When Building Your Engine, Trust No One—Even Yourself

Basketcase is the common term for the motorcycle engine I received in early January. The engine arrived in 14 boxes, each box holding a fraction of an engine ready for assembly, all the parts nestled into the torn-up wrapping paper the seller had used to wrap their kids' Christmas gifts. I'm a trusting guy. All the parts in those boxes might be perfect, but when it comes to assembling an engine, you should trust no one. It's not personal, I promise.

It's been nearly six months since the eternally optimistic devil on my shoulder convinced me to buy a really crusty 1988 Honda XR600R. I had grand plans to make the bike run again and to enjoy a season of riding before I tore it down and rebuilt it into the XR600R I've always wanted. Like any good project, things went pear-shaped pretty much immediately. Any home mechanic can relate to the various distractions, including daydreams of the project finishing itself, that kept me from any real progress for a bit.

I've been elbow-deep in nine Honda XR engines within the last few years, but the 600R is a new puzzle. After spending a night or two cozied up with the factory service manual, I decided the whole ordeal was nothing to stress over. This model had a couple small quirks compared to the XR250R and XR200 motors that have been eating up most of my time (and all of my money), but nothing too crazy.

Actually assembling the engine is not the annoying part. Instead, it is the process of getting familiar enough with a new-to-me engine. When I'm pouring in oil and preparing to kick an engine over for the first time, I want to to be absolutely certain there are no missing pieces and that the ones I put together are all what they were supposed to be. One small rubber plug, and the oil system won't work as designed-and just like that, all my parts become pretty scrap metal. I often rely on disassembling an incomplete or broken engine to teach me how everything fits. Part investigation, part 200-level course in applied engineering, disassembling a project helps my mind make sense of the drawings and wording of the service manual when I'm putting everything back together. And of course, reference photos-gotta love reference photos.

This XR600 engine gave me no such opportunity. Typically, the disassembly phase is the best time to take stock of the condition of the parts. My list of what components need special attention or replacing starts with the first parts that come off the complete engine. The powerplant in the XR600 motorcycle I bought late last year was so badly damaged that it made sense to buy an entire core engine. Better that than getting mired in the XR600R engine of Thesus and melting my credit card with eBay charges for onesy-twosy parts. And so I found a new-to-me engine that was "ready to assemble," per the very kind gentleman who shipped it to me with recycled wrapping paper.

I look for the best in people as much as possible. But you and I both know jumping straight to assembly with an unfamiliar pile of parts is a terrible idea. The payment I sent for these parts was a statement of trust that I believe everything in those 14 boxes was what the seller told me it was. Once all the parts were laid out on my bench, it was up to me to confirm and verify that what I was assembling was indeed what I thought I was.

First things first, the crankcase. Most of its spinny bits are situated between two halves of a cast case. The crankshaft came with a receipt showing it had been pressed apart to remove the connecting rod and bearing and then pressed back together. While I couldn't verify that the receipt belonged to this specific crankshaft, my calibrated hands and eyes found no reason to doubt that the work had been done, and properly.

The transmission was pretty grimy. It had been sitting uncovered on a shop bench for some time and needed a deep clean, which provided me a good excuse to check everything: gear tooth counts, spacer and bushing locations and directions, and the condition of the transmission shafts. That was when I caught problem number one: miffed threads on the input shaft.

Something about me makes XR600R engine parts strip their threads. After a trip down eBay lane for a good used input shaft, I began cleaning the gearbox. Assembly quickly followed.

One of the fun things about the XR-series engines is the parts interchangeability between multiple models. While the stock transmission of the XR600R is okay, many years ago some people much smarter than me sat down and figured out how to even out the spacing of the gears while decreasing cruise rpm. They even managed to do it all with Honda OEM parts-specifically, the second and the fifth gear from an NX650 Dominator, a bike that Honda sold alongside the XR lineup. The U.S. only got the NX650 for two years in the late 1980s, but the Euro market liked the more rally-influenced styling more than we did, enough for Honda to keep the model alive in Europe through 2003. That's a decent enough run to mean parts can still be found, but only just.

Assembling the transmission, and swapping in those NX gears, took a couple evenings and a weekend. The process took maybe a bit more time than if I had been the one to pull these parts from the engine myself, but probably not by much. It was a surprising, unique challenge to start with unknown parts that I hadn't taken apart. Next up, after the cases were closed up, were the cylinder and piston.

Measuring the bore required a trip to a friend's shop, as I don't own a dial bore gauge or a micrometer large enough to get the accurate measurements I needed. The proper amount of piston-to-cylinder clearance is critical to a long-running healthy engine. If this one was off, I would need to purchase a new piston and take a trip to the machine shop. Call it luck or just that people are good, but everything in the bore spec'd out beautifully, just as it had arrived from the friendly seller on the forums.

After getting the tools set and taking a few notes, I had said friend double-check my measurements before I headed out. You know, trust but verify-even yourself.

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