2004 Honda CRF250R Engine Rebuild (Case Repair/Bottom End)
Updated: Jan 4, 2021
Just purchased a 2004 Honda CRF250R dirt bike knowing that the bottom end was shot ("seized" per the previous owner). Upon inspection it appeared that the crank arm bearing was gone... most likely in 1000 smaller pieces in the bottom of the engine. Took the bike home and got to work on it diagnosing the problem and fixing it.
-Large Sockets (27mm, 22mm, 21mm)
-Gasket set (I recommend OEM Honda for the main case gasket)
-Crankshaft (Hot Rods Crankshaft, OEM Honda Crankshaft)
-10w-40 Motorcycle Engine Oil (trans/engine)
Possible Parts Needed:
First thing I did was remove the engine from the bike. Removed the carburetor first:
Then clutch cable, electronics wires from stator and coolant lines. After that remove the 4 bolts that hold the engine to the frame. I moved the rear swingarm backwards to ease the engine coming out of the frame.
Now the fun part. Taking the engine apart and splitting the case to diagnose the seizure.
Side note: I received my bike with the cylinder and head already off as pictured below.
I didn't take the time to make a video of the tear down, but I did use a YouTube video and found it to be very helpful to follow along, you can watch that video here.
Once I got the clutch basket off that side looked decent. I was able to do it without a puller or holder tool, but required me to use a 1/2" snap-on impact gun and I wouldn't recommend that method (600+ ft-lbs is a lot if it doesn't come freely).
Next I flipped the case over and removed the flywheel. I did not have the right flywheel puller so I rented a 3 prong universal puller from autozone and used that. Since I am replacing my crankshaft I wasn't too concerned about damaging it.
Once out it was pretty evident that the bearing fragments were all over the engine and oil screen. Splitting the case was somewhat easy and did not require much force. After that the crankshaft just pulled out.
You can see all the bearing debris in the bottom of the case.
And the metal shavings in the oil filter:
Unfortunately after clearing out all the gaskets it was clear that the case on both sides was heavily damaged from the bearing fragments and on further inspections parts of the case had broken off as well.
I attempted to grind down any burs and clean the case, but it appeared that there were cracks and damaged sections that may continue to break and cause further problems.
Looking around for the cases determined that almost every used case on ebay had the same damages as mine and were still going for $100-$200. The cases are about $200-$250 each side (new). I found it interested that so many damaged ones were for sale, made me wonder if people repair them.
Set up everything in orderly fashion while I waited on parts to arrive.
And the bike sits outside to wait:
After doing a lot of research online I decided to attempt a JB weld fix to the case. The biggest concern is the JB weld breaking free and becoming a problem in the engine. I only needed to reinforce 2-4 small cracks in a non-stress location, but there was some structural damage, however they were not in a position that was under any stress.
Following a post from someone else attempting to fix a crack similar to mine I made the decision to drill along inside the case to reinforce the cracked area and prevent it from separating. I did this by drilling a small hole through the case into the cracked section.
I first tested to make sure the syringe would be able to insert JB weld and it would. After the hole was made I clean and cleaned the case. Started with simple green and water, then propane torch, then carb and choke/brake cleaner, then repeated the process engine with the torch to try and burn off any remaining oil.
Then I mixed the JB weld and put it into the syringe and inserted it into the hole making sure that it pushed all the air out.
I then put a slight layer of JB weld on the crack and deep gorge that I had. I then repeated the process on the other case, which also had similar damage.
I then let the cases sit overnight and for at least 24 hours in a position that will allow the JB weld to flow into the cracks.
Once the JB weld had dried all I had to do was sand it down and make sure the mating surfaces were flat and even. I did pay up for the OEM Honda case gasket due to my potentially uneven surfaces and how much work it would be to get back to the case gasket if it leaked.
Here is what the case looked like before re-assembly:
The next step was to remove and replace all the bearings in the cases as well as oil seals. This was accomplished by using sockets and a blind hole bearing puller (required for a few of the bearings you cant push from behind). Soaked the whole case in PB Blaster for a while before hand. For the main bearing I used a freezing bearing/hot case method to slide it in, which worked great (YouTube videos of that method).
After that the cases were re-assembled with the new crankshaft and seals. I followed different YouTube videos to complete the re-assembly. I did use RTV gray gasket maker between the cases. Some people choose not to do this, but as my surface was somewhat uneven I chose to do it.
To remember the bolt whole pattern I cut holes in a box and the rough outline of the case. Worked great as they are different sizes.
Next put the engine in the frame and attached the side covers. Making sure all the gears were aligned (indent mark in last picture).
Once the engine was in and everything was hooked back up (carb/cooling lines/electrical/cam chain/timing). I added oil to both the trans and engine (about 1 quart or less per trans/engine). I used 10w-40 Motorcycle specific oil for both, but plan to do another oil change soon, just to make sure all the metal remnants are out of the engine.
Once all back together I was able to get it started with a lot of kicks and some starter fluid. Seemed to be running with no catastrophic failure right away and no grinding or oil leaks!!!
It seems to be running okay, but has trouble starting and does not like to idle... Next step to solve that will be carburetor tuning and valve shim adjustment (soon to come).
UPDATE: So after a lot of troubleshooting I was able to get it to start on the first few kicks. The problem could have been any one of the following things, but I think the biggest correction was the carb jetting.
-Replaced wire harness due to a short on the coil ground
-Shimmed the valves after the seats were re-cut by a professional shop
-Changed the jetting (ongoing/will update as they change). Here are the current specs for what I am running:
Main Jet - 162
Pilot Jet - 45
Leak Down - 55
Fuel Needle Turns - 2
Links to Other projects of the 2004 CRF250R: