Vintage Dirt Bike Q & A

I have come to the conclusion that my 75 MT250 needs new crankshaft seals. It is fouling plugs every single ride. What kind of costs are associated with this kind of task? I am still trying to make a decision on whether I should replace these seals, or sell the bike. The cost will help me decide. Thanks.

The crank seals are cheap. The cost is in the effort to take apart the motor to get the old ones out and the new ones in. You can do this yourself but you will need beg, borrow or steal (buy?) something to hold the clutch, a rotor remover and a crankcase splitter. Either way its a couple of hundred dollars but your then set for a significant period of time. If the motor is taken apart check the mains and lower-end at the same time.

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Michael Stiles
1979 Honda CR250R Elsinore | 2006 Husqvarna SM510R | 2007 Service Honda CR500R-AF

I just did this and it cost $38 for seals from Honda and $30 for gaskets (aftermarket). As far as tools go, if you have an impact wrench, you don't even need to hold the flywheel or clutch. If you don't, you can use a strap wrench (about $15 for decent one). I already had a $12 flywheel puller, recommend you get one. 27 mm (for the MR) clutch socket: $4.60. Just buy an auto timing gear puller and two 5" 8mm bolts to use as a case splitter--it is the exact same thing as Honda sells. Cost: $7.60. Plus gasket sealer $4.00 and quart of oil. So, if you do it yourself and have a compressor and impact wrench, you can do it for about $100, including parts.
Now, if you want to put in bearings while your at it, include another $64.
I would recommend you buy a bearing/seal set online for about $48-$54. It is waaay cheaper than buying seals from Honda and bearings from Allied bearing or other store. If you replace bearings, I recommend you have a heat gun like used to shrink aircraft fabric. Low air, high heat. Freeze the bearings, heat up the case (evenly--very important, you don't want to warp it). Bearing should drop in with slight push.

Thanks for the tips guys. I am still undecided on what to do with it. It is my first bike, and have been wanting to learn to ride, but have become frustrated because the bike fouls plugs too often for me to learn. My dad has said he would help me out with it, and he is mechanically inclined, he also has air tools, and an impact wrench. The bike has 3700 miles on it, and I believe these seals have gone bad from sitting so much. Are there things I am going to run into that will need to be replaced along the way, or things that should be replaced since I have the engine apart? Is it probably a good idea to replace the piston rings, and have the cylinder honed? Will the bearings need replacing for sure? Thanks for the huge help. I wish there was a step by step procedure for doing this task somewhere.

why is it fouling plugs? what do they look like? i have seen bikes start sucking oil out of the tranny into the crank case and burning it, fouling the plugs. that could be a probable cause too. did it just start recently? what is the history on the bike? it could also be jetting or timing causing the plugs to foul.

If you havent already, i would start with a basic tune up- clean the carb, gas tank, and petcock. change the tranny oil (if its low, you might have the problem i was talking about earlier...), check compression, new plug and points, or at least double check the timing.

I believe it is fouling plugs because it is pulling transmission oil into the crankcase because of bad crank seals. Ive gone through the carb and cleaned it out, this didnt seem to do much. The history on the bike....I am the third owner, according to the first owner, it sat around a lot.

It looks like I may end up replacing these seals, and possibly bearings. Where should I look for the lower end seals for a 1975 MT250? Also, where for a gasket set?

I have gaskets for your MT250 if you want them. $23 plus shipping email me at mailto:elsinorestuff@qwest.net The right main seal is the hard one since you have to split the cases to get to it, but the more you can do yourself the less it will cost. With receipts the bike should be worth more to sell if you show them the motor has been overhauled.

Seals are a major comitment to a bike.

It is kind of rare to have the clutch side seals go out. Much less rare to have the center case seal go south for one reason or another.

If you have pulled the left cover off to set points, you should check for any play on the crank. Just grab the flywheel and see if you can rock it even a little. There is at least one outfit that has a package deal with bearings and seals for >$50. You would still need the gasket kit from elsinore stuff but less then $75 for a new bottom end is probably well worth it.

Make sure you have the oil injection set right if you are using it. You never can tell, someone may have filled up the tank with mix and someone else set up the oil injector to high.

If it is sucking up transmission oil, your fluid level will be going down rather quickly. Just check it after a short ride. I have to admit I have drained the transmission and started a bike up to make sure: I hate splitting cases that much!

Elsinorestuff...is it an OEM gasket set?

Ricortes, does it hurt the tranny at all to drain it and start the bike?
I have checked the oil pump, and I turned it damn near all the way down, and it still had a lot of smoke while running. I purchased this bike to learn on, and I havent been able to because of the constant fouling of the plugs. Give it 20 minutes on a brand new plug, and it is fouled. End of the plug is oil covered. I dont know what else would be causing it other than it sucking in transmission oil.

Just my opinion but I have done this in the past and it doesn't seem to hurt anything.

There are parts/bearings in the transmission that spin even with the bike in neutral. The oil left on the bearings is good'nuff to keep them lubed for a little bit to check things out.

You probably have it cornered with transmission oil leak, just a question of where from. I've seen some pretty sad stuff, like pistons coming apart and poking holes in the case through to the transmission.

That is worst case senario of course.

Best case you can get with respect to cost would be center gasket. If you don't find any end play in the flywheel and are low on money, you could get away with buying a sheet of gasket material from Ace Hardware for ~$3 and making your own center case gasket and base gasket. But this is a tough job because you have to split the cases. You will need an impact driver for $10-$20 and the transmission literally *JUMPS* apart when you split the cases. I could scan/email you a diagram of the transmission which may help some, but it is a really tough job.

2nd best is just seals need to be replaced and you don't have to split the cases. That is if bearings are tight.

If you can't get it back together you can always eBay it. I could use a spare MT engine.<grin>

For those who are "transmission challenged" like myself, here is my tip for splitting cases: Keep a roll of aluminum duct tape nearby with four strips about two-feet long. Carefully remove each set of gears, including spacers and washers. Wrap the tape around them so nothing can fall off. Place each set in a separate large ziploc bag until read to reinstall. When putting the engine back together, reverse the procedure. You will still have to make sure all parts are meshing correctly, but you won't lose anything and everything will be in its proper place. I set them in the clutch side case and then slip the stator side case in place.

how do you change crank seals without splitting cases?

Right, you can only get one and not the oil filled side which is in question.

Just a short observation. Took a peak at a case half on an engine that came apart and the clutch side seal looks like a dog has been chewing it. Makes me wonder how many broken engines just had a new piston put in vs. split the cases and check the seal.