Modern Vespa

Sorry to be posting something so specific, but I figure that maybe any answers to this question could be general enough to be of use to others. Here goes!

So I blew a hole in my piston and decided to replace the crank so long as I was in there, seeing as the big bearing in the old crank was visibly full of aluminum shards and presumably some molten aluminum as well. (Both cranks are the exact same model, W5 60mm's, for regular rotary valve engines.)

Anyway, I installed the new crank, tightened the clutch on, and found that the inside edge of the crank web was scraping against the crankcase. Flummoxed, I started taking measurements of the new crank vs. the old, and found that the shoulder on the clutch side of the shaft measured 7.6mm. Meanwhile, on the old crank (but same model), the shoulder was 7.8mm. 2/10ths of a millimetre ain't much, I suppose, but in this case it's enough to get things scraping.

So my question is, is there any reliable solution here? Some sort of shim I can install? Or will I have to write this one off and just go back to my old crank, and pray that the big bearing holds out?

Cheers
Sean

New boss same as the old boss!
let your crank beds in during the running in...
Anyway did you shove clutch side bearing to its far limits?

Hey, Fastfrog, thanks for chiming in. I suppose I could just try and run it as-is, but really, it's not just skimming things. It's *scraping*. With noise and stuff. All coming from that groove alongside the rotary pad, where the flange on the edge of the crank web extends furthest, and in some cases skimming the rotary pad itself.

And yes, this is only when the crank bearing is seated all the way home, which I made sure of. If I knock the bearing out flywheel-wards about a quarter millimetres, basically to where the retainer circlip stops further movement, the crank web then clears everything. I thought briefly about leaving it in that state but then thought better of it. I mean this crank'll be flying around at 7000rpm and presumably vibrating its way back from whence it came. To say nothing of the eventual slop that'll start appearing in the bearings.

(Or is it a common thing to just let recalcitrant cranks carve out their own breathing room? Is that what you were suggesting??)

In the meantime I cleaned my old crank as best I could (holy cow there was a lot of aluminum in there) and chucked that in and it's like butter. No grinding issues there, no sir.

anyway you need new bearings, new seals. Can't you just carefully grind this 0,2mm shoulder and get the correct play? It seems very easy with your dremel or even a flat file....

Ah no -- it's 0.3mm too short, not 0.3mm too long. I'd need to fashion some sort of very delicate spacer washer to get it to seat properly. And yes, I've installed all new bearings and seals.

I suppose the question is whether or not it's unheard of to get a badly machined crank from W5 / Serie Pro. I spent a day swapping cranks in and out of my case to make sure that the problem lay in the new crank's clutch-side shaft. I'm now 100% certain that that's the case.

can you post a pic we can watch this puzzling issue?

Well, I'm about to close the cases with my old crank in there, so please enjoy the attached diagram illustrating the problem!

(My Photoshop is on the fritz so I'm reduced to using MS Paint.)

That's what crossed my mind !!!

Saw them 2 live in Glasgow 2 weeks ago & they perfomed this !!

Saw them in Paris a week ago! they litteraly set the stage on fire!! great show....
back to your crank: it's a P200 race crank shown. is that your model?

Yup. 60mm Serie Pro. 110mm conrod.

I have 2 or 3 P200 cranks home. I can check if i get different measures where the seal applies...

i checked and got more than 7 mm on both std crank and race one... I guess it's your seal that's not enough shoved down and comes in contact with your web wall...

one more question: Did you change your clutch side seal? did you put it the right way (small circular spring facing web)?

Ah nope, the seal has absolutely nothing catching on either side. The rubbing is from the outer flange catching on the groove next to the rotary pad.

no circular mark where seal hedge could rub on web side?
I see clearly the groove you're talking about. It was done to enlarge a bit sealing pad and get a larger inlet surface on 180-200 models. Your 0,2 mm difference between the 2 cranks are normal clearance.... I'm sure prob is elsewhere.... Your clutch bearing is a standard model? right in place, not too strongly shoved? there is, clutch side, sort of a case rebate. This part is sometimes broken allowing bearing to go too far clutch side...

I checked the bearing seat and there are no cracks or wear there.

The reason why I'm pretty certain this is a defective crank is because my old one installed and fit perfectly. Its replacement -- the same make and model-- scrapes against the rotary pad and groove. I put the old crank back in and the fit is perfect again.

only easy solution is to reduce a bit this outer flange you were talking about 3 posts ago and back to the street!

I thought about grinding that down a bit, but then thought better of it when it occurred to me I've never cut anything from a crank before. And I'm kind of in a hurry here, which is a bad thing for Vespa engine rebuilds, but what can you do. Anyway I ended up washing, washing, washing out the big bearing on the old crank, hitting it with air, oiling it, reinstalling it, and it seems to be ok. Knock on wood.

My other concern was, if that machined shoulder part on that new crank was off by that amount, what else could be off-kilter?

good question.... What is the next crank issue?