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Heads and Cylinders All about your porting, compression, rings, cylinder and piston modifications to your bicycles engine

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Old 02-25-2013, 10:38 AM
k.mah k.mah is offline
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Default Re: The Basics, Port & Polish and Port Matching

reed valve is a 1-way valve. the 3rd transfer port allows for more fuel flow through the engine when paired with the reeds.
a 3rd port without reed would allow all of your pressure to escape OUT the intake side, basically robbing you of your power.

anyone feel free to correct me if that's off at all.
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Old 02-25-2013, 09:48 PM
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GearNut GearNut is offline
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Default Re: The Basics, Port & Polish and Port Matching

k.mah, you are correct.
Allow me to elaborate a little.

All 2 strokes without a reed valve, rotary valve, ect. actually pass the intake charge backwards, back out of the intake port, a little bit until the piston has traveled down the cylinder enough to completely cover the intake port. (This is why the air filter is always oily when you check it for cleanliness.)
A small portion of the intake charge actually passes through the carburetor backwards and an odd side effect to this is it sprays the air cleaner with the oily intake charge rendering the air cleaner to be "self oiling".

This is not good at all when looking for performance. Each time air passes through the carburetor it picks up fuel from the fuel circuit. This is what a carburetor is supposed to do. It is working just fine.

This whole backwards/ forwards episode ends up being a triple pass before it actually enters the engine.
Once through the carb while the piston is traveling up. (normal direction)
Once again through the carb as the piston is traveling down and the port is not closed yet (backwards direction).
The double pass charge is now trapped in the air cleaner and inlet side of the carburetor. It will stay suspended there momentarily.
And finally once again it passes through the carb as the piston travels upwards again (normal direction).
Mind you this is only a small portion of the original intake charge, a now very rich small portion.
The whole process repeats itself over and over with each power cycle of the engine.
As you can imagine this can be tricky to account for while tuning the carburetor for some circumstances such as racing. The engines RPM's also affect the effects this triple "gulp" of fuel, making things even more tricky.

Now a reed valve acts like a one way valve.
As the intake charge enters the crankcase the force of it opens the reed valve.
As the piston comes to a stop at the top of it's stroke the reed valve automatically closes due to the fact that there is no more vacuum being created by the piston rising and the intake charge comes to a rest.
Now the piston travels down and the intake charge cannot pass backwards through the carb due to the fact that the reed valve is closed.

This completely eliminates the triple gulp of fuel.
This also increases the pressure obtainable inside the crankcase.

Another downside of not having a reed valve is when this small portion of the intake charge is released back into the intake port it reduces the overall pressure potential that can build up inside the crankcase, not to mention the loss of intake charge volume.

More pressure = a faster initial charge through the transfer ports into the combustion chamber and more fuel to burn. More burn = more power.

Now that the intake port is sealed off by the reed valve when the piston it traveling down it can also be used as a transfer port too if the correct cylinder and piston modifications are made.
Why not take advantage of this necessary void in the cylinder? Put it to use! It can do double duty!
"It's a chinagirl. It's a sometimes fun little engine that often will break your heart. It's disposable.

Does not come with a fortune cookie."

Last edited by GearNut; 02-26-2013 at 08:11 PM. Reason: and piston
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Old 02-25-2013, 10:49 PM
k.mah k.mah is offline
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Default Re: The Basics, Port & Polish and Port Matching

Yeah! What GearNut said
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Old 02-26-2013, 04:05 PM
Frogster Frogster is offline
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Default Re: The Basics, Port & Polish and Port Matching

Sick! thanks guys. See this is why I love this forum. Nowhere outside of the internet could I go and get access to so many knowledgeable people who are willing to give me advice. If you hadnt have explained it so concisely Gear nut, I might have been dumb enough to go and make a third transfer port when I go to port out my engine and inspect all my internals before breaking the motor in.

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Old 04-21-2013, 04:28 PM
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LAguyyy LAguyyy is offline
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Default Re: The Basics, Port & Polish and Port Matching

Not on the topic of porting, but when I popped my head off I added heli-coils for my studs while it was off. Nothing more reassuring then stainless steel threads. Drilling was not even required for the stock 8mm studs, just the included tap and insertion tool. Now I can torque to 15 ft/lbs without worry.
80cc/66cc black jet slant china girl, dynacraft cruiser, arrow torquer II exhaust, air filter and bolt/heli-coil upgrades. When she's excited she screems, "我是一个热的矮小的瓷女孩"
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Old 04-21-2013, 09:36 PM
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Venice Motor Bikes Venice Motor Bikes is offline
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Default Re: The Basics, Port & Polish and Port Matching

I'll throw this into the mix... I've built several reed valve HT engines, & I've found that they all needed to be jetted up (more gas).

