Vespa Crank Cutting Question

Mitch Bristol /

I have a Polini 65cc kit that I put on years ago without increasing the rotary duct opening or cutting the crank. So I took old ned apart one more time to do it. Still not even sure what I am going to use to cut the crank, but as I was checking the measurements against the instructions that came with the kit something didn't quite line up right. According to Polini's instructions, it seems like I'dd be cutting the crand in the area that's recessed. I suppose this shouldn't matter as long as the surface of the crank is intact and full width. But I wanted to check with pros. Does this seem right?

Also, looking at the inlet/rotary window, when extending it out 4mm on each side according to the instructions, am I just angling that back in to the intake area? I've done this on a Vespa scooter, but on those cases you are just opening it straight up and down. do I want to get in there, smooth out those ledges you see, then ramp it down to an opening that is 4mm longer on each side? Actually, though, it looks like I don't want to extend it much if at al on the right side in the photo, because i don't want to get too close to that edge, right? So realistically I'll just be extending it on the left.

Any thoughts on either thing would be helpful. Cranks!

Crank.jpg
rotary.jpg
instruct.jpg

Re: Vespa Crank Cutting Question

Instead of just randomly cutting you can figure out at what degree it’s opening and closing and for how long. I’m not sure if all cranks and cases are the same, it seems like every case I’ve pulled apart has a different shape intake port.

https://vespa-ciao.nl/how-tos/motorische-how-tos/krukas-tuning-berekenen/

At about 20 minutes in it shows how to measure at what degree it opens and closes.(edited)

Re: Vespa Crank Cutting Question

Mitch Bristol /

ugh, science. i like when the pictures tell me what to do.

Re: Vespa Crank Cutting Question

Mitch Bristol /

Thanks for the link to that page. I think I get it even with the rough Google translation. But I don't think the info on that page is helpful without the calculator it is linking to, and the link is broken. Not sure if the info on that page allows me to calculate the "inlet period" without it.

Re: Vespa Crank Cutting Question

angle grinder does the trick

Re: Vespa Crank Cutting Question

For the cases, leave 1mm to seal on all sides of the intake and gut the rest

Re: Vespa Crank Cutting Question

Joe and i just had this conversation in recent days.

Most stock vespa cases have spigots between 9mm and 12mm and intake patch ports ranging from 120-145 degrees (the wee tiny combinations are usually -20mph ciao and the biggest combos are usually for 30+ grandes).

Any stock topend will improve hp and revs by boring the spigot to 13 and matching a 13.13 carb while also extending that patch porting to about a mm seal on all sides and getting around 160 degrees long max, especially useful if you are like Joe and turn cylinders into whiffle balls with bunches of finger ports and split ports.

Even typical 60-64cc kits only really need somewhere between that same 145 and 160-ish degrees of intake duration to provide noticeable improvements because they arent really all that big of displacement and they have pretty good transfers anyway.

because a stock case (not polini or other aftermarket casting) has an intake patch that can be opened up within a mm or so of all edges and will end up being just about 160-ish degrees without ever touching the crank, you dont really need to cut the crank lobes in most cases

its the small kits with modded transfers/ports and especially the bigger kits with or without the extra transfers and port mods that really do need more than 160 degrees of intake to shine. Thats when you should consider crank cutting or anticipato aftermarket cranks and even then you really dont want to get longer duration than the 180's / 190's cause approaching 200+ you are almost for sure going to get blowback out the spigot because you are probably already ATDC beforrrre the trailing cut edge of the lobe seals off the intake patch for the compression side of the stroke

BTDC and ATDC matters when cutting and you can get some insight at known good degree benchmarks on the interwebs including: https://vespa-ciao.nl/

Re: Vespa Crank Cutting Question

dang that video! look at the size of that intake, from the factory! that must be oil injection era euro because it has the casting in there for the oil inject boss. i'm seeing a few other differences from a US case.

i had a CDI/ oil injected case from europe that i traded to tyler, the intake wasn't that big in my memory, but maybe.

i usually leave a little more than 1 mm around the perimeter of that port, i try not to go less than 1.5 mm. especially when you start swapping around aftermarket cranks with slightly different O.D. (+/- .1 mm or so) the clearance on that gap changes and its nice to have a little more cushion to ensure a healthy idle. I use bluing or paint marker on the port, then i trace the lines out with a comparator.

