminate any detonation that was encountered. Key here was that I did not set the initial map “rich” which is contrary to good engine break-in. The map should be as close as possible to what it will eventually need to be but definitely not rich, this will wash down the cyl walls and prevent proper ring seating. I have since change to a TTS Mastertune system that provides far more flexibility for tuning and does not have the quirks of the DTT. I cannot emphasize how important a good tune is to longevity of these engines. Runing rich, even moderately rich will shorten ring life and put more metal particles into the oil, impacting all other components.
Bottom line is at 35,000 miles, I do not see any excessive wear in any components. The cylinders still show a good crosshatch with no vertical scarring. The pistons skirts show the normal rubbing, again without any scarring or scraping. The cams show no wear. There was slight carbon build up on the pistons, but I could still see the HQ logo on them. The slight build up is due to all the crap that gas manufacturers now put in the gas as well as now having to deal with 10-15% methanol. There is not much info available yet about the longevity of the new style cam chain tensioners, but at 45,000 miles they show very minimal wear and should last at least twice as many miles before servicing is needed.
To answer the question “How long will a big bore last?” at this time is impossible to say. At 35,000 miles I am finding no more wear than what I would have expected to find in a stock motor at 10,000 miles. I am re-assembling the engine as is, no part changes except for new gaskets and changing to a SuperTrapp Supermeg exhaust. I am going deaf from the Borzilla!. I fully expect to have this engine go another 35,000 without finding anything different. Using well engineered and manufactured components, cleanliness, attention to detail, and a good tune will allow a large bore engine to live a long, long time.
Guy Dalton
Painesville, Ohio
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