V50 Smallframe: 43mm or 51mm Build?
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Member
1965 Vespa VBB
Joined: 20 Apr 2020
Posts: 26
Location: NYC
Sat Oct 10, 2020 6:05 am quote
I'm picking up a '76 V50 that I was going to do a full motor teardown and rebuild on, but as I will be tearing down to the crank and most likely replacing (as I want to increase displacement) I was curious if while I am replacing, should I just invest in the 51mm crank so that the V90/Primavera cylinders can be run?

I don't mind limiting myself to the 57.5mm kits on the 43mm stroke, but if installing the 51mm stroke crank will allow the ~130cc kits fit, I would go that route.

TL/DR: Looking for any opinions, especially from anyone who's done both, whether the long stroke conversion is worth my time, or if I should stand on the 43mm and tune around that, whether it be the Malossi or Polini cylinder.
Molto Verboso
Joined: 03 Nov 2011
Posts: 1091
Location: Racing Capital of the World
Sat Oct 10, 2020 6:10 am quote
Dirty30Dillon wrote:
I'm picking up a '76 V50 that I was going to do a full motor teardown and rebuild on, but as I will be tearing down to the crank and most likely replacing (as I want to increase displacement) I was curious if while I am replacing, should I just invest in the 51mm crank so that the V90/Primavera cylinders can be run?

I don't mind limiting myself to the 57.5mm kits on the 43mm stroke, but if installing the 51mm stroke crank will allow the ~130cc kits fit, I would go that route.

TL/DR: Looking for any opinions, especially from anyone who's done both, whether the long stroke conversion is worth my time, or if I should stand on the 43mm and tune around that, whether it be the Malossi or Polini cylinder.
51 opens up the door to a lot of options!

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Hooked
Joined: 22 Jul 2018
Posts: 118

Sat Oct 10, 2020 6:30 am quote
Do it!
Ossessionato
2015 GTS300, 1974 Primavera, 04 Ninja 250
Joined: 04 Apr 2013
Posts: 4503
Location: San Diego, CA
Sat Oct 10, 2020 11:13 am quote
You need to do it, there are so many better options on the better crank, but when you do it think about how hard you want to tune, you should go to the 20 or 24mm cone to allow more power. The 19mm tapers will shear if you tune much.
Member
1965 Vespa VBB
Joined: 20 Apr 2020
Posts: 26
Location: NYC
Sat Oct 10, 2020 7:49 pm quote
I'm still fairly new to the smallframe game, but as far as I gather the cones are the diameter of either stub of the crank? Does moving up to the larger cones change my needs for the clutch/clutch basket and ignition?
Ossessionato
Joined: 26 Oct 2015
Posts: 2685

Sat Oct 10, 2020 8:13 pm quote
Just set aside at least $1500 for parts if you want to build an engine up. If you spend closer to $2k-$2500 on parts, you should have a very well rounded and reliable tuned engine, ignition, clutch, transmission, and exhaust. I just figured i'd throw out some ballpark $$$$ numbers for you.
Molto Verboso
Joined: 03 Nov 2011
Posts: 1091
Location: Racing Capital of the World
Sat Oct 10, 2020 8:24 pm quote
Dirty30Dillon wrote:
I'm still fairly new to the smallframe game, but as far as I gather the cones are the diameter of either stub of the crank? Does moving up to the larger cones change my needs for the clutch/clutch basket and ignition?
Go to a 20/20. You will have to install a flywheel/ignition with 20mm cone. Everything else will be the same for you.

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Molto Verboso
Joined: 03 Nov 2011
Posts: 1091
Location: Racing Capital of the World
Sat Oct 10, 2020 8:26 pm quote
whodatschrome wrote:
Just set aside at least $1500 for parts if you want to build an engine up. If you spend closer to $2k-$2500 on parts, you should have a very well rounded and reliable tuned engine, ignition, clutch, transmission, and exhaust. I just figured i'd throw out some ballpark $$$$ numbers for you.
Probably about right on the money if you do it correctly; no short-cuts and no BS.
Ossessionato
Joined: 26 Oct 2015
Posts: 2685

Sun Oct 11, 2020 9:19 pm quote
Here’s some info from a 2010 SCOOTERING magazine...

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Ossessionato
Joined: 26 Oct 2015
Posts: 2685

Sun Oct 11, 2020 9:24 pm quote
And page 2. You can glean what you want from the article.

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Member
1965 Vespa VBB
Joined: 20 Apr 2020
Posts: 26
Location: NYC
Mon Oct 12, 2020 7:16 pm quote
That magazine article is succinct and clear.

I suppose I need to determine what my end goal withe the bike is. I think I am trying for a wide band, flat torque tourer. I have 16 other bikes in the garage right now, including a Lambretta Cento that's getting some work one, and I would love to have the two matched fairly to cruise on with the misses or a pal.

Ugh, well, always endless projects.

Is it false wisdom to think that building the bike initially on the 43mm crank with a mild kit and pipe to get the feel for it (never ridden a smallie in any trim/tune) is a good idea? I am an incessant tinkerer, so it would undoubtedly get the full treatment somewhere down the line. I just don't know if that makes sense in the long run.

Thanks everyone for the semi-spoon feeding, it's good to hear everyone's opinion to help temper my own.
Member
PK50XL,PK100S,ET3
Joined: 26 Aug 2020
Posts: 15
Location: UK
Wed Oct 14, 2020 5:50 am quote
Hi, from everything ive read, if you are going to be getting a new crank and kit etc then you might as well spend the money on the 51mm stroke crank and go 130.

I've recently built a 130 for a scooter I'm getting back on the road because i wanted it to be useable 2-up. I'm also experimenting with the 43mm stroke crank on my 50 still and playing around with porting, different kits, exhausts etc. I'm not expecting the 50 (100) engine to last particularly long.
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