Planet Minis

mexicanyella

Safely, I mean, and taking into account a possible overbore or two to accomodate wear? 50mm? 52mm, somewhere in there? And could a 49cc jug go just as far?

dirtbkr188

The last oversize piston that Wiseco has available for the XR/CRF70 is a 49mm one. I know the Kitaco 75cc bbk uses a 48mm piston, and the DRATV 85cc kit uses a 51mm piston. How big you'd want to go would be dependent upon the O.D. of the cylinder spigot, and what is considered a safe amount to have left after boring it.

Godfried

51mm (85cc) would be the limit. I used to run a kit like this years ago. Worked well, although the cylinder skirt was really thin! I don't think this would leave a lot of room for overbores though...

Cheers,
Godfried

Z

I agree with Dries. 51mm or 85cc's on the stock crank. Powroll used to bore the stock 72cc cylinders to 51mm all the time before the JDM parts became available here.

mexicanyella

dirtbkr188, Godfried and Z: thanks for the info and sorry for the slow response. Another step closer to figuring out how much of the stuff I already have can be used towards building my larger-displacement engine project.

fatcaaat

I had success in the old days of taking them out to 51.

Z

Jared - Did you ever run any of the Powroll 85cc BBK's with the delrin wrist pin caps instead of the circlips? Good times all around..

fatcaaat

I've run Delrin wrist pin caps in the past on other things but not on a mini.

The first motor I ever did up for minis was good for the times. In 2001, I took a Stock CT70 motor, bored the jug to 51mm and bored and relocated the pin so the crank was 51mm. Heated up the connecting rod and mushed it in and trimmed the skirt off the piston (and i don't remember what the piston was from).

For the head, I did port it, installed springs, and welded up the cam and ground it based off what I thought would work. Motor still runs great in my buddies bike. It is on par with your typical 106 I suppose.

mexicanyella

Z: in what way are Delrin wrist pin caps a good time? Do they taste good or something? How are they more fun than circlips?

Fatcaaaaaaaaaaaaaaat: Hold on a minute. What'd you do, work for Powroll or something? Isn't what you described their own proprietary classified stroker process? How'd you come to perform those operations yourself?

Does longevity differ between the iron and aluminum cylinders? Is it just a heat-transfer issue that inspired the aluminum ones, or is it a significant weight savings or what?

My intended use won't be a bunch of Fatcaaaaat DC Youtube street drags, like with that awesome CL70...much milder than that, but I'm heavy so I want a bunch of torque.

RcWannabe

Aluminum cylinders are half the weight dissipate heat faster and dont expand like CI.

firepower354

Cast iron expands WAY less than aluminum.

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fatcaaat

Yeller...It ain't all that hard to do it. Just make up a jig, get out the torch, and squish. Not too difficult. As a matter of fact, you don't even need to take the connecting rod out of the crank.

mexicanyella

Wow. Now, just out of curiosity, would you apply the squish compression to the heated rod by holding the crank at the BDC position and pushing on a wrist-pin-sized rod through the small end? Would you need to support the big end as well to avoid knocking the crank webs out of parallel, or do you just support the ends of the crank?

I suppose you'd use a rosebud tip for that? How much compressive force do you have to apply to the rod to get it to squish the needed amount, once it's heated?

Back--sort of--to the original topic...can a 49cc cylinder be bored as far as a 72cc one?

fatcaaat

I'll tell you how I did it: Set the crank in a set of vee-blocks and position it under your press. Once you are sure it is perfectly at BDC and perfectly parallel, bolt a set of the correct sized vee-blocks to the top so it can't rotate. Now, align it perfectly centered to the press. Put a wrist pin in there so you don't accidentally squish the rod hole. Now heat it up nice and slow toward the thicker part down by the crank throws. Don't use oxy-acy because that is WAY to fast and hot for this. Go get a propane torch and heat it up cherry red...it will take about 2-3 minutes. Keep the heat on it and then press.

To be completely honest with you, we did this operation first. I had a guy with a caliper there watching for 5mm and we got it really close. When that was cooled off, we took the rod out and relocated the pin based on how the rod was squished. Moving the pin is easy when you work in a world class machine shop. But the rod squishing was winging it.

However, if you are into vintage racing and period pieces, you can't just pop in a TB or Takegawa stroker crank...they didn't have that in the 70's so you have to do it yourself.

mexicanyella

Okay, I am officially impressed. I'd have loved to have been there to see those operations take place.

So, anyone ever experimented with how far you can bore a 49cc Honda cylinder?

arlindsay1992

I don't know how far a 39mm cylinder can go but I can assure you it is less than a 47mm cylinder. If you look at the bottom you will see the skirt is very different in design. I'll snap a picture of a spare when I get home from school if you want.

firepower354

Bottom spigot is about 47mm, so no way it'll take a 47mm bore. It looks like a breakthrough between the fins or in to the chain tunnel would happen before that anyway. 2mm over Wiseco and a stroker might be the way to go. Likely have to notch/shorten the sleeve spigot and piston skirt for rod angle. I'm sure there are guys who know, but cheaters get quiet

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Custom FPM parts and engines.
Parts and service for all makes.
Honda Trail Bikes parts and accessories.
Pitster/Piranha/Thumpstar bikes, parts, engines and accessories.

mexicanyella

Arlindsay: thanks for the offer, but no need...I have two 72cc spare jugs and one 49cc one, but when I posted the question I was not able to go check. Looks like I'll get out to where I keep my bike stuff today, so I can dig them out and compare them. I appreciate it, though.

Firepower: Thanks also to you for your continuing answers. I was just asking because I wondered if they were essentially the same casting, with different sized holes bored in them, for some reason never having held the two up side by side to compare the spigots at the bottom. I figured it might be fun, if I go the short cylinder/DrATV stroker crank route, to have the jug still say "49cc" but be a 50mm bore or something. I guess not, unless I want to buy one of those jugs from CHP with the fake-out 49cc marking on it.

Basically I'm into the idea of building whatever I can that comes closest to my needs with the castoff stuff I've collected. Perhaps that exercise will give me more concrete, ambitious performance goals for my NEXT project.