May 04, 2022 5 min read
Enjoying all the thrills and risks a dirt bike can bring you will inevitably result in the need for maintenance work. Part of that maintenance is checking, and potentially rebuilding, the bottom end of your bike’s engine. But how can you tell that you need a new one?
The bottom end of a dirt bike engine needs to be rebuilt when the rider has reached 80 hours of riding use or when they can’t get to the level of power they were before. Inspecting the spacing between the connecting rod, the crank shells, and crank webs will let the rider know when it’s time to replace those parts.
If you would like to learn how to check your bottom end to avoid unnecessary risks to yourself, how much a bottom-end overhaul would cost you, and more, then keep reading.
In order to inspect the bottom end of your dirt bike, you have to take the top end off. Once it’s off, the first thing you check is the connecting rod. First of all, make sure it’s not discolored or scorched as these indicate you need to replace the connecting rod and reball the crank. That is, replace the ball bearings in the crankshaft.
Next, hold the conrod perfectly straight in the center and tug back and forth toward you and away from you. You shouldn’t feel any wiggle room. If you do feel space, you need to rebuild your conrod and crankshaft.
Now, check the conrod’s lateral movement by pushing down on it until it’s pushed into the center. Tug back and forth toward you and away from you to see if there’s any wiggle room to make sure there is clearance between the conrod and crank shells. If the rod doesn’t move and is touching the crank shells it tells you the big end bearing has too much room and that the crankshaft needs rebuilding. That’s also what causes the scorching or discoloration I mentioned before.
You can use a dull indicator to check the lateral room of the connecting rod. If it shows there is less than 1mm of room, you should be fine. Each time you check your bottom end, using a dull indicator can help you see if the space is expanding, and therefore if the bearing is wearing down.
Finally, you should check the axis spacing of the connecting rod between the crank webs. Your manual should inform you how much space there should be between them, such as between 0.6mm and 0.7mm. You can check the space by taking the base of the conrod, pushing it to one side (toward you or away), and trying to insert feeler gauges into the gap that are made at different thicknesses, such as 0.6mm.
Essentially, what you’re doing is inspecting the most critical parts of the bottom end to see how they’re doing. Each time you take these measurements and checks, I recommend keeping a log so that you know what the state of your bottom end was the last time you worked on it.
Normally, trying to check the bottom end can be a hassle because you usually have to take off the top end first in order to examine it, and you keep having to move the bike around to keep it where you need it to be as you work.
Most dirt bike stands don’t make this any easier since they try to be a universal fit, but since every dirt bike manufacturer designs their chassis differently, and therefore are not universally designed, they’re just frustrating. That is why I and my team at Risk Racing came up with the A.T.S. Magnetic Stand. Whatever make or model dirt bike you have, you’ll be able to level it on our stand because of its innovative adjustable top. This will save your tires too.
Symptoms that tell you your bottom end is approaching the end of its life can include odd knocking noises while your bike is idling, vibrations caused by loose bearings, and leaking crank seals.
Every time you take apart your top end, I would recommend checking the measurements of your bike’s bottom end. Checking the measurements doesn’t take much time, and you will be spared from the unpleasant surprise of discovering that you should have overhauled your bottom end, say, a month ago.
If you have a two-stroke engine, you know you need to rebuild your bottom end when you notice you’re not getting the amount of power you used to or when you’ve reached between 70 and 80 riding hours. For four-stroke engines, rebuild your top end between 80 and 100 hours of riding use.
Usually, it’s not until you’ve put in more than 500 hours of riding time that you’ll have to overhaul your bottom end.
While a top-end rebuild takes care of the piston and cylinder of your engine, the bottom end is concerned with the connecting rod and crankshaft especially. Rebuilding the bottom end consists of replacing:
If you choose to rebuild the bottom end yourself, you need some specific tools that, if I am being honest, are not cheap. The tools you would need are:
As you can imagine, this is the main reason people typically go to the shop mechanics who already have these tools and the experience using them.
There are a few variables that will vary how much you’ll pay, such as if you work on the engine yourself or have a mechanic do it, if you go to an original equipment manufactured (OEM) parts dealer or not, and even which state you get repairs in.
Thanks to the in-depth research done by DIY Moto Fix, a full rebuild of the bottom end of the engine would cost you between $1300-$1500 if you do the work yourself and are using all OEM parts, which is preferred.
With a mechanic, it’s the cost of the parts in addition to the cost of labor, which can be between $60-$100 per hour. Rebuilding the bottom end can take 2 hours, not including the disassembly time, which means you could be handing over an additional $200 in labor by someone who knows what they were doing.
Needless to say, there is something absolutely priceless in being able to take care of your dirt bike yourself. I know I enjoy it!
If you consistently take care of the air filters and put quality oil in your dirt bike, you’ll be able to make your parts go as far as possible. Check both your top end and bottom end regularly since they usually have to be replaced together if anything goes wrong. In the meantime, as a little fatherly advice from me, start putting money aside for such repairs so you don’t end up with a $3,000 heart attack.
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