if it smells like gas, you gotta problem…..
a bad clutch-side oil seal on a vespa is a big problem — you lose primary compression and fill up yr tranny with gasoline (not a good lubricant). thankfully, changing it is not so bad.
1. split the cases (see the shifting cross article)
2. remove the clutch. you’ll need the clutch nut tool (duh), and you’ll have to make sure you’ve bent up the tab of the washer that’s folded over the nut. a electrician’s screwdriver works great for that purpose.
sometimes it’s really hard to get the nut off — if that’s the case, just take the motor out of the bike and bring it to yr local gas station or garage. one hit from an air impact wrench is all it should take.
3. tap out the crank — work gently and make sure to catch it as it comes free of the bearing. using a center punch (there’s a recess for one in the end of the crank) works great to drift it out. don’t hammer directly on the end of the crank with a metal hammer, as the vibes can jar the press fit of the two halves, and y’r very likely to fuck up the threads for the clutch nut. remember to put things like the oil metering drive gear (if applicable) and clutch woodruff key in safe places where they won’t get lost!
4. pry the old seal out with a screwdriver. don’t fuck up the balls in the bearing. be careful… don’t rest the shaft of the screwdriver against the case for extra leverage — you’ll only wind up cracking or damaging the gasket face. be patient and work yr way around in a circle til the seal starts to give up and come out.
5. check the new seal. first, make sure you’ve got the right kinda seal for yr bike. then, try to get piaggio (ideal) or rolf (ok) seals. lesser seals sometimes start to spin on the crank, damaging cases. note that with pseries bikes, later seals (for the px) come in 2 sizes, and the too-large seal will be near-impossible to fit into the cases, may bind on the bearing, and will likely not seal. the difference is not visibly apparent, but unless y’r a jackass and order yr seals from germany, you won’t have this problem (wanna guess how i know?). scooter centre koln actually lists dimensions in their catalog, so you can be sure y’r getting the right kind. the newest, bestest seals have brown viton rubber, which is supposed to be the bomb.
6. install the new seal. the spring side faces the crankshaft webs (not the clutch). begin by lightly wedging the seal in place (try to keep it as even and square as you can) with yr thumbs. then begin to tap around its edges, working it into the case evenly. you can cover the new seal with the old to protect it from the lite hammer blows you will use to tap it into place. tap until the raised outer edge of the seal is just flush with the ‘boss’ around it in the crankcase.
heating the case around the main bearing with a torch will make installation easier, but the main thing is to tap it in as evenly as possible. one side will want to go all the way in (flush w/ case) and the other side will want to pop all the way out. if you go on from this bad situation to drive in the ‘fully out’ half of the seal you’ll just distort it and ruin it. similarly, it’s really hard to pop the other half of the seal out of the case once it’s in. so be careful.
7. inspect and reinstall the crank. before you put the crank back in, look at the surface that the seal fits against. it’s a little raised lip with an angled surface, for want of a better term. just use some logic and stare at the crank and you’ll find it. if it’s damaged (scraped or scratched), ain’t no seal in this world gonna stop the blow-by. i know. i somehow scored this surface during my 2d attempt at rebuilding the motor and had blow-by for 2 consecutive seal jobs (duh) til i got it sorted.
if yr crank is a-ok, then heat the main bearing with a torch (don’t go berserk, and don’t try to turn the balls while y’r heating em up) evenly to make it easier to drive the crank in. work from the clutch side, and try not to heat the balls as much as the inner race of the bearing. once the bearing is too hot to comfortably touch, y’r ready. smear some grease around the seal’s lip to ensure that it doesn’t ‘roll’ when the crank arrives.
if you’ve gotten the race hot enough, the crank may just slot right into the bearing. if not, turn the crank so the piston’s at the bottom of its stroke (protecting the rod), line up the piston with the case mouth (duh), and tap the crank lightly into the bearing. this is the step that’s most likely to twist or distort the crank if you get all gorilla with it, as the crank is largely unsupported and getting a lot of sharp blows. a urethane hammer should work with a heated bearing.
8. done! now reassemble the metering gears (if applicable) and stick the clutch on there. remember to torque the clutch to spec and bend the tab washer down!! reassemble the rest of the motor according to the cruciform instructions