Replace Yamaha 2 stroke crank seals
Since a lot of friends have been getting 70’s and 80’s Yamaha 2 stroke dirt bikes, and I’ve done this a couple times now on my CT1-175 due to the motor blowing up, I figured I’d do a basic write up on how to change the crank seals.
First off… Why do you care and what’s the deal?
On 2 stroke dirt bikes, there is a rubber seal on either side of the crankshaft. Over time these dry out.
The seal on the magneto side (left side) prevents air from getting into the combustion chamber. If the seal on this side goes bad, you’ll get air leaks. Your idle will hang and/or the bike will rev erratic randomly. Bike will run lean and trying to tune it will be pointless.
The seal on the clutch side (right side) prevents crankcase oil from getting into the combustion chamber. If the seal on this side goes bad, you’ll burn a ton of oil and eventually foul plugs and suck the crankcase dry of oil.
Things you’ll need to change the crank seal which you most likely do not have or own:
– air compressor set up and air gun to loosen certain bolts for the first time in 30 years
– 26mm / 29mm / 30mm socket
– yamaha magneto puller tool
Magneto side crank seal
- Use OEM yamaha seals. Get them from BikeBandit.com
- Pull the cover off
- Bolt will be a 19mm. Unless it’s recently been loosened at some point, absolutely no way you’re getting it off without an air compressor set at above 80psi.
- Once that bolt is off, screw on the yamaha magneto puller tool
- Tighten it on there and then start to use the 2 wrenches to pop the magneto off. If this is the first time your magneto is coming off, be prepared to feel like you’re going to strip threads, or break something. Every time that it’s the first time i’m blowing off a magneto that’s been stuck on there for 30 years, it’s blows off with a crazy loud noise. Shit is seriously scary.
- Use an impact driver to get the stator screws out.
- Then take a drywall screw and hammer it gently into the old seal. Only poke it through a tiny tiny bit. Then screw it in only until it grabs. Then use a hammer to pry, and pop out the old seal. Side note – if you can get the seal out with a pick instead of a drywall screw, do it. Good luck though.
- Wipe everything dry with alcohol. Then, since this side is prone to air leaks, I use Black Gasket sealer on the outside of the seal. Never gotten air leaks by using this method.
- Push the seal in gently. Tap it in gently if you have to. Also, once the seal is pushed all the way in, put a thin coating of gasket sealer around the outer edge. Someone once recommended this. Works for me. Again, gasket sealer is up for debate but I like it on things that are prone to air leaks.
- Since it’s all apart, wipe the contact areas of the stator and magneto clean with a rag and alcohol.
- You’re done this side. Reassemble. Important note – when you put the magneto back on make sure you tap it on super hard with a rubber mallet. It needs to sit all the way into the woodruff key and the bolt needs to be tight as shit or the woodruff key will sheer off.
- Last note. These seals are directional. So put them in the right way which is little round spring facing into the combustion chamber. My images show it going in backwards, sorry. However. I put them in backwards in my IT and never had a problem … actually think it makes more sense to put them in backwards so the little spring can’t fall into the combustion chamber. I dunno, it’s your call.
Clutch side crank seal
- Pull the cover off
- Undo the clutch spring bolts
- Those 2 bolts I’m pointing to will definitely not come off unless you have an air compressor set up. Maybe if you have a clutch basket holder. Still they prob won’t come off.
- Once you do get them off, it’s easy. Pull off the clutch, and pull off the primary gear. Take a photo to ensure proper reassembly if you need to before pulling shit all apart.
- Popping the seal out on this side is the same as the other side you just did
- Wipe everything clean with alcohol, and put this seal in dry. No gasket sealer.
- Reassemble. Make sure you tighten those 2 big bolts back to the torque spec or at least make them tight as shit. Then… Drink a beer.
41 responses to “Replace Yamaha 2 stroke crank seals”
Published - Mar 3, 2011
Hey guys, thought I’d share some info on the front end swap I did over the winter on my ’72 CB350. I decided to go this route for a number of reasons, but it started out as just wanting to beef up my fork tube diameter from spaghetti noodle thin 33mm to a sturdy 35mm. […]