Piston blocking exhaust @ BDC

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k.mah

New Member
Nov 22, 2012
173
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oceanside,ca
So i have engine apart and decided to go over everything. What i noticed was that at BDC the piston is still occluding roughly 25-30% of the exhaust exit stream. I have a grubee skyhawk 66cc (the 09 version).
Wondering if this is a normal thing or if i would be better porting the exhaust stream on the head higher to increase exhaust timing.
I had roughly the same issue with the intake and decided to port the piston so at TDC it was 100% open and not occluded.
Any and all suggestions/info would be greatly appreciated
 

k.mah

New Member
Nov 22, 2012
173
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oceanside,ca
Agreed! But I am very impulsive and love tinkering/modifying, basically gives me something to do at a low cost risk.
I ended up ramping the piston at the exhaust port and the 2 transfer ports...then decided all the metal below the transfer ports was just friction and ground it out. Ill try to get some pics.
Whether it will help or make any difference or not i have no idea, but ill soon find out once i get my new gaskets in the mail!
 

k.mah

New Member
Nov 22, 2012
173
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oceanside,ca
Got the gaskets, rode it about, 40 feet and exhaust gasket blew out (tore in half). Looked at the attachment plate of the exhaust and noticed a bow in it so i ground it flat and re installed with another gasket.
Overall, what i changed was that i lengthened the intake manifold by about 3", and did all the porting/milling work to the head and piston.
All i need to say is WOW! I gained ALOT of torque, which I would say came from the intake extension. At idle the engine is VERY smooth. also, when going at wot it seems to be notably smoother than before, placebo, maybe!
With my current 44t sprocket I am able to hold 35mph on flat ground(verified by a vehicle driving right next to me) and it will pull up any hill without a hiccup. Soon to try the 36t sprocket I have!
Can't wait to see how long this little engine will take my abuse before it blows (engine still only has probably 50-60 miles on it)
Boy do I love this hobby! brnot
 

mew905

New Member
Sep 24, 2012
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Moose Jaw
I've been researching these motors for a little while and it sounds like you're doin pretty good. I ported mine to be pretty wide but I lost compression somewhere. Being winter I'm tearing the motor down to clean and modify it, I got some high quality SKF 6202-RSH/C3 bearings, I'm just waiting on getting the needle bearing. You just need to watch the weaknesses, which are essentially the bearings and studs/bolts. Intake/exhaust bolts can be replaced with M6x16 (1mm thread pitch) bolts (probably a bit longer, thats the size I bought and I'm fairly certain they dont reach the end of the stud hole, maybe M6x20?), apparently the head studs are either M6x112, M8x112 (both 1mm pitch), or M8x112 (1.25mm pitch). I tried measuring mine, but I cant get a good number, I measured the distance between two threads, 0.25mm is hard to see with the eye, then I measured 1 cm, and counted the threads between, and got 9. It's frustrating because 10 would be 1mm pitch, and 8 would be 1.25mm pitch, here I am counting right in the middle.

But, I'd say go to canadian tire if you can and grab a compression tester (for around $30), and see where you're at. I'm getting 80psi (problematically low), 100psi is normal (I think, I just got my tester so I dont know how they are out of the box), 125psi is great (achieved by shaving the head and stuffing the crank), but if you improve your bolts and grab the metal gaskets available on piratecycles, Sick Bike Parts, and other sites, you could theoretically go up to 175psi max, but you'd need a much stronger spark. If you're using the stock CDI, doing this may not spark, I'm unsure what mods you've done, but there's alot you can do to these motors to get alot more out of them :)

Reed valves will boost compression by not allowing air/fuel back out of the cylinder on the downstroke. I agree sloping the bottom of the transfers could help as being squared they block some flow (doing this may also reduce blowback on the downstroke). Shave your cylinder base gasket because if you've noticed when you pulled the cylinder off, the gasket actually blocks some of the port. Also a mod you may consider (though will require a different exhaust) is reversing the jug (cylinder). I'd think all it would take is cutting open some of the crank case where the transfers meet it. Then you get a ram air effect when at speed (this is what I plan to do).
 
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k.mah

New Member
Nov 22, 2012
173
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0
oceanside,ca
I have been needing to get replacement bearings, simply for the reason i dont trust the Chinese ones with my engine putting out this much more power than stock. You wouldnt happen to have the direct item numbers from the ones you ordered?
As far as studs, i stripped the stock ones and replaced them, much more confident in ones made out of decent materials.
I hadn't thought about modifying the bottom of the transfer ports, but next time i have it apart ill have to take a look! And i was thinking about reversing the jug but decided against it because i would rather have the majority of air against the exhaust side of the jug. I would imagine that side would have a bit more heat to dissipate than the intake side.
Now I'm curious as to my compression. I have a puch 70cc hihi head on it with 1 thin gasket (.4mm) and the jug shaved down just a bit where the head meets it.