Gearnuts 'triple gulp' explanation might just be the reason why HT reed engines need more fuel.
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Old 03-19-2014, 10:07 AM
CarpsCustoms CarpsCustoms is offline
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Default Re: The Basics, Port & Polish and Port Matching

hey BarelyAWake thanks for all that info. Im a noob who has a bikeberry motor with a pro jetpipe chamber & jaguar CDI..this really cleared up alotta stuff for me...

but Jw, after port matching, do you think I should trim my piston skirt myself??? Or am i better off buying the Cut On piston from jake's Custom's???. What do you think bro
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Old 05-22-2014, 01:13 PM
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rogergendron1 rogergendron1 is offline
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Default Re: The Basics, Port & Polish and Port Matching

Originally Posted by DanielMaia View Post
Anyone here have photos of transfer ports totally matched and polished?

I saw these photos once, but i dont know where...


i have a youtube vid posted of absoloutly perfectlyvport matched transfers on a stuffed grubee gt5 case .! its in the performance section under gt5 from the case up !
here to stay and compleatly insane ! deal with it lol
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Old 07-15-2015, 04:46 PM
Porting newb Porting newb is offline
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Default Re: The Basics, Port & Polish and Port Matching

Originally Posted by BarelyAWake View Post
Thanks Joe, and you're right silverbear... I forget the "mystery" of pullin' an engine apart for the first time... I also fear to sound "condescending" when attempting to explain something I think of as basic knowledge... overlooking the fact that it's not "basic" if it's simply unknown.

I'll start with the basic tools needed;

A Dremel is pretty much a must-have, you don't need this spiffy one: Dremel 4000-3/34 Variable Speed Rotary Tool (about $75), even a "generic" brand or the $30 base model will do just fine - yet the 4000 is such a great muti-use tool it's a worthy investment should you wish to get one.

While you can buy individual Dremel bits, they're over priced when purchased separately. Fortunately there's two lil kits perfect for the job & include almost everything ya need;

Dremel 686-01 Sanding Grinding Kit (usually $11)

Dremel 684-01 20-Piece Clean & Polish Kit (usually $11)

Other than what tools you'd need anyway to pull the motor apart (the four head bolts and the intake/exhaust manifolds, 10mm & 14mm sockets) all you'll really need beyond the above is some really fine sandpaper (400gt or finer), some soft cotton cloth... and new intake/exhaust/head & base gaskets ofc - you may be able to pull a new engine apart without damaging the gaskets, but an older, used engine is bound ta need 'em.

I should mention that ANY alteration of the engine can result in accidents and failure - DO NOT attempt this if you're not willing to say "doh" and get a new cylinder should you mess up or break something. I won't say "don't try this" if you've never done it before as this is the perfect lil engine to learn with... but I will say that even the "experts" screw up from time to time heh

Oh right - before ANY of this process is started make sure you're working in a clean, dry environment. If the engine is used, clean it thoroughly before you disassemble it. The whole point is ofc to get rid of what garbage may be in the engine - not add more lol You outa have some various containers to put parts in and I like to work on engine parts with some cardboard as a work surface - this helps prevent dinging up whatever part you're working on.

Remove exhaust
Remove intake
Remove spark plug
Remove four nuts holding the head on
Remove head
Remove cylinder

When removing the head and/or the cylinder you may find it's stuck. DO NOT "pry" at it with a screwdriver or whatever - this aluminum is soft and if you dig at it at all you'll gall it (making it hard for gaskets to seal) and/or break something. One trick is to find a piece of wood that fits in between the cooling fins to the nice, solid cylinder itself (the more surface area the better) and gently tap on the wood with a rubber mallet or it's equivalent.

The reason its sticking is the gaskets so you may need to tap a bit on one side, then some more on the other. You're trying to get the gaskets to release and they're not always cooperative (one reason I don't like insta-gasket goop). Believe it or not - this may indeed be the most difficult part of the entire procedure... stupid gaskets lol

Once the head is off the only thing holding the cylinder on is the base gasket being stuck - there's no "hidden" fasteners nor is the piston messin' with ya. If it just wont come off - hit the base gasket with some carb cleaner and let it soak... sometimes that helps. If you've tried repeatedly to get the cylinder off a used engine with no luck at all... you may wish to consider skipping the port & polish and just do the intake and exhaust manifolds. I'm not sayin' it's impossible, but this mod may not be worth broken cooling fins on a used engine - new (just) engines are cheap enough it may not be worth the effort... that's your call tho.

Now that you've gotten the base gasket free, the cylinder just slides up and off - but be wary, the piston will wanna slap up against the cylinder studs and if it's hard enough it'll leave a mark. You don't want any scoring or marring on the piston or cylinder walls - that's why we're doing alla this heh, so have a lil chunk of cardboard ready to put between the piston and the studs - you may wish to put a clean plastic bag over the piston and case just to keep crud out while yer working... you will be creating some metal dust when ya get to Dremeling.

*For a better tutorial about pullin' yer head and cylinder w/pics - check Norm's thread out: Engine top end rebuild & engine stand

Now that you've got yer engine all apart and the cylinder in hand, I'm gonna leave ya hanging for a few as I've got to go get summore parts for my bike and some foodstuffs fer lil ol' me FTW I'll continue babblin' on when I get the chance, other ppl's tips are ofc welcome

To be continued...
Can you post pictures of the bits your talking about that are needed for this job
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