|
|
Post by rocketdog on Feb 23, 2011 18:43:59 GMT -5
A note on the subject.
I've been seriously considering having Wiseco, or some other piston manufacturer, whip me up a few 47mm bore pistons for NCYs 50mm large port head. If you made a flat top with just a little more pin to piston crown lenght, and the proper valve reliefs, you could really make a 72 scream.
As far as the bottom end being able to take it, personally I think it would. I think the main problem with LARGER than 72cc kits is the reciprocating weight of the 85cc and up piston.
Great theory even if I'm wrong LOL.
RD
|
|
|
|
Post by dude on Feb 23, 2011 19:03:41 GMT -5
I thought I seen on TotalRukus where someone ordered pistons. What I rembered they where pricey. And I think they had to order x amount. I have all ways wondered about Honda xr70-75-80cc pistons.
|
|
|
|
Post by rocketdog on Feb 23, 2011 20:01:50 GMT -5
Your right on both counts Dude. But it might just be worth it to have one that fits.
RD
|
|
|
|
Post by 90GTVert on Feb 23, 2011 21:14:44 GMT -5
I think the Honda pistons might be a problem. I think their valves come in at a greater angle, so the reliefs would be off. I may be wrong though. It's been a little while since I had one apart. Not sure what size the wrist pin is either.
|
|
|
|
Post by 90GTVert on Feb 23, 2011 21:19:20 GMT -5
|
|
|
|
Post by motorhead on Feb 23, 2011 22:13:47 GMT -5
In the movie "World's Fastest Indian," Burt Monroe was a real life guy just like us, with a P.O.S. bike he wanted to make go faster. You think the QMB is slow and poorly built? This guy's bike originally had a displacement of 600cc and only did 55mph. He modded it to 950cc himself and got it over 200mph.
I bring this up because he molded his own motorcycle pistons from melted down Ford and Chevy car pistons. The guy made his own cylinders from scrap cast iron pipes and built himself home made tools. Then, he used those home made parts to set a yet unbroken speed record.
What have we become? Forget Wiseco or hunting down a close fit. Lets take off the skirts, grow a set, and mold our own proprietary high compression QMB pistons.
|
|
|
|
Post by dude on Feb 23, 2011 22:48:15 GMT -5
Ive thought that myself. Why havent someone built a racing carters that will require less work.
I seen a Junk yard melt down there own scrap. I cant even make a candle mold.
|
|
|
|
Post by stepthrutuner on Feb 23, 2011 23:26:37 GMT -5
Building a good piston is a high tech endeavor that involves many steps and and relies on 2 1/2 centuries of development. Two stroke pistons are cam ground and are actually somewhat oval shaped to compensate for irregular heating due to the exhaust side. Many factory four stroke pistons have steel anti expansion bands cast in to limit expansion and add strength. Sure you could make a good piston but I don't think it would come close giving the service in high speed use as a quality forged piece.
|
|
|
|
Post by speedy1125 on Feb 24, 2011 2:37:09 GMT -5
In the movie "World's Fastest Indian," Burt Monroe was a real life guy just like us, with a P.O.S. bike he wanted to make go faster. You think the QMB is slow and poorly built? This guy's bike originally had a displacement of 600cc and only did 55mph. He modded it to 950cc himself and got it over 200mph. I bring this up because he molded his own motorcycle pistons from melted down Ford and Chevy car pistons. The guy made his own cylinders from scrap cast iron pipes and built himself home made tools. Then, he used those home made parts to set a yet unbroken speed record. What have we become? Forget Wiseco or hunting down a close fit. Lets take off the skirts, grow a set, and mold our own proprietary high compression QMB pistons. Yep thats a great story. He had a set for sure. en.wikipedia.org/wiki/Burt_Munro |
|
|
|
Post by rocketdog on Feb 24, 2011 3:56:17 GMT -5
I wish I were that talented Motorhead. That is a wonderful story, isn't it? I tried this one GT. www.dratv.com/pili47.htmlNo luck, stock. Fly cutting the reliefs might do it though, if the piston crown is thick enough. RD |
|
|
|
Post by 90GTVert on Feb 24, 2011 7:52:46 GMT -5
The gudgeon pin size and location works though?
|
|
|
|
Post by rocketdog on Feb 24, 2011 12:18:24 GMT -5
Yeah, the pin is the same size. The pin to crown distance is a few mm longer. So I added a plate between the case and the cylinder. I tried several thicknesses. Either the valves would be too close or the compression would be no better than the standard dished 47mm BB one.
RD
|
|
|
|
Post by motorhead on Feb 24, 2011 18:05:49 GMT -5
Yeah, the pin is the same size. The pin to crown distance is a few mm longer. So I added a plate between the case and the cylinder. I tried several thicknesses. Either the valves would be too close or the compression would be no better than the standard dished 47mm BB one. RD Ditch the spacer plate. Here is what to try. Grind down the valve tappet on the rocker arms or thread the valve tappet away from the valve end. What I am suggesting is some way to shorten the valve travel to allow your experimental piston to clear the valves. In case the tapet looks like it wants to slip off the valve you could cut a woodruff key in half and weld it where the valve looks to slip at. By the way. I was hoping someone whould do this experiment with a 50cc but I'll gladly accept 70cc. Also are you tuning anything else on your scoot. Lastly, could you tell us what your top speed was with your standard 70cc kit, and what it is once this works? That way I can derive the performace gains to 50cc. Thanks, Rocket Dog. |
|
|
|
Post by 90GTVert on Feb 24, 2011 18:20:49 GMT -5
It sounds to me like RD was on the right track. I'm not so sure what you propose would work. I mean, it would work at the speeds we turn our engines over by hand, but is that gonna hold up at speed? If you just go grinding away at things, I would imagine you will create a valvetrain geometry issue. If you back them way off you're gonna have big problems too. Then there's the fact that you are trying to keep the valves from opening all the way to gain performance. Even if the valves didn't open as far and worked fine, the location of the wrist pin relative to the piston's crown would smash it into the head itself or just raise compression beyond any safe limits if I'm reading his post correctly. You can get 'em in 39mm too if you wanna try it out for a 49cc. dratv.com/z50engineparts.html |
|
|
|
Post by motorhead on Feb 24, 2011 19:47:15 GMT -5
Order two pistons. Take the first piston and cut deeper valve spaces into it until you can assemble the bike and crank the engine (with kill switch on) so you verify that it will spin with no piston contact. Then melt down the second piston and pour enough of it into piston #1 to thicken the crown back up. Trim off some piston skirt to lighten the piston after wards.
This will work. And if you doubt me I'll have to evoke some more monroe on you.
Then to get some life out of your piston I would say: Run it rich, retard spark, put a spacer on the throttle to only let it go 80% open max.
|
|
