It's not going to be gentle.
In fact, it'll probably be quite violent. You'll look down and see it. Case cracked. Ride over.
Once the life blood of your dirt bike starts leaking out you'll need to walk home unless you want to destroy the engine completely. Like a car without oil, all those vital parts rubbing against each other need the necessary lubrication and protection to prevent further damage and breakdown.
A cracked case, which encompasses the major segment of the dirt bike including the engine, clutch, crank and transmission, is probably in the Top 5 of the worst what-could-go-wrong during riding whether on the track or trail. It ends your day immediately, no simple repair exists, and fixing it ranges from a few hundred to even a thousand dollars.
No riding home after this busted case
Fortunately, it doesn't happen often and in many respects you have lots of control in preventing it. Some riders enjoy a lifetime of dirt biking without ever cracking the case and it's not because their ride sits in the garage the whole time. Preventing a cracked case it not difficult and performing routine maintenance is the #1 approach to keeping oil in your engine and not spilled on the ground.
1. Maintain the Chain
A snapped chain can whip the side of your bike so hard it cracks the case. If you read Oh Snap - My Dirt Bike Chain Broke you'll know a broken chain can happen and eventually does happen. To prevent a broken chain from cracking a case, then do your due diligence and maintain the dirt bike chain.
Lubricating the chain is the single most important part of keeping the drivetrain healthy and working. The drivetrain can wear down prematurely so ensure your pre-ride check includes a visual once-over of the chain and sprockets. If one or the other needs replacing, exchange them together. Read The Ultimate Dirt Bike Sprocket & Gearing Guide for information on wear patterns.
Secondly, ensure the chain is not too loose or too tight. The three finger method before every ride is a good rule of thumb but check the owner's manual for manufacturer's specifications as it varies between models. Read more about How to Tighten a Dirt Bike Chain.
Eventually you'll encounter a broken chain and though it's rare for one to crack the case, it's worth the investment ($35 beats hundreds in repairs) to add an extra layer of protection by installing a case saver. In the event of a broken chain, a case saver keeps the chain from whipping into your case and worse - you!
2. Install a Skid Plate
It's uncommon to see a dirt bike without a skid plate so if you're new to the sport you might think skid plates arrive like the handlebars - already on the bike. Think again. You've got to buy the skid plate separately and it's one of the first add-ons you should install.
Casing a jump (coming up short) or hitting a rock is bound to happen when riding and without a skid plate it's almost a guarantee you'll crack the case or just obliterate it.
3. Avoid Large Rocks
By large rocks we mean boulders. Unless you're specifically training for the Endurocross series leave traversing boulders to someone else. Also, as a rider don't take unnecessary risks when you encounter streams or areas where big rocks scatter the land. Even sideswiping a rock, especially one with a jagged edge, can rip a hole in the case. A skid plate is a solid defense but it can only do so much so don't test its limits.
This one might be salvageable
In some conditions, a cracked case results when a hot engine makes contact with cold water. This is more likely to happen if the engine is overheated and you ride through a cold stream or for some reason you're close to home and decide to hose down your bike too soon. Therefore, run fresh, high-quality coolant to keep your engine from overheating. If you suspect an overly hot engine let it cool off before dowsing it with water.
If you encounter a damaged case, depending on the severity, it can be fixed. However it's best to have a professional evaluate the damage and fix the crack or replace the case.
Written By: AndrewT