More on big bore kits. A "bolt on" application? Will the transfers work? Is this the one we've all been waiting for??

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Feb 15, 2019
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So if anyone read my last thread I'm looking for a spring project and I want to put a big bore top end on my pk80 race engine. I have drove myself crazy trying to find something that isn't going to.take a lot of modification like the stihl 660 and take me out of riding for weeks if not months. we'll I think I have found it. These come in two sizes, an 85cc kit (50mm piston) and a 102cc kit (55mm piston) they are for a vespa 50 2 stroke scooter and the stroke is 43 mm. With my pk80 the stroke is 40mm so with the right combination of gaskets a 43mm stroke is going to work right? And the wristpin is either 10mm or 12 depends on the year of the scooter but need to look further. But, if.it does come with a 12mm pin I can bore out my pk80 rod to 12 yeah? Or better yet put a sleeve inside the 12mm pin to make it 10mm?? What do you guys think will this work as a bolt on application for the most part?? I realize if I go for the 102cc I may have to mill out the crankcase a bit but what about the transfers? These kits have two transfers on the sides divided up into 5 or 6 I believe. It looks close. Also the 102cc version has a third boost port will this be ok? Will I have to mill out my case for it? Would love anybody's input and knowledge as this is my first time stepping into the realm of bore kits. This is about the only thing left i want or can do to my bike. And i want a little challenge but i am no welder and the stihl 660, 440, exc. Are more than I would like to take on. These kits look legit. Hopefully y'all agree or atleast agree that is might work lol. At less than $150 dollars I'd be willing to try for 102cc and share my experience.
 

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JerboaJohn

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Jul 29, 2018
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I wouldn't sleeve the wristpin. not sure but the pin on my 48cc feels like only has .0008- .0010 fit in the bore. you can get pins. they are pretty cheap

just a add-on thought: any jug swap isn't just a bolt on. mostly the transfers dont exactly line up even if you can get a 45mm bolt pattern jug. some enterprising peeps Have done a Stihl swap (660?) read as much as you can here and on the mb forums for tidbits and tips. I'd say in any case it's an advanced project. You might be way ahead to get that BGF sleeved jug and it can be bored 2-3mm easily from the looks of it. just find the piston+rings you'd need then.
if it was easy or only little bit effort, so many ppl would been running big jugs. I took on making a head from billet for this weird 42.6mm CG jug I got from BGF and it was the hardest machining project ever. lucky it had piston and rings in kit form.
 
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I know it's going to be a project That's why I had bolt on in quotation marks. Lol I meant that it is going to be way easier that the ms660 Frankenstein build where you have to completely full in the transfers and get a different rod and spacer and all that jazz. I'm looking for an easier project. I'm just trying to get some input on what people think would work and what wouldn't. I was going try putting it on my motor but changed my mind. Why ruin a perfectly good pk80 race engine built by Fred at cr machine?? I'm probably juat going to buy a stock pk80 and take my time and work on it in my free time and it's going to be a long term project or might just go out and get a 200cc 2stroke and mount on my bike. Or might go with the sleeved cylinder. I haven't completely talked myself into One or the other. Just trying to get some educated input I am actually leaning towards just getting the 200cc motor and making it a suicide shift. It's the least amount of money and work for the desired effect....more power and there are a few guys in my area.that run 110cc mx/pitbike motors and the cops don't care as long as your not screaming through town with a straight pipe.
 

Akutey

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Dec 10, 2018
Messages
32
The Malossi race pipe fits a China Girl with a little bit of filing to the bolt holes. Their may be more compatible parts. Biggest problem I can think of for the cylinder is the bottom transfer entrance, it's mismatched to the CG base. Copious amounts if JB Weld isnt too elegant and then you have the port timing issues if it isnt spaced proper. Spaced to 43mm stroke and a 50mm is a 1.163 Bore/Stroke ratio vs the 47/40CG that makes 1.175 ratio so the geometry will be nearly the same.
 

JerboaJohn

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Jul 29, 2018
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but if the stroke is not the same as the jug is designed for, port timing can be off 4- 5 degrees for every mm difference. 180/40 = 4.5deg/mm and happens twice. I think it'd be a gamble?
 

Street Ryderz

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Joined
Oct 14, 2016
Messages
2,902
I wouldn't sleeve the wristpin. not sure but the pin on my 48cc feels like only has .0008- .0010 fit in the bore. you can get pins. they are pretty cheap

just a add-on thought: any jug swap isn't just a bolt on. mostly the transfers dont exactly line up even if you can get a 45mm bolt pattern jug. some enterprising peeps Have done a Stihl swap (660?) read as much as you can here and on the mb forums for tidbits and tips. I'd say in any case it's an advanced project. You might be way ahead to get that BGF sleeved jug and it can be bored 2-3mm easily from the looks of it. just find the piston+rings you'd need then.
if it was easy or only little bit effort, so many ppl would been running big jugs. I took on making a head from billet for this weird 42.6mm CG jug I got from BGF and it was the hardest machining project ever. lucky it had piston and rings in kit form.
The 85cc cylinder shown is for a case reed induction as is the 102cc,There is no figure for the rod length so it's not known from those add's.Anything with a larger bore will require mill/machine work,bolt hole relocation,and other work.You also can't just dill or ream out the rod's little end,that bore has to be concentric and finished perfectly smooth,not easy with hardened steel.Then the geometry isn't just the stroke length but the rod's length aswell,The cranks center and the distance between it and the rods big end center differ's depending on crank circle dia and length of rod used to travel the 40mm or whatever stroke and this makes different rod anglarities,dwell and affects the time over area
but if the stroke is not the same as the jug is designed for, port timing can be off 4- 5 degrees for every mm difference. 180/40 = 4.5deg/mm and happens twice. I think it'd be a gamble?
 
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Feb 15, 2019
Messages
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Ok everyone thanks for the education. It looks like this is going to be a bust in my list of ideas for more power so I've devised a new plan of action. I think I'm going to go with a bigger motor all together. I have a photo/add below for a 100cc complete two stroke motor I think will work. And will be less of a project. I just have to figure out a mounting bracket for mounting on my frame. And I also don't think it will be so much power that I shred my back rim. I might actually change the gearing and go with a 28T rear sprocket and it probably will come with a 13t front sprocket. Manual clutch and I think 4 gears. I'm not saying I'm going to be riding balls to the wall like its a full size motorcycle but it will be nice knowing the speed and power is there if I need it, not to mention bragging right in my local mb community of having the fastest bike around. Updates to come I think I'm going to order the motor tomorrow.
 

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