I prefer to take more off the crank and leave more on the port. I also make sure to cut back the crank web far enough that it isn't meeting a restriction when it comes through the intake. Gotta remember the air coming in the intake has to turn and flow around that crank web in order to get up to the transfer ports, so radiusing that inner edge of the crank web and scalloping out the edges of the cut portion makes a huge difference in how the engine performs.

You see high timed piston port kits at a symmetrical 180-200, i've gone pretty huge on crank timing and still haven't found the wall where it starts to cause problems. The malossi 43 i built this spring had killer torque and was cut to 8/12 specs. Forgot to check the actual timing when i put it back together but i'm thinking that works out to 180ish degrees?

Re: Vespa Crank Cutting Question

I've had factory cases with patches like that. i think only the 30mph ciaos with the larger clutch bell had them.

protip: every kinetic case has a perfect intake patch

Re: Vespa Crank Cutting Question

don't spill the vespalluminati secrets, gonna up the prices on kinetic cases

Re: Vespa Crank Cutting Question

Mitch Bristol /

This is way over my head bit at least now i know it is. I'll try to do the math on it and see if lengthening the intake patch will even get me anything given i can only really make it open sooner. If by the time I do that i feel like i actually get it, then I'll consider the crank again.

Are you guys saying that it's actually ok to widen and not just lengthen the intake a long as you stay within the patch?

Thanks!

Re: Vespa Crank Cutting Question

Dirty30 Dillon /

Kinetic TFR/Magnum cases had great intake patches, but don't directly accept points ignitions.

Late model (89+? I think) cases had a beautiful intake patch. I typically won't even bother building a vespa bottom end without grabbing a stock euro bottom end to use the crank and cases. Super fucking nice units for a reliable build.

Re: Vespa Crank Cutting Question

> Dirty30 Dillon Wrote:

> -------------------------------------------------------

> Kinetic TFR/Magnum cases had great intake patches, but don't directly

> accept points ignitions.

do you mean that some of them you gotta tap the points screw hole? I didn't know if that is an urban legend or what, the set i have it is tapped. otherwise everything is points compatable or better yet just run the kinetic cdi if you got it!

yeah duder, enlargening that port is supid easy. mark it off and trace lines and just don't cross the line. that video does a nice job of showing it.

Re: Vespa Crank Cutting Question

Dirty30 Dillon /

> Graham Motzing Wrote:

> -------------------------------------------------------

> do you mean that some of them you gotta tap the points screw hole? I

> didn't know if that is an urban legend or what, the set i have it is

> tapped. otherwise everything is points compatable or better yet just

> run the kinetic cdi if you got it!

The 4-5 kinetic motors I have messed with were missing the threaded hole and the machined pivot hole as well.

NOt impossible to re-manufacture, just more difficult than modifying stock cases.

Re: Vespa Crank Cutting Question

hmm ok so not an urban legend.

i got my cases from tyler so maybe he already did it? although he was running CDI on that motor.

Re: Vespa Crank Cutting Question

Mitch Bristol /

Did all the math and in the end came up with pretty much exactly what the dude in the video had for calculations. Which basically is exactly what the Polini instructions say to do on the inlet opening end, but barely if at all cutting into the closing side.

I've abused this bike so much in taking it apart and putting it back together multiple times the poor thing is just going to grind to a halt some day. I'll just push it over to a dumpster and walk away. A rolling basket of performance parts.

Re: Vespa Crank Cutting Question

180 ? intake? gnarly but i guess rotary you can pull long intake durations. vespas are pretty neat.

Re: Vespa Crank Cutting Question

yeah dude! it asymmetrical so the bolus of air or whatever thing inertia air blob keeps moving even when the piston is turned around and you can trap way more that way.

plus its not 180 like a puch or even like a normal rotary because it doesn't open the whole port at once, the port is a long skinny oval so it takes a while to get it fully open and close

also- and this is just me onto some crazy wayne shit right here- i think having it in the back half of the crankcase helps because by the time the pressure wave (sonic) from the piston going up, or coming back down, gets all the way back to suckin' on that intake port, you are getting like 4-5 maybe more degrees of lag than you would with the port right next to the piston (at high rpm)

i dunno, i spend way tooooooo much time just thinking crazy thoughts about what happens in a vespa engine.

Re: Vespa Crank Cutting Question

My kinetic cases needed tapping but had the pivot

« Go to Topic — end of thread

Want to post in this forum? We'd love to have you join the discussion, but first:

Login or Create Account