But I digress...
Fast forward to Spring 2012, I was using the bike to commute to work and racking up the mileage. The engine was using a fair bit of oil between changes and I decided to rebuild it before the summer riding season. After doing some research, it was soon discovered that increasing the displacement would not cost much more than a standard rebuild. Lucky for me Eddie Sisneros, the guru of DRZs, had just moved his shop and was now on my way home from work. Eddie is a great guy and offered up plenty of advice from parts to procedures. Although I have rebuilt plenty of car engines, high performance bike engines have some special considerations and I was concerned about doing this right the first time.
I settled on a 435 big bore kit. This allowed me to fix the oil burning problem and up the displacement without splitting the cases. Limiting myself a 435 (94 mm piston) avoided some of the problems of the larger bore kits. Here she is going under the knife and a pile of shiny new parts:
After removing all the plastic and the fuel system, I carefully removed the cams and the head. The piston was next. Here is the old next to the new. The over fueling and burned oil is easy to see. The new piston has lots of extra material removed, I assume to keep the weights similar, although I never check for sure.
With the piston removed, I checked the bore on the wrist pin bearing and the play in the crank bearing. To my relief, both were well within spec, which meant I didn't have to split the case to replace the connecting rod bearings. Besides the larger piston, the kit came with a new cylinder jug. Boring these cylinders is not as easy as a car engine, as they are Nickel Carbide coated over aluminum instead of plain cast iron. To bore them requires re-plating and frankly that is not worth it when a new replacement is relatively inexpensive.
In addition to the big bore kit, the timing chain was at the end of its life and in need of replacement. As you can see, the chain is trapped by the clutch basket.
After removing the clutch basket, the chain was slipped off and I used the opportunity to Loc Tite the left-handed nut on the crank, which is well known for coming loose.
The previous owner had tore up/seized up the access plug to the flywheel nut. I was able to score a new one for free, but the old one was not coming out peacefully. After fighting with it for an hour and getting nowhere, I gave up and pulled the stator cover. Modifying a tool a had for a Dana 44 axle ball joint and using an impact wrench, I was able to remove the inspection plug without damage to the case cover.
In the process however I lost a small bushing for the starter counter shaft. After some reading I discovered that this is a common problem when removing the stator case cover. Oh well, lesson learned. I was able to fabricate a new bronze bushing and continue my work after checking to make sure the wayward bushing had not fallen into the engine.
On Eddie's recommendation, I had him rebuild the head with stainless one-piece valves. One of the more documented problems with the DRZs is the factory 2-piece valves separating and taking out the upper end. On any other day I would have done this job myself, but I was under the gun to get the bike back together and the tooling and parts would have cost more than having Eddie do it.
With the head done, I could start putting the engine back together. The first step was installing the new timing chain and clutch basket. Followed by the new piston and cylinder.
Since I use the bike to travel to remote places where premium fuel is not always available, I decided to stick with the stock compression ratio and therefore be able to burn low octane gasoline. With this kit it is possible to raise the compression ratio by using only one layer of cylinder gaskets instead of three. S-models have slightly lower compression than their E-model counterparts allowing them to run on low octane fuel at the cost of a few horsepower.
Installing the head and fuel system went smoothly. Special attention must be paid getting the cams timed properly, but taking my time, this was not a difficult task. The bike fired up immediately and no idle adjustment was necessary to compensate for the larger bore. There are many schools of thought when it comes to breaking in the engine. I followed the kit's instructions and kept it under half throttle and 6000 rpm for 500 miles. ( I think it was 500, but I honestly don't remember. ) Afterwards I changed the oil and filter, checked all the bolts and took it out for a spin. The kit really improved the low end torque, which works great with my 15/44 gearing. Top end HP feels about the same, but I suspect that the smaller S-model carb limits the top end HP. A head to head drag race with my friend's lighter and more aggressively geared DRZ400E proved that the big bore really did make a difference. Despite the E-model's higher compression and bigger carb, my bike was able to pull out a noticeable lead in a 1/4 mile and by then the "E" was top out and I still had plenty of RPM left in reserve thanks to my more street friendly gearing.
On the trail, the extra displacement translated to better out-of-the-hole performance. Great for low speed maneuvering in the rocks. The first trip out ended up being a 400 mile/2-day over-nighter around northern Colorado with a co-worker and his XR400. The trip was about 80% dirt and trails. The first day alone, I logged 17 hours of saddle time and it was a great shake down for the bike. ( I had it loaded with 175lbs of rider and 50+lbs of gear. )
After getting the bike home and cleaned, I inspected it again for leaks and loose bolts. To my surprise nothing had come loose. I really expected at least one loose bolt. Overall, I'm happy with how the bike turned out, especially since it burns zero oil now. I can't wait for warmer weather to return!
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