At TDC what should be going on: Piston Vs Clyinder

So I take my cylinder off,

put my piston in my cylinder at where TDC is (or at firing time 2mm BTDC i dunno you tell me)

And I look around for stuff:

If the top of my piston skirt is covering up any of my intake port should I shave it down so that's open?

If my piston is covering up any of my transfer ports do I shave that as well?

It is my understanding that the answer is yes. However:

Is this always the case for max power? Does it matter whether is reed valve vs piston port? I've done this with good results for stock puch cylinders.

Right now I'm working on an a35 cylinder. The transfers are slightly covered up. And the intake is covered up big time.


Re: At TDC what should be going on: Piston Vs Clyi

Your intake port should be exposed when you are at TDC. If not all the way exposed, then... well, hmm, I suppose you might trim it so it is fully exposed. The transfers should be completely closed at TDC, as well as exhaust. At BDC, the transfers and exhaust should be fully open.

Somebody who plays with A35 cylinders will know exactly what the port timing opportunities there are in there...

Re: At TDC what should be going on: Piston Vs Clyi

OK I'm gonna post a bunch of pics to demonstrate:

I don't know how to link it right away so there will be some posts with nothing but attached pics.


Re: At TDC what should be going on: Piston Vs Clyi

pic 2


Re: At TDC what should be going on: Piston Vs Clyi

pic 3


Re: At TDC what should be going on: Piston Vs Clyi

pic 4


Re: At TDC what should be going on: Piston Vs Clyi

yeah, im pretty sure that lenthend part of the intake is for those two boost ports.

i think your fine, the thing about reed valve is you want to time the intake to be constantly open, cuz air can only go on way on the reed.


I put the piston in the cyl at prob 2mm BTDC, for TDC imagine a little further out. You'll see how far in the next pic.

You can clearly see a large portion the intake is covered up.

This is how deep the piston is in the previous pic.

This is the piston pushed out super far so you can see the rest of the intake port

This is how far the piston is pushed out this time

Mick, the transfer are definately completely OPEN at TDC with the exhaust completely closed.

So I guess what I'm asking is, is there any downside to cuting the upper piston skirt to where the intake is completely open.

The A35 has boost ports in the piston already and this is a reedvalve. Ideas?


#CrazyWayne™ rocks. #CrazyWayne /

I like the burnt oil no the back of the piston dome .

Running lean are we ?

Ma be to much timing ?

And all of that scuffing on the cylinder and piston to Wow !

Did it hard seize or soft seize .

Good wiki pic's

Hone it with 400 wet dry sand paper


new rings and a hone dude.

porting is fine. if you want to open it up go wider on the intake and exhaust, and a little taller on exhaust.


#CrazyWayne™ rocks. #CrazyWayne /

A new wrest pin & bearing with Sr clips to.


Unless someone stuffed a Puch piston in there the two slots cut in the piston skirt facing the intake are open a long time before the bottom of the skirt uncovers the intake port. If you have no holes in your piston skirt you can make some.


Oh I reviewed the pics and you have plenty of induction with that piston. In a piston ported cylinder suction builds in the case before the piston opens up the intake port which is kind of nice cause the velocity through the carb is greater than if it where open all during the the piston stroke up. A lot of reed valve engines are kind of hybrids in that they are not inducting all the time. Because of the longer induction time and lower velocity of the intake filling on reed valve cylinders a smaller carb works pretty good. Your engine may have two pulses of pull on the carb because the holes in the piston don't extend all the way to the bottom of the skirts.


I just got the bike. It was a targa owned by a kid, who obviously let the oil injection run out at some point.

When I took it off I was really surprised that the cylinder was in such bad shape because the bike ran great and got to 35 fast with great pickup.

News rings come in the mail this week, was already planning on the 400 grit wet sand.

I took the hump out of the exhaust outlet, which removed A LOT of material without even altering the exhuast port window.

I'm going to use polini reeds, daelim intake, bigger carb, bi turbo. I hope to hit 45mph out the box.

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