head mod checklist

[paulos]

I'm going to launch into the unknown that is the engine of my ec200 to check out the condition of the top and bottom-ends and send the head off to have the squish sorted out. The bike has gone well in the 6 mths I've owned it and I've got the jetting pretty close but I want to check where I'm at for piece of mind.

I've rebuilt 2 strokes from scratch before, but they were old school air cooled engines and i just wanted to check a couple of things regarding getting the engine set up correctly before measuring the squish.

1. Piston/exhaust port height. From what i understand the optimum position for the piston is flush to the bottom edge of the exhaust port at BDC?

2. Piston/ring/cylinder condition. Again, my experience is with older engines, what tolerances should I be checking re piston/ring clearance and what should I be looking for with a plated bore with regards to wear and tear ? If I need to replace anything obviously I want to do it before I measure the existing squish.

Thanks in advance.

[Zman]

The piston being even with the bottom of the exhaust port is once piece of the puzzle but it is not as critical as the overall port timing. You can get a timing degree wheel like this to determine how your engine's port timing is set now. http://www.motionpro.com/motorcycle/partno/08-0092/

There are threads on how the port timing affects the power band so you need to figure out what you have first then what you want the engine to do differently.

[Jakobi]

Piston to bore tolerance (at tightest spot) 200 cc:
STD: 0.05 mm
Limit: 0.10 mm

As Zman said, on some engines the exhaust port has been used as a visual index for setting an engine up, but its not required. The timing is what is important. Essentially less base gaskets (lowering the ports), will move the power curve down, and vice versa. For the riding we do here in .au the goal is generally to try and get a linear curve with a meaty useable mid range. Top end screamers with weak bottoms aren't so useful in the bush, and depending on how you ride you may wish to sacrifice some top and move the power down more. I personally just use gearing once the engines setup.

The head work should improve power and efficiency across the board too, so what can happen is if you chose to lift the cylinder a touch you may find bottom end remains the same, and more gains in the mid to top. Keeping it the same you might get a better spread all over. Compression ratio will also have an effect on snap off idle and the engines ability to rev, as well as its fuelling requirements (octane).

What I have done is always tried to set the engine for a decent curve and left enough clearance and compression ratio that I can still shift the base gaskets around .1-.2mm in either direction and still have an engine that runs well. Squish between say 1mm and 1.4mm. Usually I'm somewhere in the middle with say 1.2mm squish and 13:1 UCCR.

[paulos]

Thanks guys. The bike looks to have just the one (green) base gasket at the moment, and to be honest I'm pretty happy with how the power is delivered so I might just leave well enough alone.

I will check the condition of the piston and rings as well as the bore. I'm thinking it's all good though as the bike runs really well I just want to get the head mod done to clean up the jetting and improve the fuel economy